Thesis Laser Clad

7/31/2019 Thesis Laser Clad

1/177

LASER CLADDING WITH POWDER

Marcel Schneider

effect of some machining parameterson clad properties


2/177

De promotiecommissie is als volgt samengesteld:

Voorz itter en s ecretaris :

Pr of.d r .ir . H.J . G root en b oer (Un iver s it eit Twen t e)

Prom otor:

Pr of.d r . L.H.J .F . Beck m a n n (Un iver s it eit Twen t e)

As s is te n t-p rom otor:

Dr .ir . J . Meijer (Un ivers iteit Twen te)

Le d en :

Prof.d r . W. Wei (Un ivers iteit Twen te)

Prof.d r .ir . J .-P.G. Kru th (KU Leu ven )

Prof.d r .ir . J .B. J on ker (Un ivers iteit Twen te)

t it le: La s er cla dd in g

Ph .D. Th esis Un ivers ity of Twente, En sch ede,

Th e Netherlan ds

March 199 8

a u th or : M.F. Sch n eider

ISBN: 90 365 1098 8

su bject h eadings: las er clad ding

pr in ted by: Pr in t Pa r tn ers Ip s ka m p, En sch ede


3/177

LASER CLADDING WITH POWDER

PROEFSCHRIFT

effect of some machining parameterson clad properties

ter verkrijging van

de graad van doctor aan de Universiteit Twente,

op gezag van de rector magnificus,

prof.dr. F.A. van Vught,

volgens besluit van het College voor Promoties

in het openbaar te verdedigen

op donderdag 19 maart 1998 te 15.00 uur.

door

Marcel Fredrik Schneider

geboren op 19 juli 1968

te Haarlem


4/177

Dit proefschrift is goedgekeurd door de promotor:

prof.dr. L.H.J.F. Beckmann

en de assistent-promotor:

dr.ir. J. Meijer


5/177

Summary

Laser cladding is performed to improve the surface properties of metallic ma-

chine p arts locally. A clad ding m aterial with th e d esired propert ies is fu sed onto

a substrate by means of a laser beam. The mixing between the two materials

must be as small as possible to ut i l ise the propert ies of the coat ing material

m ost effectively. By im pr ovin g a tech n ical su rface locally with a d edicated m a-te r ia l, one can u se an ordin ary cheap b ase m ater ia l for the su rface tha t is n ot

bein g exposed to h igh loads.

Las er clad ding is con sidered as a s t rategic techn iqu e, s in ce it can yield s u rface

layers that , compared to other hard facing techniques, have superior propert ies

in te rms of pureness , homogenei ty , hardness , bonding and micros t ruc ture .

Hence, it was decided to in trodu ce th is techn iqu e in th e Dutch ind u stry.

The project consisted of three parts . One part covered the achievement and en-

hancement of ins ight in phenomena tha t occur in laser c ladding. The secondpart involved the development of tools that facili tate the use of laser cladding.

Th e third p art cons isted of th e developm ent of pra ct ical appl icat ions . Some a s-

pects of th is p roject are discu ss ed in th is th esis .

This thesis is directed to laser cladding with powder and a CO2

l aser as hea t

source . The laser beam intens i ty prof i le turned out to be an important pa-


6/177

6 S um m ary

rameter in laser cladding. A numerical model was developed that a l lows the

predict ion of the surface temperature dist r ibut ion that is a t ta ined with an arbi-

t rari ly shaped intensi ty profi le . Input parameters for this model are laser ma-

chining parameters and propert ies of the laser beam, as wel l as material prop-

ert ies and the absorpt ion of laser energy at the surface. The thermal material

propert ies an d th e ab sorpt ivity were derived from experimen ts.

An an alysis th at was performed with th is m odel on several s ta n da rd las er

beam intensi ty profi les , showed that an oblong laser spot with a homogeneous

energy dist ribut ion in the direct ion perpendicular to the direct ion of motion

yield the most uniform temperature dist r ibut ion. That property is useful for la-

ser c ladd ing, becau se su ch a tem pera tu re dis t r ibu t ion resu lt s in u n iform m ate-

rial properties along the track width. Especially for the cladding of larger areas,

th is is a very us efu l featu re.

An other rea son for the u se of in tegrators is th e ach ievemen t of a laser s pot with

well defined dimensions and an energy dist ribut ion that is independent of theapplied laser source. This is essential for an effective transfer of cladding re-

su lt s from on e laser s ys tem to an other .

Therefore, such a l ine integrator has been developed. The required homogene-

ous energy dist ribu t ion was ach ieved.

Experiments performed on laser cladding with preplaced powder proved that

this method is sui table for obtaining single clad t racks. The powder is mixed

with a chem ical binder to form a p as te wh ich is spr ead over the su bs trate .

The tempera ture on the inte rface be tween coa t ing mater ia l and subs t ra te i s avery important parameter in laser cladding. If this interface temperature re-

mains too low, wetting of the substrate by the liquid clad material is l imited. In

that case, i rregularly shaped t racks with a lot of cracks, porosi ty and a poor

bon ding, are produ ced. However, if too high in terface temp eratu res a re reach ed,

severe melting of the substrate occurs. The high degree of mixing between ele-

men ts of the c ladd in g mater ia l and the s u bs t ra te can de ter iora te the c lad prop-

erties.

Some m elt in g of th e bas e ma terial is requ ired to at t a in a s t ron g fu sion bon d. In

the appl ied material combinat ion, a cobal t base powder appl ied on a hot work-ing st eel, a r elatively la rge dilu tion of 15 % pr oved to b e th e m ost effective for

ach ievin g clad layers with a good h ard n ess an d a perfect bon ding. It is poss ible

to reduce thi s dilu t ion, bu t as a consequ ence the h ardn ess i s redu ced a s well.

The preheat ing of the substrate to 100-200 C reduces the cool ing rates. This

reduces the res idua l s t ress and enhances the wet t ing and the bonding. Tem-

pera tu res of 30 0 C an d a bove are n ot app licable, becau se of evap orat ion of th e


7/177

S um m ary 7

chemical binder. Without a binder the powder is being blown away by the

shielding gas.

Th e preplaced powder m ethod is n ot su itab le for th e clad ding of larger a reas by

making several adjacent part ly overlapping t racks. Cracking of the cladded ar-

eas could not be prevented with the appl ied material combinat ion. More seri -

ously is the occurrence of a severe degree of dilution, that is inevitable. Since

the n ewly produced t rack cont ra c ts du r in g cooling, the b are s u bs t ra te a rea di-

rect ly next to it is m olten when performing th e n ext las er scan .

Laser cladding with powder injection did result in good quality clad areas with-

out severe di lut ion. Powder part ic les are injected into the laser generated melt

pool . These part ic les are heated during their f l ight through the laser beam.

Melt ing occurs only when they enter the melt pool . The supplied material en-

larges the melt and forms a layer on the surface. Compared to the preplaced

powder m ethod, th e melt depth is ea sier to contr ol an d p orosity is less likely tooccur .

The part ic les form a cloud that a t tenuates the laser beam. In the appl ied con-

figur at ion , the m easu red a t tenu at ion by a clou d con sist in g of pa rt ic les Stel lite 6

was less than 5 %. The at tenuated laser power heats the powder part ic les. This

heat ing was described in a physical analyt ical model . Most important parame-

ters for the heating of a particle are the particle size, the powder velocity and

th e las er power.

I f the powder i s hea ted to such an extent tha t a meta l vapour i s formed, a

plasma can be formed. This plasma effect ively absorbs al l laser radiat ion andstops the process. A plasma can only be formed over a certain cri t ical powder

density level (~ 2106

W/ cm2). In the applied configuration, this level is reached

in the focal area only. Plasma formation could be prevented by exchanging the

initially applied lens with one with a larger focal length. This increased the

bea m waist in th e focal region . Hence, th e crit ical power den sity level cou ld was

not exceeded anymore.

The applicabili ty of laser cladding was shown successfully with the development

of several industrial applications.


8/177

Samenvatting

Lasercladden wordt toegepast om de oppervlakte-eigenschappen van metalen

machine-onderdelen plaatsel i jk te verbeteren. Het cladmateriaal wordt met be-

hulp van een laser als warmtebron opgelast op het basismateriaal . Daarbi j is

he t essent iee l de vermenging tussen he t c ladmater iaa l en he t bas i smater iaa lzoveel m ogelijk te berp erken om zodoen de de eigen sch ap pen van h et c lad ma te-

riaal opt imaal te benut ten, terwij l toch een sterke smelthecht ing wordt verkre-

gen. Door al leen ter plaatse van de optredende belast ing een cladlaag aan te

brengen en verder te vols taan met een eenvoudig bas ism ater iaa l , kan een aa n-

zienl ijke b espa ring op m ateriaalkosten worden bereikt .

De eigenschappen van laser gecladde lagen zijn bijzonder goed in vergelijking

met andere technieken die harde deklagen opleveren: een zeer goede hecht ing,

een fi jnkorrel ige microstructuur, een betere homogenitei t , een geringere mate

van porosi teit , een grotere h ard h eid en de m ogelijkh eid om bi jzon dere m etasta -biele fases te verkri jgen. Lasercladden wordt daarom beschouwd als een veel-

belovende techniek, die voor bepaalde bedri j fs takken van strategisch belang

kan zi jn. Daarom werd besloten het lasercladden ook in de Nederlandse indus-

trie te in trodu ceren.


9/177

10 S am envattin g

Het project dat da artoe werd opgezet beston d u it dr ie delen. Het eers te deel be-

trof h et verkri jgen en vermeerd eren van procesken n is. Het tweede deel omvatte

he t ontwikkelen van hulpmiddelen en gereedschappen die he t toepassen van

h et las erclad den in de p rak t ijk m oeten vereenvoud igen . Het laa tste gedeelte be-

ston d u it het on twikkelen van echte toepas singen. Asp ecten van deze drie delen

worden beh an deld in di t proefsch rift .

Dit proefschrift r icht z ich op het c ladden met poeders, waarbi j een CO2

laser

dient a ls warmtebron. Het intensi tei tsprofiel van de laserbundel bl i jkt in be-

langri jke mate het c ladresul taat te benvloeden. Er is een numeriek model ont-

wikkeld dat de temperatuurverdel ing aan het oppervlak ten gevolge van een

willekeurig gevormde intensiteitsverdeling kan berekenen. Als invoerparameters

voor he t model fungeren verder machine- en laserbundelparameters ,

( thermische) materiaalparameters en de absorpt ie . De waarden van de thermi-

sche materiaalparameters en de absorpt ie zi jn experimenteel bepaald. Eenanalyse van enkele standaard intensi tei tsprofielen met di t model toonde aan

dat een zg. l i jnbron, dat is een laser spot met een langwerpige homogene ener-

gieverdeling in de richting loodrecht op de bewegingsrichting de meest uniforme

temp eratu u rverdel in g over de b reedte van h et c lad sp oor geeft . Daar m ee worden

dan ook de meest uniforme materiaaleigenschappen verkregen. Vooral voor het

c ladden van vlakken, wat he t aanbrengen van meerdere sporen naas t e lkaar

vereist , is dit erg n u ttig.

Een tweede reden voor het gebruik van integratoren is dat di t resul teert in een

goed b egren sd e sp ot op h et werkstu k m et een gedefin ieerde en ergieverdelin g dieonafhankelijk is van de laserbron. Dit is noodzakelijk voor een doeltreffende

overdracht van c ladresul ta ten tu ssen lasersys temen.

Een dergelijke l ijn in tegrator is da arom ontworpen en gereal iseerd. De verwach te

energieverdeling bleek inderdaad gerealiseerd te worden.

Een ui tgebreide serie experimenten toonde dat het c ladden met vooraf aange-

brach t poeder een geschikte methode i s voor h e t aan brengen van enkelvoud ige

clad lagen. Het poeder wordt gemen gd met een bind m iddel en d aa rn a in de vorm

van een p as t a aa ngebrach t op he t sub s t raa t .De tempera tuur op de inte rface tussen c ladlaag en subs t raa t i s bepa lend voor

het resul taat . Een te lage temperatuur belemmert een goede bevocht iging van

het substraat door het vloeibare cladmateriaal . Daardoor ontstaan onregelmati-

ge lagen m et veel sch eu ren , porosi teit en een s lechte h echt ing. Bij te hoge tem-

peratu ren sm elt h et bas ism ateriaal te veel en on tsta at te veel m enging.


10/177

S am envatting 11

Voor een goede clad laa g is een gerin ge ma te van in sm elt in g in h et ba sisma teri-

aal noodzakelijk voor het verkrijgen van een sterke smelthechting. Bij de ge-

bruikte materiaalcombinat ie , een poeder op basis van cobal t met een warm-

werkstaal, blijkt een relatief grote vermenging van ongeveer 15 % te resulteren

in de beste eigenschappen: een goede hecht ing en een hoge hardheid over de

clad laa g. Het is wel mogelijk m in der vermen gin g te verkri jgen , m aa r da n n eemt

de h ardh eid a f.

Voorverwarmen van het substraat tot 100 200 C vermindert de afkoelsnel-

heden, waardoor de spanningen in de c ladlaag afnemen. Voorverwarmen be-

vordert tevens het ui tvloeigedrag en de hecht ing. Bij temperaturen van 300 C

of h oger verda m pt ech ter h et bin dm iddel en wordt h et poeder weggeblazen d oor

he t schu tgas .

Het aanbrengen van meerdere sporen naas t e lkaar ten e inde grote vlakken te

cladden bleek niet succesvol met deze methode. Scheurvorming was met deze

materiaalcombinat ie niet te voorkomen. Bovendien is een te grote menging in-herent aan he t proces , doorda t he t subs t raa t di rekt naas t een gevormd spoor

bloot kom t te l iggen door contr act ie t ijden s afkoelin g van di t sp oor en da ard oor

door het laserlicht wordt besch enen bi j h et aan bren gen van een volgen d sp oor.

Goede resu ltaten met h et c lad den van grotere vlak ken zijn wel beh aa ld m et be-

h u lp van clad den m et poederinject ie . Daarb ij worden poederdeelt jes in een d oor

de laser gevormd sm eltba d geblazen. Deze deelt jes worden voorverwarm d in de

laserbundel , maar worden pas in het smeltbad vol ledig gesmolten. Het toege-

voerde materiaal vergroot het volume van het smeltbad en vormt na afkoeleneen deklaag op het substraat . Door het andere procesverloop bi j het c ladden

m et poederin ject ie is de insm elt in g in h et ba sism ateriaal is eenvoud iger te con-

troleren dan bi j de voorgeplaatst poeder methode, z i jn de restspanningen lager

en vermindert de aanwezigheid van porositeiten. Bovendien is het goed mogelijk

grotere vlak ken m et cladlagen te bedekk en.

De opwarming van poederdeel t jes t i jdens hun vlucht door de laserbundel is

gemodelleerd. De opwarming van het poeder bleek sterk afhankel i jk van de

deelt jesgroot te , de p oedersn elh eid en h et laserverm ogen. In de gebru ikte con fi-

gurat ie is de gemeten verzwakking van het laservermogen door een st room vandeel t jes Stel l ie t 6 minder dan 5 %. Het intensi tei tsprofiel van de laserbundel

wordt derhalve nauwelijks benvloed door de poederwolk.

Indien poederdeel t jes te s terk verhi t worden bi j hun gang door de laserbundel ,

kan een plasma worden gevormd. Dit plasma absorbeert de inval lende st ral ing

en stopt daardoor het c ladproces. Voor de vorming van een plasma is een be-

paalde kri t ische vermogensdichtheid van ca. 2106

W/ cm2

nodig. Het blijkt dat


11/177

12 S am envattin g

deze in de gebru ikte ops te llin g s lechts in de b u u r t van h e t bran dpu nt wordt ge-

real iseerd. De vorming van een plasma kon worden voorkomen door een opt iek

met een langere brandpuntsafs tand te kiezen, waardoor de bundeldiameter

rond he t brandpunt toeneemt en de kr i t i sche vermogensdichthe id nie t meer

overschreden ka n worden.

De prakt ische b ru ikba arh e id van h e t laserc ladd en is su ccesvol aan getoon d met

de on twikkel in g van enk ele in du strile toepass in gen .


12/177

List of symbols

A

C

cp

dc

D

Dpn

E

Ea,c,r,ra,t,0

fv

g

H

hh

c

I

k

lz

L

m

absorptivity [-]

density powder cloud [kgm-3

]

sp ecific hea t [J kg-1

K-1

]

clad d epth [m ]dimen sion las er s pot on workpiece in t raverse direct ion [m]

diam eter powder n ozzle [m ]

sp ecific ener gy (E=P/ vD) [J m-2

]

pa rticle en ergy (a : absorbed; c: convective loss ; r: reflective loss;

ra : ra diative loss; t: t rans mi t ted; o: in ciden t) [J ]

volu m e fra ction of powder p ar ticles in th e powder st rea m [-]

gravitat ion al acceleration [m s-2

]

sp ecific enth alpy [J kg-1

]

grid s tep s ize [m m ]clad h eight [m ]

beam power density [Wm-2

]

th erm al cond u ctivity [Wm-1

K-1

]

dista n ce from top of powder cloud to pa rticle [m ]

laten t h eat of fu sion [J kg-1

]

mass [kg]


13/177

14 Lis t of s y m bols

mp

p

P

Pa t

q

q

R

rp

s

t

tc

ti

tin

tou t

TT

0

Tm a x

v

vp

wc

wof

xin

xou t

x ,y ,z

powder m as s flow rate [kgs-1

]

pres su re [Nm-2

]

laser power [W]

at ten u ation of laser p ower [W]

power of poin t s ou rce [W]

h eat s our ce [Wm -3 ]

re flect ivity [-]

ra diu s of powder p ar ticle [m ]

dista n ce from exit powder n ozzle [m ]

time [s]

total clad h eight [m ]

intera ction time [s]

in sta n t par t ic le enters laser beam [s]

in sta n t pa rt icle leaves laser b eam [s]

temp eratu re [K]initial tem pera tu re [K]

ma ximu m tempera tu re [K]

feed r ate workp iece [m s-1

]

velocity powder particles [ms-1

]

clad width [m ]

waist of focused laser beam [m]

x-posit ion wh ere part ic le enters th e las er beam [m]

x-position where particle leaves the laser beam [m]

Cartesian co-ordin ate system [m]

t

T

x

p

h eat exch an ge coefficien t to en viron m en t [Wm-2

K-1

]

th erm al expan sion coefficien t [K-1

]

extinct ion coefficien t (=0

fv) [m

-1]

time interval [s]

tem pera tu re d ifferen ce [K]

dis tan ce cent re laser beam an d cent re powder s t ream [m]

angle powder injection nozzle relative to the workpiece []

angle of particle movement relative to the workpiece []

th erm al d iffu sivity [Wm

-2

]fluid viscosity [kgs

-1m

-1]

kinem atic viscos ity(=/)[m2s

-1]

m ater ial den sity [kgm-3

]

thermal t ime constant [s]

h alf an gle of divergence p owder s trea m []


14/177

Contents

Su m m a ry 5

Sa m en va t t in g 9

Lis t of sym bols 13

1 Prefa ce 171 .1 In trodu ct ion 17

1 .2 Overview of th e th es is 18

2 La s er cla dd in g: s ta te-of-th e-a r t 21

2 .1 La s er cla dd in g a n d oth er la s er s u rfa ce t rea tm en ts 21

2 .2 Applica b ility of la s er cla dd in g 24

2 .3 La s er cla dd in g vers u s con ven t ion a l m eth ods 26

2 .4 La s er cla dd in g m eth ods 27

2 .5 Cla d la yer p roper t ies 322 .6 Ma ter ia l p roper t ies 35

2 .6 .1 Powders on a s teel s u bs t ra te 36

2 .6 .2 Non -ferrou s s u bs tra tes 39

2 .6 .2 .1 Nickel ba s e s u bs t ra tes 39

2 .6 .2 .2 Alu m in iu m a n d t it a n iu m ba s e s u bs t ra tes 39

2 .7 Effect of p roces s pa ra m eters 40


15/177

16 Conten ts

2 .7 .1 Exper im en ta l m eth ods 40

2 .7 .2 Ph ys ica l m odels 45

2 .8 Proces s con trol 50

2 .9 Op tica l s ys tem 52

2.10 Preview 55

3 Tem pera tu re p rofile on a cla d s u rfa ce 57

3 .1 Nu m e r ica l m e th o d for ca lcu la t in g t h e t em p er a tu r e d is t rib u t ion 5 8

3 .2 Dis cu s s ion 66

4 Des ign a n d developm en t of a lin e in tegra tor 67

4 .1 Two d im en s ion a l ka leidos cope-type bea m in tegra tor 68

4 .2 Op tica l des ign lin e in tegra tor 70

5 La s er cla dd in g with p rep la ced powder 795 .1 Exper im en ta l s et -u p 79

5 .2 Exper im en ta l r es u lts 80

6 La s er cla dd in g with powder in ject ion 95

6 .1 Exper im en ts 95

6.2 Modellin g of pa rticle h eating in a CO2

la s er bea m 101

6.3 Ion is a t ion of powder pa r t icles 114

7 Exa m ples of a pp lica t ion s 1197 .1 Dies el en gin e in let va lve 119

7 .2 Extru der s crew 124

7.3 Wirelin e cylin der h ea d 129

8 Con clu s ion s , review a n d recom m en da t ion s 135

8 .1 Con clu s ion s 135

8 .2 Review 138

8.3 Recom m en da t ion s a n d fu tu re res ea rch 143

Appen dix 1 : Ma ter ia l p roper t ies 145

App end ix 2: Determinat ion of th e ab sorpt ion of las er energy 14 7

App en dix 3 : Exp er im en ts on la ser cla dd in g with p rep la ced powder 1 51

App en dix 4 : Powder -s ub stra te com bin a t ion s repor ted in litera tu re 1 59

Da n kwoord 165

Referen ces 167


16/177

Chapter 1

Preface

1.1 Introduction

To improve the surface propert ies of metal l ic mechanical parts , such as the re-

s i s tance aga ins t wear and corros ion, severa l thermal surface t rea tments a reavai lable; for instance, f lame spraying, plasma spraying and arc welding are

estab lish ed techn iqu es. Cha racteris t ic for thes e tech n iqu es is th e app licat ion of

a surface layer with the required propert ies on top of a cheap material without

those pr oper t ies . Depend ing on the a ppl ied techn iqu e , comm on problems are a

combina t ion of a p oor bond in g of th e ap pl ied su rface layer to th e ba se m aterial,

the occurrence of porosi ty, the thermal distort ion of the workpiece, the mixing

of the surface layer with the base material and the inabili ty of a very local

t rea tment .

One of the techniques that overcomes these problems is laser cladding. Laserclad ding ha s been defin ed as a pr ocess wh ich is u sed to fu se with a las er beam

another material which has different metal lurgical propert ies on a substrate ,

whereby only a very thin layer of the substrate has to be melted in order to

ach ieve metal lu rgical bon ding with m in im al dilu t ion of ad ded m aterial an d s u b-

strate in order to maintain the original propert ies of the coat ing material

[Komvopou los, 19 90 ].


17/177


18/177

Preface 19

subs t ra t e on t he o the r hand . Thi s means t ha t t he hea t i npu t by t he l a se r

source must be well-controlled to achieve an acceptable degree of mixing.

It also followed from the li terature that the properties of produced clad layers

depend strongly on the appl ied temperature cycle . The favourable propert ies of

clad layers are not only the resul t of the pureness of the surface layer, but are

also caused by the high cool ing rates that occur. This so-cal led quenching re-

sul ts in the formation of favourable fine-grained microstructures or in meta-

s table phases .

Th ese two reason s, i .e . the contr ol of th e degree of m ixin g an d th e depen den cy

on the temperature cycle , make i t important to control the laser energy dist ri -

bu t ion a nd the resu lt ing tempera tu re dis t r ibu t ion over the su rface . In chap ter 3

a n u merical m odel is d escribed wh ich al lows th e calcu lat ion of th e su rface tem-

perature dist r ibut ion that is imposed by an arbi t rari ly shaped moving laser

spot. An evaluation of the effect of several common laser beam profiles showed

tha t a l ine shaped laser beam wi th an uni form power dens i ty di s t r ibut ion i sbest su ited for the ach ievemen t of a m ore or less u n iform tem pera tu re dist r ibu -

t ion and therewith constant c lad propert ies over the width of the t rack. This

conclu sion is su pported in th e literatu re by several au th ors.

In order to cover larger areas i t can be necessary to apply several adjacent

t racks . Th is enh an ces the n ecessi ty to apply a laser bea m with a u n iform p ower

den si ty dist r ibu t ion. However, th e power den si ty dist r ibu t ion is a cha racteris t ic

property of a laser system. Therefore, dedicated opt ics must be used to change

the original laser power density distribution into a uniform one. A line integra-

tor which was developed to perform that task, is demonstrated in this thesis inchapter 4 . The use of integra tors makes c ladding resul t s independent of the

appl ied laser source. Therefore, resul ts can be exchanged more easi ly between

different laser systems. This aspect agrees well with the intended facili tation of

t ran sfer of clad ding resu lts to the Du tch indu stry.

Ad. 2 Th e secon d par t of the process i s the add it ion of a c ladd ing ma ter ia l to

the substrate . Various methods are avai lable , but according to the l i terature

the most common method is the appl icat ion of powder. This can be done prior

to the process (preplaced) or during the process (powder feeding). The effect ofprocess parameters on clad layer propert ies was invest igated for laser cladding

with preplaced power as well as for laser cladding with powder injection. These

two methods have a qui te different process course. When cladding with pre-

placed powder, the m elt pool is formed on top of th e clad ding ma terial an d p ro-

ceeds downwards to the su bs t ra te . On ly when th e su bs t ra te has b een melted a

clad layer can be formed . However, th e powder inject ion meth od s tarts with th e


19/177

20 Chapter 1

forma tion of a melt pool in th e su bs trate in wh ich clad ding powder is b ein g fed.

Thus a clad layer is formed almost instantaneously. The preplaced powder re-

su lt s a re presented in ch apter 5 .

In th e case of las er clad ding with powder inject ion, powder p art ic les a re t ra n s-

ported by a gas st ream and injected into the melt pool . The powder part ic les

absorb laser energy on their way through the laser beam. Therefore, they are

preheated on arrival in the melt pool . In the melt pool , they exchange heat and

mix with the elements already present . The powder part ic les might also affect

th e las er power den si ty. If th is h ap pen s indeed, th is effect mu st b e in corporated

in the model described in chapter 3. The interact ion between powder and laser

beam is s tu died in ch apter 6 .

Exper imen ts sh owed tha t plasm a format ion can occur in the process a rea . This

is a c r it ica l ph enomen on, becaus e the plasm a can sh ie ld th e su bs t ra te from th e

laser beam and thus s top the c ladding. This phenomenon i s di scussed in

cha pter 6 as well.

As mentioned before, the work described in this thesis was ini t ia ted to intro-

duce the laser c ladding technology in the Dutch indus t ry . Emphasis was put

on the achievement of good qual i ty metal c lad layers on top of s teel surfaces.

Having gained enough insight in the process mechanisms, the final s tep which

is necessary to convince industry to apply laser cladding themselves is the de-

velopment of pract ical appl icat ions. Some examples of such appl icat ions that

were developed in th e au th or 's laboratory are sh own in ch ap ter 7.


20/177

Chapter 2

Laser cladding: state-of-the-art

2.1 Laser cladding and other laser surface treatments

Laser cladding is one of the laser surface treatments. The positive effects of la-ser su rface t rea tmen ts a re bas ed on a chan ge of the m icros t ru c ture or the m a-

terial com posi t ion of th e su rface layer du e to a th erma l cycle which is indu ced

by a m ovin g las er sou rce. Th e process is s h own sch ema tically in figure 2 .1.

Th e t reated a rea is h eated by ab sorp t ion of energy delivered by th e las er bea m .

The heat input due to a high power laser beam is wel l confined and very in-

tense. Hence, heat ing rates in the surface layer are high. The heated layer is

self-quenched after passing of the laser beam by diffusion of heat to the cold

bulk. The high heat ing and cool ing rates in the surface layer resul t in grain re-

fin ement a n d in th e format ion of metas ta ble ph ases an d/ or a lte red m icro-s t ruc ture s .

Larger areas can be t reated by making several adjacent t racks. Some processes

requ ire th e u se of a s hielding gas to a void corrosion of th e t reated a rea.


21/177

22 Chapter 2

Fig. 2.1 The w ork piece perform s a relative movem ent to the las er bea m . The

area exposed by the laser beam is treated. Larger areas can be treated by ap-ply ing s evera l a d jacen t track s .

In our r esearch a 2 kW CO2

las er was u sed a s h eat sou rce. Th is p ower level can

be seen as a minimum requi rement for laser surface t rea tments . In most in-

dustrial appl icat ions the laser power is above 5 kW. This al lows the t reatment

of larger areas in one process step. The wavelength of 10.6 m causes signifi -cant reflect ion losses. At room temperature only 6-10 % of the radiat ion is ab-

sorbed in a sol id low al loy steel surface [Grnenwald, 1996; Stern, 1990]. The

absorpt ion can be increased by using special (graphi te) coat ings [Dausinger,

1988]. Under cladding condit ions the absorpt ion of CO2

radiat ion on a mild

steel sur face is con siderably larger: abou t 30 % [Gr n enwald, 199 6].

Recently, Nd:YAG lasers have been developed to laser powers of several kWs,

making them useful for laser surface t reatments. The wavelength of 1.06 m ismore efficient : ab sorpt ion on a solid steel su rface is ab ou t 30 % [Stern , 19 90 ].

Under cladding condit ions the absorpt ion increases to 60 % [Grnenwald,

1996]. Another advantage over the CO2

laser is the possibi l i ty to t ransport the

laser beam through opt ical f ibres, a l lowing flexible beam handling systems.

Taking into account both advantages , one can thus say tha t th i s l aser type i s

very us efu l for fu tu re las er su rface t reatm ents .

Three groups of laser surface t reatments are dist inguished: those without

m elt in g, those with m elt in g of th e su rface a n d th ose with melt in g of th e su rface

an d ad di t iona l m aterial.

The working pr inc iple of the m ost common laser s u rface t rea tmen ts is su mm a-

rised in figur e 2.2.


22/177

La s er clad d ing: s ta te -of-th e-art 23

The effects of laser remelt ing and laser t ransformation hardening are based on

a microstructural change of the surface layer. However, i t can be necessary or

chea per to ap ply a layer with a d ifferent comp osit ion on top of th e ba se m aterial

if th e ba se m ater ial itself can n ot be imp roved s u fficien tly.

Three laser techniques tha t can enhance surface proper t ies by changing the

material composi t ion in the surface can be dist inguished: laser al loying, laser

dispersing and laser cladding. These techniques are shown schematical ly in

figu re 2.3 .

All three methods involve the formation of a melt pool to which material is ap-

pl ied. Depend in g on th e ach ieved degree of m ixin g between th e ad ded m aterial

and the base material in the surface layer, one can dist inguish laser al loying

and laser di spers ing on the one hand, and laser c ladding on the other hand.

Th e first c las s is cha racterised by a comp lete mixin g an d/ or react ion of th e

added elements with the base material . In contrast , laser cladding generates a

su rface layer th a t h ardly conta ins e lements of the s u bs t ra te on top of the b ase

Fig. 2.2 Laser surface treatm ents a re distingu is hed w ith res pect to the su rface

temp erature (s olid pha se / m elt pool) an d to the ad dition of ma terial.


23/177

24 Chapter 2

m aterial. J u st en ou gh m ixin g is a llowed to ach ieve a st ron g bonding. Hence,

th e propert ies of th e produ ced clad layer depen d en t irely on th e ap pl ied coat ing

material .

2.2 Applicability of laser cladding

Laser c ladding i s a technique used in order to produce hard [Amende , 1990;

Folkes, 1994; Lang, 1994; Lugscheider, 1990], wear resistant [Brenner, 1996;

Choi , 1994; Eiholzer , 1985; Fischer , 1996; Gassmann, 1992; Liu, 1994;

Nowotny, 199 4; Wolf, 199 5] an d/ or corrosion resistan t [Bru ck, 19 87; Fellowes,

1990; Fouquet , 1993; Gasser , 1996; Lugscheider , 1994; Singh, 1987; Wang,1993] surface layers. Laser cladding can also be appl ied to produce thermal

barriers [Damborenea, 1993; Pei , 1995; Smurov, 1992; VandeHaar, 1988], to

ach ieve layers s u itab le for ap pl icat ion in n u clear p ower-stat ions [Corchia, 19 87 ;

Li , 1992] or to obtain surface layers that prevent s tacking [Frenk, 1991]. The

techn iqu e is u sed to produ ce high qu al ity su rface layers on top of n ew parts , to

produ ce ent ire par t s [Gebh ardt , 1 996 ; Haferkam p, 19 95; Kreutz , 1995 ] an d, to

Fig. 2.3 Las er alloying, dispe rsing an d clad d ing. The right pa rt of the pic-

ture ind icates the d istribution of the ad de d elemen ts m eas ured top-dow n

in th e centre of the track.


24/177


refurbish and improve used parts [Fl inkfeldt , 1994; Knig, 1994; Riabkina-

Fish ma n, 1996 ].

Table 2.1 gives some examples of reported industrial applications of laser clad-

ding. Clad ding is ap pl ied on m echa n ical pa rts th at a re exposed to an aggressive

environment: thermal cycles with high heat ing and cool ing rates, corrosive

gases, high tem peratu res, abra sive pa rt icles an d/ or cavitat ion erosion .

Most of these parts only require local t reatment that can be performed with a

single clad track. An example is the cladding of a diesel engine exhaust valve

with a Ste llit e 6 c lad layer tha t h as a width of abou t 2 .5 m m an d a thickn ess of

1 .0 m m. Su ch layers can be app lied with a 2 kW CO2

laser system.

On the other hand, the cladding of an extruder screw [Schneider, 1995; Wolf,

1995] with a 10 mm wide, 2 mm thick layer over a length of more than 3 m is

an example of a non-local t reatment which requires the use of a 10 kW laser.

Laser cladding is not the most economic al ternat ive for the coat ing of such a

rath er large area in term s of coverage rate or pr odu ct ion costs . However, in th iscase al l other techniques would fai l to achieve a st rong fusion bond between

coat in g an d b as e ma terial , or would resu lt in s evere distort ion of the screw.

For the cladding of even larger areas, very high power CO2

lasers with a laser

power of more than 20 kW are avai lable . They can be used to produce 44 mm

wide an d 5 .5 m m h igh t ra cks in on e step [Volz, 199 4].

Table 2.1 Ind us trial ap plications of las er clad d ing.

Source Part/industry MaterialRolls Royce [Macintyre, 1983]

Pratt & Whitney [Eboo, 1983]

Combustion Engineering

[Eboo]

Fiat [Eboo, 1983]

GM [Eboo, 1983]

Rockwell [Eboo, 1983]

Westinghouse [Eboo, 1983][Bruck, 1987]

[Lubbers, 1994]

[Schneider,1995]

[Wolf, 1995]

[Haferkamp, 1994]

high pressure gas turbine blade

shroud

interlock

turbine blade

parts of off-shore drilling heads

cylinder and valve

automotive parts

aerospace

turbine bladeturbine blade, plough blade

diesel engine valve

extruder screw plastic machinery


(steel 1.4541)

deep drawing tool (cast iron GGG60)

Triballoy on Nimonic

CrC, Cr, Ni on cast iron

on cast iron

Stellite, Triballoy T-800

Stellite, Colmonoy

Stellite 6, Stellite SFStellite 6

Stellite 6

LC2.3B (Ni-base)

Al-bronze

Stellite SF6


25/177

26 Chapter 2

Source Part/industry Material

[Weerasinghe, 1987]

[Amende, 1990]

[Kpper, 1990]

[Blake, 1985]

[Blake, 1985]

[Amende, 1988]

[VandeHaar, 1988]

[Bruck, 1988]

[Bruck, 1988]

[Bruck, 1988]

[Bruck, 1988]

[Ritter, 1991]

[Wissenbach, 1991]

[Gasser, 1996]

[Gasser, 1996]

[Fischer, 1996]

[Nowotny, 1996]

[Lang, 1994]

[Li, 1992]

[Knig, 1992]

[Amende, 1988]

Pratt&Whitney [Duhamel, 1986]

[Bergmann, 1994]

[Corchia, 1987]

drainage plough blade

leading edge steam turbine blade

valve in combustion engine

(X45CrSi9)

aircraft engine turbine blade Z-notch

leading edge turbine blade in industrial

gas turbine in power plant

deformation tool

gas turbine airfoil thermal barrier

valve seat

stainless steel seal runner

stainless steel gate valve

leading edge steam turbine blade

valves

valve in combustion engine

(X45CrSi9)


(14CrMoV6 9)

moulding die (45NiCr6)

exhaust valves large Diesel engines

(NiCr20AlTi ~ DIN 2.4952)

camshaft

compressor blade (Ti-6Al-4V)

nuclear valve (AISI 304)

blowing mould


jet engine turbine blade notch (PWA

Alloy 1455)

die for production of glass bottles

components of nuclear plants (AISI

304)

Stellite 6

Stellite 6

Triballoy T-800

Stellite 6

Stellite 6

Inconel 625 + CrC

AISI 410

Stellite 6

Stellite; induction heating

Stellite 6,F

Stellite 21

Ni-Cr-Al-Y

Co-Cr-W-C

Ti-6Al-4V + cubic BN

Stellite 6

Ni-Cr alloy

PWA Alloy 694

Stellite

Stellite 6, Colmonoy 5

2.3 Laser cladding versus conventional methods

Laser cladding is one of the coat ing techniques that are in use for improving

surface propert ies of mechanical parts . Examples of other thermal coat ing

methods are flame spraying, plasma spraying and arc welding. The coat ing that


26/177


is appl ied on the base material must provide the required surface propert ies ,

such as resistance against corrosion or wear. The use of a dedicated material

on top of a ch eap b as e ma terial resu lts in cons iderab le ma terial cost sa vin gs.

The coat ing material can be preplaced on the substrate to be covered or i t can

be provided to i t during the process. All processes apply a heat source to melt

the coa t in g mater ia l and/ or the ba se m ater ia l. Depend ing on th e tech nique , a

fu s ion bon d or an adh es ive bond is a chieved be tween th e su rface layer and the

base material .

In general , the use of a laser beam in surface t reatments offers several advan-

tages over convent ional heat sources [Knig, 1989; Molian, 1990; Oberlnder,

199 2; Webber, 19 87]:

- the energy supply can be well controlled;

- very local treatm en t is poss ible;

- th e total heat inp u t is low, resu lt in g in m in ima l distort ion;

- the heat ing and cool ing rates are high, resul t ing in a f ine microstructurea nd / o r m e ta s t a b le pha s e s ;

- the t reatment is a non-contact process. There is no tool wear, nor act me-

cha n ical forces on th e workpiece;

- th e process depth is well defin ed.

Atamert [1989], Cai [1990], Li [1994], Monson [1990] and Oberlnder [1992]

describe some more advantages of laser cladding over convent ional coat ing

techniques. The combinat ion of a control led minimal di lut ion of the substrate

by the coat ing material and nevertheless a very st rong fusion bond between

them, is a unique feature of laser cladding. Porosi ty in the coat ing can be pre-vented en t irely and a h omogeneous dist r ibu t ion of elemen ts can be ach ieved.

2.4 Laser cladding methods

Two material appl icat ion principles can be dist inguished: The two stage

(preplaced) techn iqu e an d th e one sta ge (in -situ ) techn iqu e, wh ich a re both dis-

played in figure 2.4. These two methods are both used in pract ice. The coat ing

m aterial is predom in an t ly su ppl ied in th e form of powder pa rt ic les.

T w o s t a g e p r o c es s

Several meth ods of las er clad ding with preplaced m aterial are in u se. Exam ples

are the melt ing of f lame and plasma sprayed layers [Cuetos, 1993; Gasser,

19 96 ; J u ch, 19 94 ; Li, 1992 ; Lu gscheider, 1990 ; Mordike, 19 94 ; Reichel t , 19 96 ;

Smurov, 1992], the melt ing of preplaced powder [Arl t , 1994; Belmondo, 1979;

Bru ck, 198 8; Flin kfeldt , 19 94 ; Hirose, 199 5; Lu gscheider, 1992 ; Matth ews,


27/177

28 Chapter 2

198 3; Nu rminen, 1 985 ; Sakam oto, 1995 ] an d the melt in g of preplaced p la tes or

chips [Du h am el, 198 6; Tosto, 199 4].

Techn ical ly speak in g, the clad ding of flam e an d plasm a sp rayed layers is in fact

remelt ing. This remelt ing may be necessary, because the propert ies and per-

formance of convent ional ly appl ied coat ings may be seriously degraded due to

poor inter-part ic le bonding, considerable porosi ty or extensive chemical inho-

m ogeneity [Li, 19 92 ].

Laser cladding of plasma sprayed layers requires some special a t tent ion. The

sprayed layer must have enough bonding to the subs t ra te to prevent i t f rom

peel ing off the substrate due to expansion during the second stage of the proc-

ess .

Laser cladding with preplaced powder is the most common two stage cladding

meth od. Th e appl ied powders a re the sa me as those u sed in flam e an d plasm a

spraying. The powder must be mixed with a chemical binder to ensure that i t

Fig. 2.4 Tw o s tage (preplaced) an d one sta ge (in-situ) las er clad d ing.


28/177


will s t ick to the substrate during the process; I t has to be given enough struc-

tural strength to withstand forces imposed by gas flows or gravitation [Arlt ,

1994]. The chemical binder evaporates during the process. This can resul t in

some porosity in the clad layer.

The second stage of the process starts with the formation of a melt pool in the

surface of the coating material. The melt pool propagates to the interface with

the su bs t ra te . Con t in u ed hea t in g ens u res tha t the m elt pool is exten ded to the

subs t ra te and tha t a s t rong fus ion bond i s achieved. The hea t input must be

well control led to prevent deep melt ing of the substrate and the therefrom re-

su lt in g severe dilu t ion on the on e h an d, an d to a chieve a s t rong fu s ion b ond on

the othe r han d .

The two stage methods are part icularly useful for parts that can be t reated in

one single t rack. I t is of course possible to apply several adjacent t racks, but

th is will resu lt in an in creased di lu t ion. This can be i llu stra ted with figure 2.5.

Before the fi rs t c lad t rack is produced, the ent i re area is covered by the pre-placed ma terial . A pa rt of th e coat in g is m olten by th e las er bea m . After pa ss in g

of the beam, the molten material contracts due to surface tension. Hence, the

substrate area direct ly next to the produced clad layer is not covered with pre-

placed m ater ia l anymore . Wh en th e next t rack i s ma de , the n on covered par t of

the substrate is direct ly i rradiated by the laser beam. Deeper melt ing of the

su bs trate will occu r in th is a rea.

On e s t a ge p r oc e s s

The one stage process starts with the formation of a melt pool in the substrate .

Sim u ltan eous ly, coat in g ma terial is fed into th is p ool an d melts; a s t r ong fu sion

bon d between coat in g material an d su bs trate is achieved im m ediately.

Fig. 2.5 The m olten coating contracts d uring the s olid ification. Hence, the

s ubs trate nex t to the clad lay er is e xpose d to the las er bea m du ring the

ap plication of the n ext track.


29/177

30 Chapter 2

Th e one stage process h as s everal oth er advan tages over the two stage process :

- larger areas which require the appl icat ion of several adjacent t racks can be

produ ced with less di lu t ion. Con tract ion of th e clad layers on coolin g down

st i ll occu rs, bu t becau se m aterial is fed to the su bs trate n ext to th e previou s

t rack n o par t of the s u bs t ra te i s ir radia ted u n necessar i ly;

- the coat ing thickness can be varied on-l ine by control l ing the material feed

ra te ;

- products with a complex geometry can be t reated, because material is fed

continuously to the interaction zone. Therefore, flowing out of the molten

m aterial by gravitat ion is not problematic an ymore.

Two methods of material feeding are available: wire feeding and powder injec-

tion.

Wire feeding [Burchards, 1990; Draugelates, 1994; Hensel , 1992; Hinse-Stern,

19 92 ; Steenb ergen , 199 3] can be u sefu l for the cladding of rotat iona lly symm et-

ric products that can be clad in one cont inuous t rack. The posi t ioning of thewire to th e su bs trate is a crit ical aspect of the p rocess for two reason s:

- the wire is usual ly in direct contact with the melt pool in the substrate . In

order not to dis tu rb th a t m elt pool the pr ocess requ ires th e u se of an accu-

rate wire posi t ioning system an d a n accu rate con trol of th e wire feedin g rate

[Burch ards , 199 0; Steenb ergen, 19 93];

- th e su bs trate is pa rt ly sh ielded from th e las er beam by th e solid wire. To al-

low a good qu al ity clad layer, i.e . on e with a good b ond in g to th e s u bs trate ,

flown out an d a sm ooth su rface , a ra t io be tween beam diam eter a n d wire di -

ameter of at least a factor three must be used [Burchards, 1990]. The wiremust be well a l igned to the centre of the laser spot to ensure the formation

of a s ymm etrical clad layer.

Th e wire feedin g nozzle mu st be close to th e processing a rea to a llow an accu -

rate feeding of the wire in the melt pool. Therefore, the machining of a curved

produ cts , concave sh ap es in p art icular, is difficu lt .

The use of rotat ing wire feeding heads al low cladding with a constant angle

between wire and substrate as wel l as a constant angle between the inject ion

direct ion a n d th e relat ive movemen t of th e su bst rate .

Th e process efficiency can be increas ed by th e ap pl icat ion of an in du ct ion u n itto preheat th e wire [Bur cha rds , 1990 ; Hins e-Stern, 199 2].

The inject ion of powder into a laser generated melt pool is a much more com-

mon method compared to wire cladding [Frenk, 1993; Grnenwald, 1992; Pel-

letier, 1993; Volz, 1994; Wolf, 1995]. The method is more flexible [Hensel,

1992]: i t allows the on-line variation of clad dimensions and clad composition

and, many more elements and al loys are avai lable as powder then as wire.


30/177


Powder in ject ion clad ding is a m ore robu st m ethod th an wire clad ding, becau se

there i s no di rec t contac t wi th the mel t pool and, the laser beam can pass

th rou gh th e st ream of powder par t ic les instead of bein g obs tru cted by the wire.

Powder feeders can be based on various working principles. Two such princi-

ples are shown in figure 2.6. The left picture shows a feeder consisting of a

powder container from which powder flows by gravity into a slot in a rotating

disk. The powder is t ransported to a suct ion uni t from which i t is t ransported

by a gas st ream to a powder nozzle . The volumetric powder feed rate is con-

trolled by th e dimens ions of th e slot an d th e sp eed of th e disk [Carvalh o, 1995 ].

Another method for powder dosing is the use of a pneumatic screw feeder

[Weck, 1994; Weerasinghe, 1984] (figure 2.6 right). The powder is taken from a

container to the powder pick-up and t ransported to the powder nozzle . The

powder feed rate is control led by the rotat ional speed and dimensions of the

screw.

A different approach to powder feeding is the appl icat ion of a hopper and acombinat ion of pneumatic and vibrat ional forces. A standard ul t ra-sonic

cleaning device is suitable for delivering the vibrational forces. The powder is

taken from th e hopper by mean s of a press u re diffe rence and t ran spor ted to the

pr ocessin g zon e [Metzbower, 19 86 ; Macint yre, 198 3].

The development of new powder feeders for laser cladding is directed to the di-

rect con trol of th e flow rate: con trol of m as s flow in stea d of volu m etric flow an d

instantaneous flow variat ions [Carvalho, 1995; Nowotny, 1996; Schiere, 1996;

Weck, 19 94 ].

The p owder m u st be t ran spor ted from the feeder to th e process a rea . This canbe done by the use of a carrier gas (argon, hel ium, ni t rogen) or s imply by

gravita tion . Su bs equ en tly, it is d irected to th e melt pool by a p owder n ozzle.

The powder nozzle can have several configurat ions. Two basic lay-outs are

shown in figure 2.7. The co-axial supply of powder (figure 2.7 left) [Nowotny,

1996; Pel let ier , 1994; Tucker, 1984; Vet ter, 1994] can be integrated with the

opt ical system. An advantage of a co-axial powder supply is the independence

of the powder supply of the direct ion in which the workpiece moves. Another

ad van tages of th e co-axial powder su pply are th e contr olled h eat in g of th e pow-

der b efore it en ters th e m elt pool an d th e h igh powder efficiency [J ou vard ,19 97 ; Lemoine, 199 4; Li, 19 95 ; Vet ter, 199 4]. However, n ot a l l produ cts ca n be

access ed with a co-axial powder n ozzle.

The lateral supply of powder (figure 2.7 right) allows the treatment of all kinds

of shapes by applying dedicated powder nozzles. Basically, lateral powder noz-

zles a re jus t tu bes with the p roper length, sh ape a nd diam eter . Lensch [1996 ]

developed a cu rved tu bu lar n ozzle to clad in n er diameters of tu bes.


31/177

32 Chapter 2

2.5 Clad layer properties

Th e pr operties of clad layers ar e clas sified in fou r grou ps (tab le 2.2 ).

Some of th ose propert ies ma y be in ter-related. The wear resistan ce can , for in -

s tan ce, be a ffec ted by the h ardn ess , the m icros t ruc tu re , th e nu mb er of c racks

and the i r depth and di rec t ion, the bonding be tween base mater ia l and sub-

stra te , e tc.

Fig. 2.6 Tw o pow d er d osing principles.

Fig. 2 .7 Co-axial (left) an d lateral (righ t) pow d er s upp ly.


32/177


In practice, i t is difficult to produce a clad layer which meets all requirements.

Usu al ly a ba lan ce mu st be fou n d between several propert ies . An examp le is th e

reduct ion of crack formation in the clad layer by preheat ing the substrate . The

prehea t ing reduces th e coolin g ra tes an d th e resu lt ing res idua l s t ress . Con se-

quent ly, crack formation is avoided and, the hardness is reduced as wel l

[Brenner, 1996; Eigenmann, 1995; Evers, 1993; Frei tas , 1993]. Crack preven-

t ion is important , because cracks ini t ia te corrosion fracture and reduce fat igue

s t rength.

d i lu t ion

Laser cladding requires the achievement of a s t rong fusion bond between the

cladding material and the substrate , which, in i ts turn, requires the formation

of a melt pool in the substrate . I t has to be noted however, that the depth of

thi s mel t must be as smal l as poss ible in order to obta in a pure surface layer

which is n ot dilu ted by the ba se m aterial.

The di lut ion of the produced clad layer by elements of the substrate is used to

characterise the clad qual i ty. I t can be measured in two ways [Bruck, 1987].

The first method is based on the clad layer geometry (figure 2.8). The dilution is

then defined as the rat io of the clad depth (dc) in the substrate over the total

clad height (tc) . This geometr ica l approach assumes an homogeneous dis t r ibu-

tion of elements over the clad cross-section.

The s econd meth od is bas ed on an an a lys is of the m ater ia l comp osit ion in the

clad layer (ch em ical or volu m etric d ilut ion). A com pa rison is m ad e be tween

the material composi t ion of the pure coat ing material and the composi t ion of

the substrate is made. This method al lows the determinat ion of a variat ion of

th e di lu t ion over th e clad depth an d is preferred over th e geometrical app roach .

Tab. 2.2 Properties of clad lay ers.

geometrical

properties

mechanical

properties

metallurgical

properties

qualitative

properties

clad dimensions

dilution

roughness

hardness distribution

residual stress

wear resistance

tensile strength

microstructure

dilution

grain size

homogeneity

corrosion resistance

porosity

cracking


33/177

34 Chapter 2

In chapter 5 (laser cladding with preplaced powder) two examples are given to

illustrate the effect of dilution on some coating properties. The first example

sh ows h ow the m easu red geometr ica l dilu t ion re la tes to th e ha rdn ess d is t r ibu -

t ion. Th e second exam ple sh ows th at th e chem ical dilu t ion is th e most u sefu l of

th e two, becau se n ot al l c lad layers h ave a u n iform d istribu t ion of elemen ts.

p oros i t y

The presence of holes in the clad layer is being referred to as porosity. Porosity

can be cau sed b y several reason s. First , it m ay be th e resu lt of th e forma tion of

gas bu bb les th at a re t rap ped in th e solidifyin g melt pool. Th is ph enom enon can

be decreased by a vibration of the workpiece (frequency: 25 Hz; amplitude: 20

m ) [Powell, 19 81 ]. It was obs erved th at th is vibr o las er cladd in g also r edu ces

inte rn a l s t resses an d cracking.

Secondly, if solidification proceeds in different directions, some regions in the

m elt can be en closed. A contra ct ion occu rs u pon solidificat ion of th ese en closed

regions. That contract ion causes tensi le s t ress in the layer and may even lead

to th e forma tion of h oles. Th ese two kin ds of porosi ty are to be fou n d in th e clad

layers.

Two other kinds of porosi ty are confined to the substrate-clad interface. The

first one is caused by the presence of minor flaws, such as grease which influ-

ences the surface tension and therewith the bonding of the coat ing material to

the sub s t ra t e.The last type of porosi ty appears when overlapping t racks are appl ied as indi-

cated in figure 2.9. This so-called in ter-ru n porosi ty can occu r when too mu ch

powder is s u pp lied. Th is ca n be a voided with a width -heigh t ra t io of more th an

five [Steen , 19 87 ].

Fig. 2.8 Cross -s ection of a s ingle clad lay er w ith d efinition of the clad geom e-try: clad height (h

c), clad depth (d

c), total clad height (t

c) an d clad w idth (w

c).


34/177


Some exam ples of porosi ty which occu rred in actu al las er clad ding experim ents

are given in ch ap ter 7.

2.6 Material properties

Laser cladding is used to improve the surface propert ies of metal l ic machine

parts. A wide variety of commercial metallic or ceramic powders is available.Those powders were developed for the use in plasma and flame spraying. They

are a lso fit for u se in las er clad ding, becau se th e in tend ed fu n ct iona l propert ies

are the same. In appendix 4 an extensive overview is given of combinat ions of

powders a nd base m ater ia l s as repor ted in th e lit e ra ture .

An essen t ial asp ect of las er clad ding is th e ach ievemen t of a s t rong fu sion bon d

over the ent i re interface between the substrate and the clad layer. A good wet-

t ing between the coat ing material and the substrate is therefore required. Be-

cau se of the poor wetta bility of ceram ics t o m etals, i t is very difficu lt to clad ce-

ram ics on top of a m eta l su bs t ra te [de Hosson, 199 5; Lu gscheider , 1994 ; Zhou ,19 91 ]. Th e ap pl icat ion of sa n dwich layers between th e in tend ed su rface coat ing

an d th e su bs t ra te [Cuetos , 1993; Li, 1992 ; Liu , 1994 ; Smu rov, 1992 ] or the u se

of binders can be helpful [Lang, 1994; Lugscheider, 1994; Nowotny, 1994;

Singh, 1987; Zhou, 1991]. Preheat ing may also help to enhance wett ing [El l is ,

1995].

The basic requirement for a fusion welding is that the two coupling materials

are solub le in ea ch oth er an d, there is qu est ion of a p h as e equi libriu m b etween

them [Li , 1992]. For most al loys this requirement poses no difficul ty, but pure

m etals ca n som etim es be d ifficu lt to clad on top of each other. Alu m in iu m an d

cobal t , for in sta n ce, are n ot solu ble in each other, so th ere is n o ph as e equ ilib-

riu m . Ph as e equi librium s exist on ly between a lu miniu m a n d som e in termetal-

l ics or between cobal t and other intermetal l ics . So, the only way to bond them

together is to s an dwich th em with interm etallics .

Mu lt i- layers can also be app lied in meta l-metal com bina t ions to redu ce residu al

stress due to repeated heat input [Frenk, 1993] and to form a funct ional ly gra-

Fig. 2 .9 Inte r-run porosity .


35/177

36 Chapter 2

dien t layer with a low dilu t ion a n d th e in tend ed propert ies on top [J as im , 1993 ;

Sepold, 198 9].

Convect ion is the single most important factor influencing the geometry of the

pool, in clu ding pool sh ap e an d ripples. Convect ion is a lso the m ain m echa n ism

for mixing added material with the substrate [Chan, 1984; Picasso, 1994].

Sin ce th e microstru ctu re an d ch emical compos it ion of th e clad st ron gly depend

on the degree of mixing and the cool ing rates during sol idificat ion and conse-

quent cool ing, the parameters cont rol l ing these mechanisms are important to

th e las er clad ding process [Komvopoulos, 19 90 ]. Material prop ert ies th at in flu -

ence these mechanisms are , among others , the mel t ing point and the thermal

conductivity.

The melt ing point of the substrate must preferably be higher than that of the

coating material. If this is not the case, then it is possible that during solidifica-

t ion a n d su bsequ ent cooling of the c lad layer , the s u bs t ra te region ju s t u n der-

n eath it can be h eated to a tem pera tu re over th e melt in g poin t . Th is r egion willsu bs equ ent ly lose its s t ren gth. Und er th e effect of th e st res s of th e clad ab ove,

h ot tear will occu r a lon g this region [Li, 199 2]. Su ch a s itu at ion will in an y cas e

result in porosity along the interface.

Th e form ation of cracks in th e clad layer is m ainly cau sed b y the th erma l s t ress

crea ted b y the h igh th erma l gradient bu ilt u p d u r in g the cooling s tage and the

difference between the thermal expansion coefficients [Frenk, 1991, 1993; Li,

1992; Pi l loz, 1990; Vasauskas, 1996; Zhang, 1994; Zhou, 1991]. Especial ly

layers that are characterised by the presence of hard and bri t t le part ic les, such

as carb ides, a re pron e to cracking [Lu ft , 19 95 ]. Ceram ic layers a re a lso vu ln er-able to cracking, because of their l imited ductil i ty combined with the difference

in th erma l expan sion coefficients compa red with m etals [Van deHaar , 198 8].

Residu al s t resses ca n b e redu ced by a redu ct ion of th e coolin g rate . Th is can be

achieved by preh ea t in g [Brenn er , 1996 ; Evers , 199 3; Glu ma nn , 199 4; Schn ei-

der, 199 3].

The different base materials can be classified into two groups. The first group

conta in s a ll s tee ls . The second grou p conta in s n on-ferrous bas e ma ter ia ls , su ch

as a lumin ium and t it an ium.

2.6.1 Powders on a steel substrate

Cobalt bas e p ow d ers (S tellites )

Coba lt b as e su pera lloys (Stellites) ar e very popu lar with regard to th e im-

provem ent of th e wear r esistan ce of m echa n ical par ts , es pecial ly in h ost ile en vi-

ronments [de Hosson, 1996]. Those powders are mixtures of cobal t and other


36/177


e lements like n icke l, chromium , tun gs ten, carbon an d m olybdenu m. Ch romiu m

is a dded to form ca rbides an d to provide st ren gth to th e coba lt ma trix as well as

to enhance the res i s tance aga ins t corros ion and oxida t ion. Tungsten and mo-

lybden u m h ave large atom ic sizes a n d give, therefore, addi t iona l s t ren gth to th e

matrix. They also form hard bri t t le carbides. Nickel is added to increase the

ductility.

The predominant carbide found in Stel l i tes is of the chromium rich M7C

3

(M=metal) type. These carbides (2200 Hv) are responsible for the hardness of

th e clad (~ 550 Hv) an d for th e wear res ista n ce. In low-carbon al loys other car-

b ides such a s M6C a nd M

23C

6are abu nd an t [de Hosson , 1996].

If the wear properties of a given cobalt-base powder mixture is not sufficient,

then hard par t ic les , such as carbides , n i t r ides and bor ides , can be added di -

rec t ly to thi s mixture [Gassmann, 1992; Glumann, 1994; Grnenwald, 1992;

Nowotny, 1994; Volz, 1994]. Those hard part ic les usual ly have a high melt ing

temperature. The flow in the melt pool must ensure that they are uniformlymixed with the other e lements an d b ecome embedded in the m at r ix provided b y

th e molten coba lt -bas e ma terial .

Gassm an n [199 2] descr ibed the add it ion of tu ngs ten carbide (WC/ W2C) to a

Stellite powder in ord er to en h an ce the ab ras ive wear r esista n ce. Tu n gsten ca r-

bide is dist inguished by a minimal plast ic deformation capaci ty, a low thermal

expan sion an d a h igh wettabi lity by molten m etals , esp ecial ly cobalt . Du e to th e

low free formation enthalpy tungsten carbide is dissolved in the sol id state by

molten cobal t . The dissolut ion increases with the temperature of the melt and

the interact ion t ime. Depending on the carbon concentrat ion in the melt , dis-solved tu n gsten carbide recrystal lises eith er to WC, or with low carbon concen-

trat ions, to W2C or br i t t l e phases such as Co

3W

3C and Co

6W

6C. It is therefore

important to keep the temperature in the melt as low as possible . Not only re-

sul ts this in a low carbide dissolut ion, i t a lso ensures a dense coat ing with

lower tensile stresses.

By far the most popular commercial cobalt base powder is Stelli te 6 (table 2.1).

Stelli te 6 can be applied on e.g. dies, inserts, valves, feeder screws in plastic

machinery and turbine blades. The working temperature is l imited to 500 C.

For many of the appl icat ions in table 2.1 i t is possibly not the best avai lablecoat ing. However, i t is selected most ly because i ts propert ies and clad behav-

iour are well known and because i t has establ ished i tself in other hardfacing

techniques .

Nick el b as e a lloy s


37/177

38 Chapter 2

Nickel ba se al loys a re su ited for app licat ion in pa rts th at a re exposed to a n ag-

gressive atmosphere at e levated temperatures. They have a good high tem-

perature corrosion and oxidat ion resistance. Nickel base al loys can also be

used as a subs t i tute for cobal t . This may be important in the future , because

coba lt is a relatively rare a n d expa n sive elem en t, wherea s n ickel is widely avail-

ab le and m u ch cheape r.

An example of an appl icat ion is the cladding of a high pressure gas turbine

blade or the improvement of a feeder screw in plast ic machinery [Amende,

1988].

Elements tha t a re comm only mixed with nicke l a re ch romiu m, boron, carbon,

si l icon and aluminium. The formation of hard borides and si l icon carbide im-

proves the wear resistance and hardness. However, a too large presence of

these hard phases makes the coat ing very bri t t le [Arl t , 1994; Becker, 1991;

Grnenwald, 1992; Lugscheider, 1990]. Hard part ic les can also be mixed with

the added elements [Pelletier, 1994]. Luft [1995] describes the addition of tung-sten carbides to a mixture of Ni-B-Si in order to achieve a solidified nickel rich

solid s olu tion with finely distribu ted Ni3B an d dissolved tu ngs ten.

Th e ad di t ion of boron an d si licon imp roves th e wett in g beh aviou r. Hen ce, very

smooth surfaces can be achieved [Wolf, 1995].

Aluminium can be added to fur ther increase the hardness . The hardness in-

crease is due to the formation of intermetallic phases (NiAl3

and Ni2Al

3)

[Gr n enwald, 19 96; Marsd en, 1 990 ] or an oxide layer (Al2O

3).

Iron bas e a lloy sAlthough the selection of an iron base alloy for improving surface properties of

an iron bas e sub s t ra te may n ot be the m ost obvious choice, some research h as

been don e to this s u bjec t.

It has been repor ted tha t a mixtu re of iron, chromium , carbon an d m an gane or

tungsten has superior wear propert ies compared to Stel l i te 6 [Choi , 1994; Ei-

holzer, 1985; Komvopoulos, 1994]. The elements that are added to i ron ensure

the formation of carbides, contribute to the oxidat ion and corrosion resistance

and promote the sol id solut ion st rengthening. The main carbide type found in

th is k ind of clad layer is M6C in stead of th e M7C 3 type fou n d in Stellite 6.Others have reported on the appl icat ion of austeni t ic corrosion resistant s teel

layers on top of ordinar y low carb on s teels [Fouqu et , 199 4; J as im, 1 98 9]. Th e

corrosion resistance of those layers can be further improved by increasing the

m olybdenu m cont ent in it [Hu an g, 199 5].


38/177


2.6.2 Non-ferrous substrates

2 .6 .2 .1 Ni ck el ba s e s u bs t ra t es

Nickel base al loys retain their mechanical propert ies up to high temperatures

and are corrosion resistant . Therefore, they are especial ly useful for appl ica-

t ions tha t undergo high tempera ture corros ion and wear , such as exhaus tvalves of large Diesel engines for ship propulsion or power generation [Fischer,

1996] as wel l as for je t engine turbine blades [Duhamel , 1986]. However, the

wear properties of nickel base alloys are not very good. The resistance to wear

can be improved by using cobal t base al loys which have a bet ter wear endur-

an ce th an nicke l [Cooper , 1989; Duh am el, 1986 ; Gass ma n n, 1 992 ; Macintyre ,

1983], by making use of a protect ive oxide layer such as chromium oxide

[Fellowes, 19 90 ] an d zirconium oxide [Dam boren ea, 19 93] or by th e a dd it ion of

hard par t ic les in a binder a l loy [Gassmann, 1992; Jas im, 1993; Zhu, 1993] .

Oxide layers ar e not only very ha rd, th ey also form a th erm al an d ch emical ba r-rier between th e meta l an d th e aggressive en viron men t [Fellowes, 19 90; Singh,

1987; VandeHaar, 1988; Vasauskas, 1996]. A problem with ceramic layers is

th at th ey often h ave a poor adh erence to the coat in g. React ive elemen ts s u ch a s

hafnium and yt t r ium can improve thi s adherence [De Damborenea , 1993;

Sin gh, 198 7; Sircar, 19 89].

One of the appl icat ions of laser cladding on a nickel base substrate that has

established itself in practice is the cladding of notches of jet engine turbine

blades [Duhamel , 1986]. A high wear point exists a t the notch between neigh-

bou rin g blades forming the ou ter sh rou d of je t en gin e tu rbine sta ges. Th e ma te-rial in the notch region experiences rubbing contact with the adjacent blade

during engine operat ion. The wear resistance could be increased by the appl i-

cat ion of a coba lt ba sed alloy (PWA a lloy 69 4).

2 .6 .2 .2 Al u m iniu m a n d t i t a n iu m ba s e s ubs t ra t es

Aluminium and t i tanium al loys (predominant ly Ti-6Al-4V) are popular materi-

a l s for cons t ruc t ing mechanica l par t s in aerospace and automot ive indus t ry ,

because of the combinat ion of low weight and high st rength. Unfortunately, a t

h igh tempera tu res the m echan ica l proper t ies an d th e wear res is tan ce a re poor .

These propert ies can be enhanced by applying a nickel base layer [Kar, 1988;

Liu, 1994; Sal lamand, 1993; Sircar, 1989]. Thus the advantage of l ight weight

is combined with a good high tem peratu re mecha n ical beh aviour .

Nickel , t i tanium and aluminium form several bri t t le intermetal l ic compounds,

su ch a s Al3Ni, Al

3Ni

2and AlNi. The formation of these compounds on the inter-


39/177

40 Chapter 2

face leads to cracking. Suitable sandwich layers l ike Ni-Al bronze can prevent

th is [Liu, 1 99 4].

Firmly bonded aluminium oxide layers could be obtained by cladding an alu-

miniu m al loy with a mixture of alum in iu m an d s ilicon oxide [De Hosson , 19 95 ].

In th is rea ction coating th e followin g reaction occur red:

2Al+3/ 2SiO2=Al2O3+3 / 2S i.

The presence of sil icon in the produced oxide layer seems to be favourable for

th e wett in g by liqu id a lu m in iu m.

The hardness of an a luminium or t i t anium a l loy can a l so be increased by the

in ject ion of a m ixtu re of h ard pa rt ic les a n d a n alum in iu m resp ect ively t itan iu m

alloy. Especial ly si l icon carbide and t i tanium carbide seem to be useful hard

pa rticles [Folkes , 19 94 ; Hegge, 199 0; Ricciar di, 19 90 ]. Lan g [19 94 ] in vestigated

the injection of a very hard cubic boron nitride and Ti-6Al-4V mixture on a Ti-

6Al-4V compressor blade.

2.7 Effect of process parameters

Th e ph ysical and process variables tha t affect the clad r esu lts a re sh own in fig-

u re 2.1 0 [Ollier , 19 95 ]. In order to b e ab le to pr edict c lad ding resu lts , th e rela-

t ionships be tween these var iables and the c lad resul t s must be known and

solved s im u ltan eous ly. At th e m omen t n o exact solu t ions are a vai lable .

2.7.1 Experimental methods

Man y auth ors ha ve s tu died the laser c ladd ing process by mean s of a s imp le pa-

rameter variation for a fixed material combination [Komvopoulos, 1990; Kreutz,

1995; Nurminen, 1983] . This parameter var ia t ion can he lp to enhance the

process kn owledge an d resu lt s in tables s u ch a s ta ble 2 .3 , which sh ows qu ali-

ta t ively how clad propert ies are influ enced b y the pr ocess pa ram eters .

Parameter variat ion can also yield process diagrams l ike the one shown in fig-

u re 2.11 [Kreu tz, 199 5]. Th is d iagram in dicates th e clad h eight a s a fu n ct ion of

the powder injected per unit of length for two different feed rates and two pow-

der densi t ies . The clad height increases with power densi ty and powder mass.

The properties of the clad are insufficient for low and high powder feed rates

origin atin g eith er from d ilu tion or lack of fu sion.


40/177


Fig. 2.1 0 Variab les in las er clad d ing w ith pow d er in jection [Ollier, 199 5].

Tab. 2.3 Reported effects of increas e of param eter values on clad lay er proper-

ties (+: increas e; -: red uction).

properties

parameters

clad

height

melt

depth

geometrical

dilution

hardness/

cracking

maximum

clad

thickness

laser power

feed rate

quantity of coating material

laser spot

-

+

+

+

+

-

-

-

+

-

-

-

-

+

+

+

+

-

+

-


41/177

42 Chapter 2

Others [Blake, 1985; Chen, 1989; Engstrm, 1988; Kar, 1988, 1989; Komvo-

poulos, 1990; Molian, 1988; Singh, 1985] relate laser cladding resul ts to the

specific energy E:

where P is the laser power, v is the feed rate of the workpiece and D is the di-

am eter of th e las er spot .

The m aximu m coat ing th ickness tha t can be ap plied du r ing laser c ladd ing de-

pen ds on th e sp ecific en ergy [Chen , 19 89; Marsd en, 1 99 0]. Below a certain level

of the specific energy no fusion bond can be formed, whereas beyond another

level th e dilu tion b ecomes too lar ge [Molian , 198 9]. Kar [19 89 ] an d S in gh [19 85 ]

su ggest th at a d ecrease of th e specific energy du e to an in crease of th e feed rate

can en h an ce the s olid s olu bility of alloyin g elem en ts.

In this thesis i t is shown that the specific energy alone is not a sui table pa-

rameter for explaining laser clad propert ies: the thermal cycle in the surface

layer will be different for each specific parameter combination that results in a

certa in valu e of th e sp ecific ener gy. Th is op in ion is s h ar ed b y Ellis [19 95 ] an d is

Fig. 2.11 Exam ple of a process d iagram a chieved b y pa ram eter variation

[Kreutz , 199 5]. 25 k W/ cm2: : 350 m m/ min , +: 300 m m/ min; 18 kW/ cm

2:

o: 350 mm / m in, : 300 mm / min. Pow der: Stellite 6; Su bstrate: C45.

Eq . 2 .1E

P

v D=


42/177


supported by the exper imenta l resul t s which a re presented in chapter 6 . I t i s

also su pp orted by figure 2 .12. Th is f igur e was com posed by tak in g values of th e

interact ion t ime and power densi ty as reported in the l i terature for laser clad-

ding of coba lt b as ed powders on low alloy steels. According to th is figu re, a ll ex-

per imen ts a re in th e range of 0 .5 - 15 kJ / cm2. Average valu es for int era ction

t ime an d powder dens ity are respect ively 0.5 s a n d 15 k W/ cm 2.

Ex p er im en t a l d es ign

The factorial design method is one of the methods that can be used for a sys-

tematical ly performed parameter evaluat ion. I t was introduced by Box [1978]

an d J ohn son [197 7]. It is a mu lt i para meter an a lys is m ethod which is m ost

u sefu l for appl ica t ions in which a process depend s on nu merou s p aram eters . In

such appl icat ions i t is hardly possible to draw conclusions from graphs and ta-

bles th at a re ach ieved with ordinar y one-at-a-t ime experimen ts, esp ecial ly when

taking into account that interact ions and higher order effects between parame-

ters can exist .

Th e pa ram eters , a lso called factors , a re varied in a system atical way from a low

value to a high level (figure 2.13). In this example each parameter is evaluated

Fig. 2.1 2 S ome va lue s of the s pecific en ergy as reported in the literature for

las er clad d ing of cobalt bas ed coatings on low alloy s teel. Each d ot repre-

s ents a d ifferent pa ram eter se tting.


43/177

44 Chapter 2

on three levels . For each possible parameter combinat ion an experiment is

performed an d the process respon se is m easu red.

The factorial design method is more efficient than an ordinary one-at-a-t ime

variat ion of parameters , because i t requires less experiments to achieve the

same accuracy. For example, the simplest possible factorial experiment is one

with two factors that are both evaluated on two levels. In a factorial design ex-

periment , the set-up requires four experiments. A one-at-a-t ime variat ion re-qu ires s ix experimen ts in order to obtain th e sa m e level of accu racy.

The factorial analysis gives a statistical significance to the observed relation-

sh ips , which n ot only al lows th e with dra wal of in sign ifican t p rocess p ara m eters

from further experiments, but a lso al lows a sensi t ivi ty analysis of the process

[Box, 19 78 ].

The calculated relationships between factors are only valid between the lower

an d th e higher leve l of the p ara meters . Man y processes in n a tu re sh ow an ex-

ponent ial course, which can not be revealed by this method. However, in a

small interval such a curve can be approximated by a st raight l ine or a seconddegree polynom ial.

Van Sprang [1992] and Eiholzer [1985] appl ied this method to laser cladding

with prep laced powder. Th ey varied th e las er power den si ty (I), the feed rate of

th e workpiece (v) an d th e coa t in g thickness (d) . Thereupon, they studied the re-

sponse of clad layer propert ies to these variat ions. Van Sprang evaluated al l

three factors on three levels for which he performed 27 experiments. The

Fig. 2.1 3 S et-up for a three param eter factorial exp erim en t w ith all pa -

ram eters at three levels .


44/177


ach ieved res u lts of h is an alysis (tab le 2.4) are sh own in tab le 2.5. Thes e valu es

can be u sed to comp ose a process wind ow of ma chining para meters .

As on e can s ee in tab le 2.5, the average micro ha rdn ess in th e clad layer is 6 80

Hv. Th e ha rdn ess is redu ced by eith er an in crease of th e power den si ty or a re-

duction of the feed rate. This agrees with the findings of others (table 2.3) and

can be explained b y the redu ced coolin g rate . Th e feed rate h as a larger imp act

on the ha rdn ess tha n th e power dens i ty.

In this case, only the l inear effects of the factors on the hardness, the heat af-

fected zon e an d th e di lu t ion were sign ifican t . In teract ions an d s econ d order ef-

fects are im portan t for the mea su red abs orpt ion on ly.

2.7.2 Physical models

An al tern at ive to the p rocess a n alysis with experim enta l meth ods is to develop

models which are based on physical laws. Models that are based on physical

laws cont r ibute to a be t te r ins ight in the process . Exper iments must be per-

formed to ver ify the ass u mp t ions on which the m odel is b ased an d to a dap t th e

Tab. 2.4 Inpu t param eters for factorial de s ign an alys is of las er clad d ing.

Level power density[kW/cm

2]

feed rate [mm/s] coating thickness[mm]

-1

0

1

18

23

28

2

4

6

0.4

0.6

0.8

Tab. 2.5 Proces s resp ond s a s a chieved w ith a three level factorial experim ent for

three factors . All factors s caled betw een -1 a nd 1. Ba s e m aterial: C45; Clad d ing

material: Metco 18C.

Property parameter function

hardness H [Hv 0.2] H = 680 - 338 I + 452 v 55

heat affected zone HAZ [mm] HAZ = 0.76 + 0.25 I - 0.43 v - 0.30 d 0.05

dilution DIL [%] DIL = 35.8 + 39.0 I - 21.0 v - 76.0 d 4.0

absorption A [%] A = 28.9- 3.2 I + 0.7 v - 0.5 v2

+ 0.8 d + 0.8 v d + 0.3 d 1.1


45/177

46 Chapter 2

parameters in i t . A good model supports the necessary experimental research

for th e developm en t of n ew app lications .

Few models for laser cladding have been proposed. They can be dist inguished

in to two group s. The firs t grou p of m odels is a imed a t pred ict in g the a m ou n t of

mater ia l tha t can be deposi ted, based on the assumpt ion tha t a perfec t c lad

has no di lut ion with the substrate , without calculat ion of the temperature fie ld

in th e melt [J ou vard, 19 97; Lemoin e, 199 3, 19 94 (2x), 199 5; Li, 199 4; Mars den ,

19 90]. An alyt ical solu t ions for th e resu lt in g clad geometry ca n be ob tained be-

cau se convect ion is n ot accoun ted for.

A typical exam ple of su ch m odels is th e work of J ou vard [199 7]. As a firs t st ep

he d e termines th e laser p ower which i s t ran sm it ted throu gh th e c loud formed

by injected powder particles (P-Pa t

in figure 2.16) by application of the Beer-

Lam bert law. For detai ls , on e is referred to ch ap ter 6.2, where th is law is u sed

to calcu late th e tempera tu re rise of pa rt ic les du rin g their f light th rou gh th e la-

se r beam.The second step involves the calculat ion of the power required to produce a

melt pool in the substrate . Combined with the power at tenuat ion factor, the

th erefore required laser p ower can be determ in ed (th resh old 1 in figure 2 .14).

If more power than this f i rs t threshold is suppl ied, a clad layer can be bui l t up

on the subs t ra te . During the f l ight through the laser beam the par t ic les a re

heated but not molten. Melt ing occurs when the powder reaches the melt pool

tha t ha s been ini t ia ted on the su bs t ra te .

The mass f low tha t can be mol ten only depends on

Thesis Laser Clad

Documents

Transcript of Thesis Laser Clad