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Ways to Intensify Laser Hardening Technology
Ways t o Intensify Laser Hardening Technology V o l o d y m y r S. Kovalenko Laser Technology Research Institute, National Technical University of Ukraine, Kyiv, Ukraine Submitted by Dirk F. D a u w (an Active M e m b e r ) Received o n January 10, 1998
Abstract A t this stage of t h e laser material hardening development, f e w techniques t o intensify t h e process and t o Improve t h e quality of the hardened surfaces m a y be proposed: 1 ) the development of n e w absorption coatings t o increase laser radiation absorption efficiency of t h e surface t o be hardened; 2) the development of devices t o measure material absorptivity and instant temperature in working zone at laser irradiation t o control treatment conditions; 3) t h e n e w compositions development for alloying and cladding; 4) t h e development of combined techniques The study results of these and other techniques are presented and discussed Keywords : Hardening, Laser, Quality Improvement
I. Int ro ducti on =
In recent years Laser hardening technology has become quite spread in industry thanks t o a number of advantages it has in comparison with conventional hardening technology. M a i n of t h e m are t h e followingt h e ability t o improve t h e component surface quality locally, where such improvement is needed, thus avoiding overheating and hence component deformation, t h e ability t o harden o n a d i s t c i c e w i t h o u t mechanical action on t h e surface; t h e ability t o change the material surface properties in wide range, t h e great flexibility of t h e laser hardening technology, w h i c h allows t o perform quite a large variety of processes transformation hardening, local surface alloying, surface glazing, shock hardening, cladding [21 Nevertheless there are still unused reserves w h i c h allow t o intensify t h e laser hardening technology and t o avoid s o m e drawbacks w h i c h m a y arise at laser treatment like n o t thick enough hardened layer, cracks formation, porosity, hardness non uniformity, etc A m o n g such reserves there are f e w techniques w h i c h had been developed a t t h e Laser Technology Resei rch Institute (LTRI) and Laser Technology and Material Science Department o f t h e National Technical University of Ukraine " Kiev Polytechnic Institute"
2. Development of n e w absorption coatinqs. Up t o n o w t o perform hardening operations t h e CO2 and Y A G lasers delivering IR radiation are mainly used w h i c h causes large p o w e r loses o n reflection f r o m metal surface. T o decrease these losses different techniques are used t h e m o s t efficient being t h e preliminary surface coating with absorption substances. These coatings have t o be cheap, non toxic, easy t o be coated, temperature resistant, and should increase absorptivity significantly. A t LTRl different types of such coatings have been developed and studied. They have been ranged correspondingly t o their efficiency coefficient q (Table 1 ) .
Annals of the ClRP Vol. 47/1/1998
hc1hw.c. ,
where hc and hw.c. is depth of hardening w i t h coat irig and w i t h o u t coating correspondingly .
I I I I
Coating Cr Without coating Cd C ZnO Zn3(P O 4 2 s i 0 2 , A1203,, C Fe304
rl
0.6 1 .o
2.0 3.0
I I
I I
4.5 5.1
I
6.5
I
6.7
I
I
3 . Absorption and Temperature Control at Laser Interaction with Surface. Laser surface treatment is a multyfactor process w h i c h depends upon more than 30 factors. It is clear that measuring and controlling all these factors is practically very difficult or even impossible. So it is feasible t o determine t h e main factors w h i c h influence other factors and t h e process in general. A m o n g such factors are t h e following: surface temperature, duration and speed of heating, cooling speed. Surface temperature itself depends very m u c h on surface absorptivity. To control and t o increase t h e hardening efficiency one has t o find t h e w a y t o control the surface absorptivity and temperature simultaneously w i t h t h e irradiation process. A t LTRl w e developed systems and devices t o measure and t o control t h e mentioned parameters One of t h e first versions of such device w a s based on the calorimeter t y p e s y s t e m . The device has a double wall hemisphere w i t h central opening t o pass
133
t h e laser b e a m The internal surface of hemispriere has an absorbing coating. Special gas working m e j i a is pumped b e t w e e n t h e walls. Reflected f r o m t h e irradiated surface laser beam causes gas media heating and t h u s gas pressure changes b e t w e e n t h e walls of hemisphere. These data serve as initial information t o measure absorptivity changes during t h e irradiation N e x t version of t h e device uses a pyroelectric detector as a sensor (Fig 1 , i t e m 41, fixed at a focusing s y s t e m over t h e treated surface and electrically connected with t h e information developing block (IDB) and t h a n with t h e register device. The IDB is connected with a laser p o w e r meter as w e l / (Fig. 1 , i t e m 2).Surface absorptivity is measuied as a esult of comparison of t w o signals proportional t o t h e intensity of incident and reflected radiation and is s h o w n a t t h e computer monitor. The program developed enables t o analyze t h e absorptivity dynamically The t i m e constant of t h e developed device is 10 s The results obtained s h o w t h e w i d e opportunities t o use such devices installed in t h e s y s t e m s for processing adaptive control [7]. Further improvement of t h e device is t h e s y s t e m for measurement not only t h e absorptivity but instant temperature in t h e irradiated zone as well. In this device t h e problem of separation of reflected laser radiation and radiation f r o m heated surface is salved with help of their modulation and b y use of special filters (Fig. 1 ) Tests of t h e device have s h o w n that it m a y be used in automated control s y s t e m for t h e process of laser surf ace t reatnient
Fig. 1 . Scheme of t h e device t o measure absorptivity and instant temperature in irradiated zope. P r - incident radiation; Pr - reflected'radiation; Pre reemmited radiation. 1 - partly reflecting mirror; 2 - detector; 3 - modulator; 4 - detector o f reflected power; 5 - filter; 6 attenuator; 7 - detector of reemmited power; 8 hemisphere; 9 - irradiated surface.
3. New Composition for Laser Allovinq and Cladding. From 1-2 elements composition for local surface alloying in t h e past [I] for cladding t h e multielements compositions are used n o w . In m a n y Yases
134
these compositions originally have been developed for plasma cladding The m o s t w i d e spread powders are produced o n nickel base The main drawbacks of such powders are high price, gripping of cladded layer at dry friction and formation of significant tensile stresses N e w compositions have been developed at LTRl based on Fe-B-C-alloy Due t o iron base t h e tensile stresses formed in cladded layer are usually lower t h a n in case of Ni-based compositions Boron coating of carbon steel gives higher wear resistance but has very l o w plasticity So t o increase t h e boron plasticity t h e extra elements (Si and Cr) have been added t o t h e powder. Test has s h o w n t h e following advantages of t h e developed compositions for laser cladding [61 they are 4-5times cheaper t h a n N i based alloys and 8-9 times cheaper t h a n Co based alloys, n e w alloys are n o n t o x i c unlike Ni based alloys w h i c h f o r m a t cladding some toxic oxides because of presence of phosphor, cladded layer hardness is not l o w e r than that for Ni based alloys (62-63 HRC), adhesion of n e w alloys is better because of cladded matrix material similarity. As it w a s mentioned earlier t h e main disadvantages of laser cladding are porosity and cracks formation Such defects lead t o flaking and abrasive wear of the cladded layer during t h e work of t h e component being cladded Porosity m a y be reduced quite easily a t cladding with self-fluxing p o w d e r s Experiments have s h o w n that it w a s enough t o dry and t o screen the powder for cladding and sometimes t o preheat t h e powder before cladding u p t o t h e temperature 100-150°C Crack formation depends o n cladding conditions, chemical content o f cladded material and chemical content of m a t r i x material The decrease of crack formation m a y be achieved b y . base metal preheating; proper choice of laser cladding conditions, proper choice of cladded p o w d e r chemical content. The first w a y is n o t a l w a y s appropriate because of loosing t h e advantage of local heat influence on metal base The second w a y is wide spread and still has a lot of opportunities. T h e third w a y m a y be considered quite prospective t o decrease crack formation In accordance with this w a y t h e choice of the material chemical content for t h e layer t o be cladded has t o be made taking into account t h e structure of the matrix material This c o m e s f r o m consideration of the whole picture of thermal tensions generated during cladding process. This consideration proves t h a t the reason for crack formation a t laser cladding is the action of residual tensile stresses, exceeding the ultimate rupture strength of t h e cladded metal The technique proposed is directed t o decrease the crack formation a t laser cladding with self fluxing powder materials. In accordance with this technique t h e decrease of internal tensile stresses in t h e cladded layer IS achieved b y their relaxation in plastic alloying additives and with equalization of t h e temperature field differences across t h e depth of the s y s t e m "cladding material - m a t r i x material" [5] The developed procedure includes the following stages. First t h e temperature distributions across the depth of t h e cladded layers were observed and
alloying additives t o cladding metal t o decrease :rack formation were identified empirically. Based o n temperature distribution analysis t h e thermo-physical problem and solution of l o w alloy steel laser cladding with multi-component alloys o n nickel base w e r e proposed Than the adequacy of suggested mathematical model t o practical results w a s prfived. Thus based on t h e solution of this thermo-physical problem it w a s possible t o suggest t h e procedure t o choose t h e thermo-physical coefficients of cladded metal through calculation of t h e temperature and stresses distribution of t h e s y s t e m "cladded material matrix material" at k n o w n chemical content of t h e matrix material. Getting thermo-physical coeffic ients of t h e cladding material and its necessary hardness t h e chemical content of t h e material car? be identified. During t h e experimental phase of t h e developed procedure s o m e special additives were m i x e d with standard p o w d e r compositions, used for plasma cladding. The addition t o some standard conipcsition of ferroalloys FeV and FeTi (up t o 3%), or pure Ti u p t o 2 3 - 2 . 5 '/o decreases t h e tendency of cladded material t o crack's formation practically t o zero. By adding t o s o m e compositions Ti or FeTi (about 2 - 3 YO), FeV or V u p t o 7 % t h e full guaranteed cracks elimination is achieved. Quite significant decrease of crack formation is observed at addition of Cu u p o 10 % and Pb u p t o 5 %. Based o n experimental data t h e plotted dependencies, s h o w n o n Fig. 2, were obtained.
0.2
Laser Plastic Deformation Hardeninq (LPDH) combines t w o processes in one - laser hardening and thermo plastic deformation hardening The benefits of such combination are t h e following first, possibilities t o get hardened layer with specific structure for laser quenching, second, possibilities t o get guaranteed compressive stresses favorable for increasing fatigue strength and wear resistance In the developed process the plastic deformation of material is caused with roller during laser irradiation of t h e surface [31 The main problem w a s h o w t o determine the magnitude of deforming force w h i c h depends on deformation temperature, resistance t o deformation and value of material surface cold working For studied steel 4 5 ( 1 0 4 5 ) t h e value of deforming force w a s accepted a t t h e level 5 0 0 - 6 0 0 N Using t h e calculated temperature distribution in t h e irradiated zone t h e distance f r o m t h e center of irradiated spot and roller position w a s found These data were used t o design t h e device for surface laser plastic deformation. The study of t h e process and analysis of t h e obtained results revealed t h e following -due t o simplicity and high efficiency LPDH is a prospective method t o increase the fatigue strength and wear resistance of machine components, - in comparison with laser hardening, at LPDH the micro hardness is 1 5 0 0 MPa higher and t h e hardened layer depth is 1 0 0 - 2 0 0 p m larger, Laser Ultra-Sonic Hardeninq (LUSH) combines laser irradiation with vibration plastic deformation at ultrasonic frequency. Such combination m a y cause t h e increase in micro hardness u p t o 3000 4000 MPa Surface topography is improved as well, due t o such treatment. Laser Hardeninq in Liquid Nitroqen (LHLN) is a quite efficient technique because a t this process t h e heat removal f r o m irradiated zone is improved due t o higher temperature gradient and conditions for better nitrogen diffusion into m a t r i x material are created This gives t h e substantial increase of treated material micro hardness (Table 2 )
Steel Fig. 2. Influence of alloying elements o n cracks formation index a : 1 - FeV; 2 - Ti; 3 - Fe; 4 - Cr; 5 - CU; 6 - Pb; 7 - B. The special index a w a s excepted as a total amount of cracks per length unit of cladded material. The developed procedure of t h e cladding composition selection w a s introduced into practice at laser cladding of crank shafts for combustion engines (matrix material - special alloying steel or high alloyed cast iron), for laser cladding of t h e shafts before thread cutting and for wear resistance of other components. 4. Combined hardeninq processes. T o intensify t h e hardening effect a t laser surface treatment different combinations of laser hardening technology with other techniques of material treatment have been developed at LTRI: Laser Plastic Deformation Hardening, Laser Ultra Sonic Hardening, Laser Hardening in Liquid Nitrogen, Laser Cladding with Electro Magnetic Agitation.
Laser hardening (HP, MPa) 9760 9760 9340
Laser hardening in liquid nitrogen, (Hp MPa) I 10700 11190 10700
Carbon(l.2Y0) High alloy High speed cuttins Laser Claddinq with Electro Maqnetic Agitation (LCEMA). In spite of t h e development of different n e w compositions for laser cladding t h e researcher usually has t o find t h e compromise in choosing the proper additives for alloying compositions, because some of t h e m brings cracks resistance increase but at t h e expense of wear resistance decrease. Other additives w o u l d increase t h e hardness and wear resistance of cladded layer b u t at t h e same time will bring d o w n t h e cracks resistance. So t h e compromised solution w a s found by development of t h e combined process w h i c h includes t h e action of C W C 0 2 laser radiation, electric arc and variable magnetic field o n t h e material (Fig, 3).
135
A n electric arc creates current in t h e m e l t arid is considered as additional source of energy. The variable magnetic field interacting with the current in t h e melt provides t h e electromagnetic agitation of t h e melt. Thus t h e hydrodynamic processes in the melt are t h e superposition of thermo capillary process that is typical for laser cladding, electromagnetic agi, ation and electric arc pressure [ 6 ] .
0.4
0.45
5
4 Fig. 4. A m o u n t of cracks per bead length unit ( a )VS t h e level of magnetic field intensity ( B ) , ( l a -arc
1
ri ""I
_L
z
V ro? v i e w of expertmental I
___ 11
Fig. 3. Schematic setup for combined process of LCEMA. 1 - surface t o be cladded; 2 - alloying powder feeder nozzle, 3 - working gas; 4 - electro - magnetic (EM) coil; 5 - focusing lens; 6 - EM p o w e r supply; 7 - laser beam; 8 - electric arc p o w e r supply, 9 - arc electrode working gas nozzle; 1 0 - cladded layer: 1 1 - electric arc. Fig 4 represents t h e influence of magnetic field intensity B o n index a (cracks number per length unit) Layers have been C 0 2 laser cladded a t radiation p o w e r P = 1,4 kW, cladding speed V = 3 . 0 m m i s and p o w d e r mass rate M = 5 gis. It I S w o r t h t o note that for b o t h additive materials (1 - standard f o r plasma alloying, 2 specially developed for laser cladding, experimental) t h e use of electric arc ( l a = 4 0 A ) a t t h e absence of magnetic field (B = 0) brings already t h e significant M i c r o structure analysis of reduction of index a layers obtained w i t h usual laser cladding (but w i t h o u t electric arc action) has revealed t h a t crack are formed mainly a t t h e bead edge close t o p o w d e r particles w h i c h didn't dissolved entirely, because during cladding these particles have been at t h e laser beam periphery. A t cladding with laser + electric arc technology such stress concentrators are practically absent, because t h e injected particles melting starts already in t h e arc coaxial with laser beam. W h e n E M A is added t o t h e process a critical value of magnetic field intensity Bcr is observed Only after acceding this value t h e influence of E M A on cracks formation begins (Fig. 4 ) . For standard composition t h e critical value Bcr is significantly l o w e r t h a n that for t h e d e v t oped experimental composition for cladding Other combined processes of laser hardening technology are still under development in LTRI.
136
I " ,
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1 - standard p o w d e r composition 2 - special composition @ - l a = 0; la = 40 A . C1 - la = 4 0 A .
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la
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5. Conclusions 1. Devices t o measure surface absorptivity and temperature has been developed w h i c h allow t o control t h e processing paranieters and t o create t h e adaptive control system. 2. Alloying compositions t o improve the quality of cladded material have been developed. Means t o l o w e r porosity and cracks formation have been suggested. 3. The combined processes t o improve t h e laser ha r d e ni ng techno I o g y w it h d if f e re nt t e c hni q ues m a k e it possible t o improve significantly t h e quality of the treated surfaces
6.
References
(1) Draper C., 1 9 8 2 , Surface Alloying: the State of A r t , Journal of Metals, 3 4 , 4 : 2 4 - 3 2 (2) Heuvelman C.J., et al, 1 9 9 2 , Surface Treatment Techniques b y Laser Beam Machining, Annals of t h e CIRP, 4112: 6 5 7 - 6 6 6 ( 3 ) Kovalenko V., Golovko L., 1 9 9 3 , Laser ThermoDeformation Material Hardening, Proc. Int. Conf Electron Beam and Laser Proc., Reno, USA. (4) Kovalenko V. et al, 1 9 9 3 , Wear resistant Laser Cladding with Boron Containing Powder Steel, Proc. ICALE0'93, Orlando, USA. (5) Kovalenko V., Haskin V., 1 9 9 5 , The Selection of Self-Fluxing Powder Materials f o r Laser Cladding, lnformatization and N e w Technology, 1 : 3 6 - 3 9 . 9 9 9 (6) Kovalenko V., Lutay A., Anyakin M., Gas Powder Laser Cladding with Electro-Magnetic Agitation, 1 9 9 7 , Proc. of I C A L E 0 ' 9 7 , San Diego, USA. (7) Mazunder J., Conde O., Villar R., Steen W., (eds.), 1 9 9 4 , Proc. of N A T O ASI, Sessimbra, Portugal
Abstract A t this stage of t h e laser material hardening development, f e w techniques t o intensify t h e process and t o Improve t h e quality of the hardened surfaces m a y be proposed: 1 ) the development of n e w absorption coatings t o increase laser radiation absorption efficiency of t h e surface t o be hardened; 2) the development of devices t o measure material absorptivity and instant temperature in working zone at laser irradiation t o control treatment conditions; 3) t h e n e w compositions development for alloying and cladding; 4) t h e development of combined techniques The study results of these and other techniques are presented and discussed Keywords : Hardening, Laser, Quality Improvement
I. Int ro ducti on =
In recent years Laser hardening technology has become quite spread in industry thanks t o a number of advantages it has in comparison with conventional hardening technology. M a i n of t h e m are t h e followingt h e ability t o improve t h e component surface quality locally, where such improvement is needed, thus avoiding overheating and hence component deformation, t h e ability t o harden o n a d i s t c i c e w i t h o u t mechanical action on t h e surface; t h e ability t o change the material surface properties in wide range, t h e great flexibility of t h e laser hardening technology, w h i c h allows t o perform quite a large variety of processes transformation hardening, local surface alloying, surface glazing, shock hardening, cladding [21 Nevertheless there are still unused reserves w h i c h allow t o intensify t h e laser hardening technology and t o avoid s o m e drawbacks w h i c h m a y arise at laser treatment like n o t thick enough hardened layer, cracks formation, porosity, hardness non uniformity, etc A m o n g such reserves there are f e w techniques w h i c h had been developed a t t h e Laser Technology Resei rch Institute (LTRI) and Laser Technology and Material Science Department o f t h e National Technical University of Ukraine " Kiev Polytechnic Institute"
2. Development of n e w absorption coatinqs. Up t o n o w t o perform hardening operations t h e CO2 and Y A G lasers delivering IR radiation are mainly used w h i c h causes large p o w e r loses o n reflection f r o m metal surface. T o decrease these losses different techniques are used t h e m o s t efficient being t h e preliminary surface coating with absorption substances. These coatings have t o be cheap, non toxic, easy t o be coated, temperature resistant, and should increase absorptivity significantly. A t LTRl different types of such coatings have been developed and studied. They have been ranged correspondingly t o their efficiency coefficient q (Table 1 ) .
Annals of the ClRP Vol. 47/1/1998
hc1hw.c. ,
where hc and hw.c. is depth of hardening w i t h coat irig and w i t h o u t coating correspondingly .
I I I I
Coating Cr Without coating Cd C ZnO Zn3(P O 4 2 s i 0 2 , A1203,, C Fe304
rl
0.6 1 .o
2.0 3.0
I I
I I
4.5 5.1
I
6.5
I
6.7
I
I
3 . Absorption and Temperature Control at Laser Interaction with Surface. Laser surface treatment is a multyfactor process w h i c h depends upon more than 30 factors. It is clear that measuring and controlling all these factors is practically very difficult or even impossible. So it is feasible t o determine t h e main factors w h i c h influence other factors and t h e process in general. A m o n g such factors are t h e following: surface temperature, duration and speed of heating, cooling speed. Surface temperature itself depends very m u c h on surface absorptivity. To control and t o increase t h e hardening efficiency one has t o find t h e w a y t o control the surface absorptivity and temperature simultaneously w i t h t h e irradiation process. A t LTRl w e developed systems and devices t o measure and t o control t h e mentioned parameters One of t h e first versions of such device w a s based on the calorimeter t y p e s y s t e m . The device has a double wall hemisphere w i t h central opening t o pass
133
t h e laser b e a m The internal surface of hemispriere has an absorbing coating. Special gas working m e j i a is pumped b e t w e e n t h e walls. Reflected f r o m t h e irradiated surface laser beam causes gas media heating and t h u s gas pressure changes b e t w e e n t h e walls of hemisphere. These data serve as initial information t o measure absorptivity changes during t h e irradiation N e x t version of t h e device uses a pyroelectric detector as a sensor (Fig 1 , i t e m 41, fixed at a focusing s y s t e m over t h e treated surface and electrically connected with t h e information developing block (IDB) and t h a n with t h e register device. The IDB is connected with a laser p o w e r meter as w e l / (Fig. 1 , i t e m 2).Surface absorptivity is measuied as a esult of comparison of t w o signals proportional t o t h e intensity of incident and reflected radiation and is s h o w n a t t h e computer monitor. The program developed enables t o analyze t h e absorptivity dynamically The t i m e constant of t h e developed device is 10 s The results obtained s h o w t h e w i d e opportunities t o use such devices installed in t h e s y s t e m s for processing adaptive control [7]. Further improvement of t h e device is t h e s y s t e m for measurement not only t h e absorptivity but instant temperature in t h e irradiated zone as well. In this device t h e problem of separation of reflected laser radiation and radiation f r o m heated surface is salved with help of their modulation and b y use of special filters (Fig. 1 ) Tests of t h e device have s h o w n that it m a y be used in automated control s y s t e m for t h e process of laser surf ace t reatnient
Fig. 1 . Scheme of t h e device t o measure absorptivity and instant temperature in irradiated zope. P r - incident radiation; Pr - reflected'radiation; Pre reemmited radiation. 1 - partly reflecting mirror; 2 - detector; 3 - modulator; 4 - detector o f reflected power; 5 - filter; 6 attenuator; 7 - detector of reemmited power; 8 hemisphere; 9 - irradiated surface.
3. New Composition for Laser Allovinq and Cladding. From 1-2 elements composition for local surface alloying in t h e past [I] for cladding t h e multielements compositions are used n o w . In m a n y Yases
134
these compositions originally have been developed for plasma cladding The m o s t w i d e spread powders are produced o n nickel base The main drawbacks of such powders are high price, gripping of cladded layer at dry friction and formation of significant tensile stresses N e w compositions have been developed at LTRl based on Fe-B-C-alloy Due t o iron base t h e tensile stresses formed in cladded layer are usually lower t h a n in case of Ni-based compositions Boron coating of carbon steel gives higher wear resistance but has very l o w plasticity So t o increase t h e boron plasticity t h e extra elements (Si and Cr) have been added t o t h e powder. Test has s h o w n t h e following advantages of t h e developed compositions for laser cladding [61 they are 4-5times cheaper t h a n N i based alloys and 8-9 times cheaper t h a n Co based alloys, n e w alloys are n o n t o x i c unlike Ni based alloys w h i c h f o r m a t cladding some toxic oxides because of presence of phosphor, cladded layer hardness is not l o w e r than that for Ni based alloys (62-63 HRC), adhesion of n e w alloys is better because of cladded matrix material similarity. As it w a s mentioned earlier t h e main disadvantages of laser cladding are porosity and cracks formation Such defects lead t o flaking and abrasive wear of the cladded layer during t h e work of t h e component being cladded Porosity m a y be reduced quite easily a t cladding with self-fluxing p o w d e r s Experiments have s h o w n that it w a s enough t o dry and t o screen the powder for cladding and sometimes t o preheat t h e powder before cladding u p t o t h e temperature 100-150°C Crack formation depends o n cladding conditions, chemical content o f cladded material and chemical content of m a t r i x material The decrease of crack formation m a y be achieved b y . base metal preheating; proper choice of laser cladding conditions, proper choice of cladded p o w d e r chemical content. The first w a y is n o t a l w a y s appropriate because of loosing t h e advantage of local heat influence on metal base The second w a y is wide spread and still has a lot of opportunities. T h e third w a y m a y be considered quite prospective t o decrease crack formation In accordance with this w a y t h e choice of the material chemical content for t h e layer t o be cladded has t o be made taking into account t h e structure of the matrix material This c o m e s f r o m consideration of the whole picture of thermal tensions generated during cladding process. This consideration proves t h a t the reason for crack formation a t laser cladding is the action of residual tensile stresses, exceeding the ultimate rupture strength of t h e cladded metal The technique proposed is directed t o decrease the crack formation a t laser cladding with self fluxing powder materials. In accordance with this technique t h e decrease of internal tensile stresses in t h e cladded layer IS achieved b y their relaxation in plastic alloying additives and with equalization of t h e temperature field differences across t h e depth of the s y s t e m "cladding material - m a t r i x material" [5] The developed procedure includes the following stages. First t h e temperature distributions across the depth of t h e cladded layers were observed and
alloying additives t o cladding metal t o decrease :rack formation were identified empirically. Based o n temperature distribution analysis t h e thermo-physical problem and solution of l o w alloy steel laser cladding with multi-component alloys o n nickel base w e r e proposed Than the adequacy of suggested mathematical model t o practical results w a s prfived. Thus based on t h e solution of this thermo-physical problem it w a s possible t o suggest t h e procedure t o choose t h e thermo-physical coefficients of cladded metal through calculation of t h e temperature and stresses distribution of t h e s y s t e m "cladded material matrix material" at k n o w n chemical content of t h e matrix material. Getting thermo-physical coeffic ients of t h e cladding material and its necessary hardness t h e chemical content of t h e material car? be identified. During t h e experimental phase of t h e developed procedure s o m e special additives were m i x e d with standard p o w d e r compositions, used for plasma cladding. The addition t o some standard conipcsition of ferroalloys FeV and FeTi (up t o 3%), or pure Ti u p t o 2 3 - 2 . 5 '/o decreases t h e tendency of cladded material t o crack's formation practically t o zero. By adding t o s o m e compositions Ti or FeTi (about 2 - 3 YO), FeV or V u p t o 7 % t h e full guaranteed cracks elimination is achieved. Quite significant decrease of crack formation is observed at addition of Cu u p o 10 % and Pb u p t o 5 %. Based o n experimental data t h e plotted dependencies, s h o w n o n Fig. 2, were obtained.
0.2
Laser Plastic Deformation Hardeninq (LPDH) combines t w o processes in one - laser hardening and thermo plastic deformation hardening The benefits of such combination are t h e following first, possibilities t o get hardened layer with specific structure for laser quenching, second, possibilities t o get guaranteed compressive stresses favorable for increasing fatigue strength and wear resistance In the developed process the plastic deformation of material is caused with roller during laser irradiation of t h e surface [31 The main problem w a s h o w t o determine the magnitude of deforming force w h i c h depends on deformation temperature, resistance t o deformation and value of material surface cold working For studied steel 4 5 ( 1 0 4 5 ) t h e value of deforming force w a s accepted a t t h e level 5 0 0 - 6 0 0 N Using t h e calculated temperature distribution in t h e irradiated zone t h e distance f r o m t h e center of irradiated spot and roller position w a s found These data were used t o design t h e device for surface laser plastic deformation. The study of t h e process and analysis of t h e obtained results revealed t h e following -due t o simplicity and high efficiency LPDH is a prospective method t o increase the fatigue strength and wear resistance of machine components, - in comparison with laser hardening, at LPDH the micro hardness is 1 5 0 0 MPa higher and t h e hardened layer depth is 1 0 0 - 2 0 0 p m larger, Laser Ultra-Sonic Hardeninq (LUSH) combines laser irradiation with vibration plastic deformation at ultrasonic frequency. Such combination m a y cause t h e increase in micro hardness u p t o 3000 4000 MPa Surface topography is improved as well, due t o such treatment. Laser Hardeninq in Liquid Nitroqen (LHLN) is a quite efficient technique because a t this process t h e heat removal f r o m irradiated zone is improved due t o higher temperature gradient and conditions for better nitrogen diffusion into m a t r i x material are created This gives t h e substantial increase of treated material micro hardness (Table 2 )
Steel Fig. 2. Influence of alloying elements o n cracks formation index a : 1 - FeV; 2 - Ti; 3 - Fe; 4 - Cr; 5 - CU; 6 - Pb; 7 - B. The special index a w a s excepted as a total amount of cracks per length unit of cladded material. The developed procedure of t h e cladding composition selection w a s introduced into practice at laser cladding of crank shafts for combustion engines (matrix material - special alloying steel or high alloyed cast iron), for laser cladding of t h e shafts before thread cutting and for wear resistance of other components. 4. Combined hardeninq processes. T o intensify t h e hardening effect a t laser surface treatment different combinations of laser hardening technology with other techniques of material treatment have been developed at LTRI: Laser Plastic Deformation Hardening, Laser Ultra Sonic Hardening, Laser Hardening in Liquid Nitrogen, Laser Cladding with Electro Magnetic Agitation.
Laser hardening (HP, MPa) 9760 9760 9340
Laser hardening in liquid nitrogen, (Hp MPa) I 10700 11190 10700
Carbon(l.2Y0) High alloy High speed cuttins Laser Claddinq with Electro Maqnetic Agitation (LCEMA). In spite of t h e development of different n e w compositions for laser cladding t h e researcher usually has t o find t h e compromise in choosing the proper additives for alloying compositions, because some of t h e m brings cracks resistance increase but at t h e expense of wear resistance decrease. Other additives w o u l d increase t h e hardness and wear resistance of cladded layer b u t at t h e same time will bring d o w n t h e cracks resistance. So t h e compromised solution w a s found by development of t h e combined process w h i c h includes t h e action of C W C 0 2 laser radiation, electric arc and variable magnetic field o n t h e material (Fig, 3).
135
A n electric arc creates current in t h e m e l t arid is considered as additional source of energy. The variable magnetic field interacting with the current in t h e melt provides t h e electromagnetic agitation of t h e melt. Thus t h e hydrodynamic processes in the melt are t h e superposition of thermo capillary process that is typical for laser cladding, electromagnetic agi, ation and electric arc pressure [ 6 ] .
0.4
0.45
5
4 Fig. 4. A m o u n t of cracks per bead length unit ( a )VS t h e level of magnetic field intensity ( B ) , ( l a -arc
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Fig. 3. Schematic setup for combined process of LCEMA. 1 - surface t o be cladded; 2 - alloying powder feeder nozzle, 3 - working gas; 4 - electro - magnetic (EM) coil; 5 - focusing lens; 6 - EM p o w e r supply; 7 - laser beam; 8 - electric arc p o w e r supply, 9 - arc electrode working gas nozzle; 1 0 - cladded layer: 1 1 - electric arc. Fig 4 represents t h e influence of magnetic field intensity B o n index a (cracks number per length unit) Layers have been C 0 2 laser cladded a t radiation p o w e r P = 1,4 kW, cladding speed V = 3 . 0 m m i s and p o w d e r mass rate M = 5 gis. It I S w o r t h t o note that for b o t h additive materials (1 - standard f o r plasma alloying, 2 specially developed for laser cladding, experimental) t h e use of electric arc ( l a = 4 0 A ) a t t h e absence of magnetic field (B = 0) brings already t h e significant M i c r o structure analysis of reduction of index a layers obtained w i t h usual laser cladding (but w i t h o u t electric arc action) has revealed t h a t crack are formed mainly a t t h e bead edge close t o p o w d e r particles w h i c h didn't dissolved entirely, because during cladding these particles have been at t h e laser beam periphery. A t cladding with laser + electric arc technology such stress concentrators are practically absent, because t h e injected particles melting starts already in t h e arc coaxial with laser beam. W h e n E M A is added t o t h e process a critical value of magnetic field intensity Bcr is observed Only after acceding this value t h e influence of E M A on cracks formation begins (Fig. 4 ) . For standard composition t h e critical value Bcr is significantly l o w e r t h a n that for t h e d e v t oped experimental composition for cladding Other combined processes of laser hardening technology are still under development in LTRI.
136
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1 - standard p o w d e r composition 2 - special composition @ - l a = 0; la = 40 A . C1 - la = 4 0 A .
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5. Conclusions 1. Devices t o measure surface absorptivity and temperature has been developed w h i c h allow t o control t h e processing paranieters and t o create t h e adaptive control system. 2. Alloying compositions t o improve the quality of cladded material have been developed. Means t o l o w e r porosity and cracks formation have been suggested. 3. The combined processes t o improve t h e laser ha r d e ni ng techno I o g y w it h d if f e re nt t e c hni q ues m a k e it possible t o improve significantly t h e quality of the treated surfaces
6.
References
(1) Draper C., 1 9 8 2 , Surface Alloying: the State of A r t , Journal of Metals, 3 4 , 4 : 2 4 - 3 2 (2) Heuvelman C.J., et al, 1 9 9 2 , Surface Treatment Techniques b y Laser Beam Machining, Annals of t h e CIRP, 4112: 6 5 7 - 6 6 6 ( 3 ) Kovalenko V., Golovko L., 1 9 9 3 , Laser ThermoDeformation Material Hardening, Proc. Int. Conf Electron Beam and Laser Proc., Reno, USA. (4) Kovalenko V. et al, 1 9 9 3 , Wear resistant Laser Cladding with Boron Containing Powder Steel, Proc. ICALE0'93, Orlando, USA. (5) Kovalenko V., Haskin V., 1 9 9 5 , The Selection of Self-Fluxing Powder Materials f o r Laser Cladding, lnformatization and N e w Technology, 1 : 3 6 - 3 9 . 9 9 9 (6) Kovalenko V., Lutay A., Anyakin M., Gas Powder Laser Cladding with Electro-Magnetic Agitation, 1 9 9 7 , Proc. of I C A L E 0 ' 9 7 , San Diego, USA. (7) Mazunder J., Conde O., Villar R., Steen W., (eds.), 1 9 9 4 , Proc. of N A T O ASI, Sessimbra, Portugal