Laser beam welding hard metals to steel

Laser beam welding hard metals to steel

Journal of Materials Processing Technology 141 (2003) 163–173 Laser beam welding hard metals to steel Alexandra P. Costaa, Luı´sa Quintinoa,*, Martin...

825KB Sizes 7 Downloads 88 Views

Journal of Materials Processing Technology 141 (2003) 163–173

Laser beam welding hard metals to steel Alexandra P. Costaa, Luı´sa Quintinoa,*, Martin Greitmannb a

Instituto Superior Te´cnico, Technical University of Lisbon, Av. Rovisco Pais, Lisbon 1049 001, Portugal b State Material Testing Institute (MPA), Stuttgart University, Stuttgart, Germany Received 3 January 2001; accepted 24 September 2002

Abstract Laser beam weldability of hard metals to steel was examined with high power (cw) CO2 laser, (cw) Nd:YAG laser and (pw) Nd:YAG laser. Steel/hard metal joints, with 2.5 mm thickness, welded by the three laser systems, for comparison, will be presented. Two different hard metals compositions (K10 and K40) were examined. Power (P), speed (s) and vertical focal point position (f.p.p.) were investigated. To reduce the problem of porosity and crack formation in the hard metal, the parameter, tw, horizontal distance from the beam to the joint was also investigated. Weld bead size, microstructure, bending tests and hardness were evaluated. # 2002 Elsevier B.V. All rights reserved. Keywords: Laser welding; Parameters; Hard metals; Microstructure

1. Introduction Hard metals are hard, wear-resistant, refractory materials in which carbide particles are bound together by a ductile metal binder. The first cemented carbide to be produced (1920) was tungsten carbide (WC) with a cobalt binder, by sinter technology. Over the years these materials have been modified to produce a variety of hard metals with a variety of properties, used in a wide range of applications [1–4]. Due to its great hardness and good wear properties they have been used mostly as cutting tools, where high strength at high temperatures is required. Since hard metals are brittle and expensive materials when compared with tough steels, only the active part of the tool is usually made of hard metal. Consequently one of the industrial problems is the joint of the hard metal to the steel holder. Traditionally, brazing is the joining method used. However this method has several disadvantages, namely when high temperatures are achieved during cutting operations, giving rise to tips detachment. Mechanical clamping is another alternative, with great success, due to the fact that tips can be substituted and all the cutting edges can be used. However there are cases in which mechanical attachment is not possible due to geometric limitations. In those cases a welding joint can be the best solution.

*

Corresponding author.

0924-0136/$ – see front matter # 2002 Elsevier B.V. All rights reserved. PII: S 0 9 2 4 - 0 1 3 6 ( 0 2 ) 0 0 9 5 4 - 8

In order to minimise residual stresses caused by the welding process, the joining method should minimise the HAZ (heat affected zone) and the heat input in the welding zone. Having this in mind, laser beam welding could be a good alternative, to hard metals joining, since it is possible to have a control of the heat input and obtain limited HAZ. Research work done with ceramic materials, mainly with Al2O3 [5–8], show some potential in this field. Also studies done with diamond impregnated materials, in a cobalt matrix, were successful and the method is already applied in industry [9,10]. Research done with hard metals/steel joints [11,12] was encouraging and opened the doors to further development considering that much work is still needed for a clear understanding of the mechanisms involved. Hard metals have a mechanical behaviour similar to ceramic materials, although the presence of cobalt in their composition (even at low level) increases the welding capabilities. However it is known that hard metals are brittle, have low ductility, are thermal shock sensitive and have low heat expansion coefficient. Previous research work done with several hard metals compositions and a CO2 laser beam [13] shows that it is possible to melt the hard metal achieving full penetrations; however in most of the cases some porosity is formed which can lead to crack formation. Due to this fact, in a dissimilar weld, it will be fundamental to analyse the laser beam position (distance) relatively to the joint (tw). This study examines the weldability of different hard metals compositions to steel, by different laser sources.

164

A.P. Costa et al. / Journal of Materials Processing Technology 141 (2003) 163–173 Table 1 Specimens chemical composition and geometry Material

Chemical composition

Geometry (length  width  thickness)

Hard metal K40 K10

88% WC, 12% Co 94.4% WC, 5.6% Co

60 mm  4:2 mm  2:5 mm 60 mm  4:2 mm  2:5 mm

0.25% C, 0.32% Cr, 0.16% Si, 1.23% Mn

60 mm  25 mm  2:5 mm

Steel 1.7182

Fig. 1. Vertical f.p.p. and horizontal f.p.p. (tw).

The laser parameters power (P), speed (s), vertical focal point position (f.p.p.) and horizontal f.p.p. (tw) influence are investigated (Fig. 1). Weld bead size (penetration and width) and HAZ are measured. Bending and hardness tests are presented. Microstructure is evaluated. (pw) Nd:YAG laser, (cw) Nd:YAG laser and (cw) CO2 laser are compared.

2. Experimental conditions Dissimilar laser beam butt welds, were performed with different laser systems using hard metals qualities K10 and K40 (ISO 513) to steel holders (1.7182 according to DIN 17007), in specimens 2.5 mm thick. Experiments were done with three different laser systems: (cw) CO2 laser, (cw) Nd:YAG laser and (pw) Nd:YAG laser. 2.1. Specimens Table 1 presents the specimens chemical composition and geometry. 2.2. Procedures The (cw) Nd:YAG laser welds were done using a Haaslaser-model HL3006D, with 3000 W maximum power, transverse electromagnetic mode (TEM) multimode, 150 mm focal distance and 480 mm beam diameter at the f.p.p. This laser allowed a 3D spatial control movement due to its connection to an 8-axis weld robot. Power, speed, vertical and horizontal f.p.p. were varied. The (pw) Nd:YAG laser welds were performed with a Haas-laser equipment, with 300 W of average power, maximum pulse power of 6 kW, maximum pulse energy of 54 J, 150 mm focal distance, 150 Hz maximum frequency and 50 mm beam diameter at the f.p.p. (spot size). Pulse power (Pp), pulse time (Pt), pulse frequency (f), speed, horizontal and vertical f.p.p. were varied. The (cw) CO2 laser welds were done using a ROFINSINAR laser with maximum power of 6000 W, TEM multimode, focal distance of 150 mm and 300 mm beam diameter at the f.p.p. Power, speed, vertical and horizontal f.p.p. were varied.

The ‘‘f.p.p.’’ is considered zero when the laser spot is positioned at the specimen surface, it is positive out of the specimen and negative when it is focused inside the specimen. The f.p.p was varied from 1 mm to þ0.5 mm. tw is considered zero when the laser spot is positioned exactly into the joint steel/hard metal and is positive to the steel side (Fig. 1). Due to the higher ductility of the steel and the general hard metals behaviour, the laser was positioned more for the steel part. The tw varied from 0 to 0.4 mm. The steel specimens were polished for oxide removal, rectified and ultrasonically cleaned before welded. The hard metal/steel specimens were then mechanically clamped into position to weld, without gap, by a specially designed device. The welds started and finished outside the specimens. Some tests conducted with the laser weld starting and finishing inside the specimen conducted to the hard metal cracking. All the experiments were done with argon as protecting gas. Before starting the welding experiments, some preliminary welds were made to find a group of parameters leading to good results. Just after this optimisation, the experimentation started varying the weld parameters around those values, to observe their influence on weldability. After welding the specimens were observed to conclude about the fillet aspect and penetration (full or not) followed by bending tests. Also, for each group of parameters, a specimen was cut and metallographically prepared (Appendix A) for microscopy observation, weld bead measurements (penetration, width and HAZ) and hardness tests. For microstructure analysis, scanning electron microscopy (SEM) and Auger electron spectroscopy (AES) were conducted on several samples. The bending tests were conducted in a universal-testing machine from Zwick with a maximum 10 kN load cell and a test speed of 0.5 mm/min. Fig. 2 shows the test procedure. The steel part was clamped and the applied load was centred with the hard metal tip width and length. Vickers hardness tests were conducted according to ISO Standard 6507 and ISO Standard 3878 (hard metal hardness tests). A 98.07 N (10 HV) load was applied during 15 s. Each hardness value was determined from the average of three indentations.

A.P. Costa et al. / Journal of Materials Processing Technology 141 (2003) 163–173

165

Fig. 2. Bending test procedure.

3. Results and discussion 3.1. Macroscopic analysis 3.1.1. (cw) Nd:YAG laser welds The beads obtained with the (cw) Nd:YAG laser met the following aspects: narrow regular beads, full penetration and good surface appearance. The transverse cross-sections showed a conical shape close to the surface and a cylindrical shape in the interior (Fig. 3A). The bead width at half thickness varies between 0.5 and 0.7 mm. The HAZ are parallel to the cylindrical part of the melt zone and have a width varying from 0.3 to 0.6 mm. Beads microscopic evaluation usually show lines due to Marangoni heat convection flow. The joints welded with tw ¼ 0 mm present higher hard metal deformation than the ones welded with tw ¼ 0:2 mm. The K10 samples (5.6% Co) show some cracks, probably due to the lower ductility of this hard metal type. Despite the fact that the steel has high carbon content and can present cold cracking, this was not observed in any of the samples. The cracking, when it happens, always occurs in the hard metal. The fusion zone as well the steel HAZ presents a martensitic structure. However the aspect of the fusion zone is different from the HAZ structure, due to the different carbon content in both the zones. Carbon diffusion during cooling and multiple carbide formation leads to a martensite of the lath type in the fusion zone (martensite carbon depletion) while the HAZ presents a plate martensite type. 3.1.2. (pw) Nd:YAG laser welds These welds presented bad surface finishing, high width and large HAZ, due to the high heat input used, conical geometry, in some cases bead depression and incomplete penetration (Fig. 3B), probably due to the fact that the average power available was very low. The bead width at half thickness varies between 0.7 and 1 mm. HAZ presents an average width of 1 mm. The tips exhibit high deformation after welding and the K10 quality (5.6% Co) high cracking occurrence. As in the latest case no cracking was observed in the steel and both the fusion zone and HAZ present a martensitic structure. In most of the samples, particles

Fig. 3. (A) K10 sample welded with (cw) Nd:YAG laser, tw ¼ 0:15 mm (macro analysis); (B) K40 sample welded with (pw) Nd:YAG laser, tw ¼ 0:15 mm (macro analysis); (C) K40 sample welded with (cw) CO2 laser, tw ¼ 0:1 mm (macro analysis).

spread into the bead appear (these particles react in few seconds with Murakami’s solution, thus indicating being multiple tungsten carbides). The Marangony heat convection flow lines were not observed. 3.1.3. (cw) CO2 laser welds The welds obtained show irregular sections with a conic tendency, being in same cases larger at middle thickness (Fig. 3C). The bead width at half thickness varies between 0.5 and 0.8 mm and the HAZ as an average size between 0.4 and 0.6 mm. As in the latest welds, the smaller is tw, the higher is the hard metal deformed. K10 tips (5.6% Co) at times showed some cracks. Almost all the specimens presented full penetration. In general, the fusion zone and the steel HAZ present a martensitic structure. In some samples the Marangony heat convection flow lines were observed.

166

A.P. Costa et al. / Journal of Materials Processing Technology 141 (2003) 163–173

Fig. 4. K10 sample welded with (cw) Nd:YAG laser, tw ¼ 0:2 mm (SEM analysis).

Fig. 6. K10 sample welded with (pw) Nd:YAG laser, tw ¼ 0:1 mm (SEM analysis).

3.2. SEM and AES analysis Four specimens were selected from each kind of laser for the microscopic analysis: two K10 samples (5.6% Co) and two K40 samples (12% Co). Tw influence was observed (tw ¼ 0 and 0.2 mm) which seems to have a strong influence in the joint mechanical behaviour. 3.2.1. (cw) Nd:YAG laser welds SEM samples observed with 5000 amplification show dendritic structures at the interface hard metal/weld bead, which are surrounded by branches (inter-dendritic structures), which are scattered on the bead (long and thin filaments). At the interface near the hard metal boundary appears a ‘‘border’’, where these inter-dendritic structures have origin (Fig. 4). This border, with variable width, shows an eutectic structure. Samples welded with tw ¼ 0 mm show larger borders than the ones welded with tw ¼ 0:2 mm. This is probably due to the fact that for lower tw the higher is the interaction between the laser beam and the hard metal, conducing to higher tungsten diffusion to the weld bead with the consequent eutectic zone formation and multiple carbide rising in the bead. It seems that due to the hard metal heating, some WC particles at the boundary deform and loose W. This element diffuses to the bead, giving rise to the eutectic structures. The remaining particles present the usual angular aspect (Fig. 4). Fig. 5 shows an AES analysis; here four quadrants can be observed, representing carbon, cobalt, iron and tungsten content, starting by the top left and going on the clockwise direction. In each quadrant it can be observed two distinct

Fig. 5. K10 sample welded with (cw) Nd:YAG laser (AES analysis).

areas: on the left the weld bead (steel) and on the right side the hard metal. In the middle, the eutectic zone (interface) is shown. The grey colour gradient indicates the presence, or not, of the element being analysed. The lighter colours reveal the presence, in high level, of the element. The darker colour indicates that the element is present in low quantity. As darker is the grey colour, lower will be the element content. The AES analyses show, in general, some iron (Fe) diffusion to the hard metal (near the boundary). The inter-dendritic zones were identified and are constituted by Fe–W–C (multiple carbides) and sometimes with some cobalt also. These filaments appear in higher quantity near the hard metal/bead boundary, but also very thin ones appear spread over the bead. 3.2.2. (pw) Nd:YAG laser welds Using SEM, it was observed that the hard metal/bead boundary is not very clear. The interface is irregular and diffuse, between the two materials (Fig. 6). Dendritic structures were not detected neither eutectic zone at the boundary. In the middle of the bead it is frequent to observe multiple carbide particles with a half-angular aspect (Fig. 6). These particles have W (which diffused to the bead) in their composition. AES analyses show that K40 samples, welded with tw ¼ 0 and 0.2 mm have some iron content in the hard metal and some cobalt in the bead (in the matrix not in the particles). Samples welded with tw ¼ 0 mm show particles spread into the bead which are composed by Fe, W and C (Fig. 7). W appears in high quantity. K40 samples welded with tw ¼ 0:2 mm show low level content of W in the bead. The filaments observed near the interface, to the bead side, have W, Fe and C. Sample K10 welded with tw ¼ 0:3 mm shows large iron content in the hard metal boundary and some cobalt in the steel side (bead). The particles and filaments in the bead have W and Fe. 3.2.3. (cw) CO2 laser welds As in (cw) Nd:YAG laser welds, these welds present in the bead near the interface hard metal/bead, dendritic structures, which are surrounded by filaments (inter-dendritic structures) which are spread into the bead. Those filaments have

A.P. Costa et al. / Journal of Materials Processing Technology 141 (2003) 163–173

167

Fig. 7. K40 sample welded with (pw) Nd:YAG laser (AES analysis). Fig. 9. K10 sample welded with (cw) CO2 laser (AES analysis).

higher lengths for smaller tw, probably due to the higher tungsten diffusion that happens in these cases. A border in the bead boundary (near the hard metal) is also observed where the inter-dendritic structures have origin (Fig. 8). Also, this border with variable width has an eutectic structure. Samples welded with tw ¼ 0 mm show larger borders than the ones existing in the samples welded with tw ¼ 0:2 mm. Not all the specimens present filaments spread all over the bead. This fact could be due to the beam spot size. The (cw) CO2 laser beam has a spot size smaller than the (cw) Nd:YAG laser, so when the CO2 laser is positioned at a certain distance from the hard metal, the material is less affected than when the (cw) Nd:YAG laser is positioned in the same place (with the same heat input). This fact can lead to lower W diffusions to the bead with the CO2 laser. AES analyses show that K40 samples welded with tw ¼ 0 and 0.2 mm have some iron in the hard metal boundary and some cobalt in the bead. W, Fe and C compose the interdendritic structures. Sample K10 welded with tw ¼ 0:2 mm did not show cobalt in the bead. The inter-dendritic structures are also composed by W, Fe and C (Fig. 9). 3.3. Bending tests Figs. 10–12 present the bending tests results achieved with the specimens welded by (cw) Nd:YAG laser, (pw) Nd:YAG laser and (cw) CO2 laser, respectively. Each laser system has its own characteristics, e.g. wave length, TEM,

Fig. 8. K10 sample welded with (cw) CO2 laser, tw ¼ 0 mm (SEM analysis).

spot size, focal distance and maximum power available. Due to this fact, for each laser used, the parameters were optimised and the work conducted afterwards was based on modification of the parameters around the optimised values. Therefore, for each laser, the bending tests results show the joints behaviour in the range of parameters defined as the best for each laser. 3.3.1. (cw) Nd:YAG laser welds The first obvious conclusion is that there is a true difference between the K40 (12% Co) and the K10 (5.6% Co) quality specimens. As already expected, the hard metals with higher cobalt content present better weldability to the steel, since the cobalt is a metal and its affinity with steel is higher. The (cw) Nd:YAG laser does not seem to show a strong dependence of the f.p.p. Regarding the horizontal f.p.p. (tw), we can state the notorious influence of this parameter. The results show an optimum point for tw ¼ 0:15 mm and a decrease in quality for the other cases. This kind of result agrees with previous research in this field, since if the laser beam is too far from the hard metal, no joining is achieved, and if the laser is too close to the hard metal it leads to crack formation in this material. The curves of power and speed agree with the general welding curves where for higher power deeper penetrations are achieved and better joint resistance. For higher speeds, less energy is delivered, the penetration has a tendency to decrease leading to lower joint resistance. 3.3.2. (pw) Nd:YAG laser welds As in the latest case, it was observed that K10 specimens present, in general, lower joint strength when compared with K40 samples. For K40 samples f.p.p. presented an optimum point for f:p:p: ¼ 0:8 mm. K10 samples presented better results with f:p:p: ¼ 0:4 mm, showing full penetration. Examining the horizontal laser distance to the welding joint, it was observed that for lower tw the higher is the hard metal deformation. As tw increases the joint strength increases. This is probably due to the fact that for higher distances from the laser to the hard metal the residual stresses developed in this material are not so strong. It is important to have in mind

168

A.P. Costa et al. / Journal of Materials Processing Technology 141 (2003) 163–173

Fig. 10. Bending tests results for specimens welded by (cw) Nd:YAG laser: (a) vertical f.p.p. influence on maximum bending load; (b) horizontal f.p.p. influence on maximum bending load; (c) power influence on maximum bending load; (d) speed influence on maximum bending load.

that the weld beads and HAZ obtained with this laser have a considerable width due to the high heat input used. The weld depth increased with pulse time increasing for K40 samples, but in terms of strength the better point was obtained with Pt ¼ 2 ms. In K10 samples the influence of the pulse time (in the range tested) on the joint strength was not observed. The speed influence for K40 samples agrees with general behaviour. For lower speeds (higher heat input) the weld depth increased and consequently the joint strength. For K10 samples it was obtained an optimum point to s ¼ 47 mm/ min, with a full penetration weld bead. 3.4. (cw) CO2 laser welds Relatively to K10 and K40 f.p.p. influence, the samples are similar (bead appearance) and the evolution of the mechanical behaviour is identical. However, like in the other cases, the K40 samples exhibit a mechanical strength much higher than the K10 samples. The mechanical strength increases a little bit when f.p.p. is placed slightly below the surface. As in the latest cases, the hard metal deformation increases with smaller tw. K10 hard metal did not deform as much as K40 hard metal. For tw ¼ 0:2 and 0.3 mm, hard metal deformation was not observed and the joint strength increased. For tw ¼ 0:4 mm, the ‘‘bond’’ between steel and

hard metal seems to decrease, but the joint strength increases, probably due to residual stresses reduction in the hard metal. In K40 samples, the weld depth increased with power. However the strength optimum point was obtained to a middle power of 2200 W and a speed of 1.75 m/min. In K10 samples, higher power (higher heat input) leads to higher joint strength, although all beads present full penetration. Regarding the speed influence in K40 samples it can be observed that a higher strength value occurs for a speed of 1.75 m/min (which corresponds to the same optimum point, in terms of heat input, observed in power influence). After this point the resistance decreases (as expected for higher speeds). For K10 samples the mechanical behaviour is better for higher heat inputs (lower speeds). This could be due to the fact that K10 tips have higher thermal conductivity, requiring higher heat input. Fig. 13 shows the tw influence on the behaviour of K40 samples when welded by the three different laser systems (Nd:YAG (cw), Nd:YAG (pw) and CO2 (cw)). It is clear that Nd:YAG (cw) laser present better results for smaller tw (0.1– 0.2 mm). By contrary the (pw) Nd:YAG and (cw) CO2 lasers exhibit better results for higher tw, due to the high heat input used and higher spot size, respectively. During the bending tests it was possible to observe the cracking mode. Figs. 14 and 15 show the two kinds of

A.P. Costa et al. / Journal of Materials Processing Technology 141 (2003) 163–173

169

Fig. 11. Bending tests results for specimens welded by (pw) Nd:YAG laser: (a) vertical f.p.p. influence on maximum bending load; (b) horizontal f.p.p. influence on maximum bending load; (c) pulse time influence on maximum bending load; (d) speed influence on maximum bending load.

fracture that takes place in these joints. Fig. 14 shows the cracking for high resistant joints. Fig. 15 shows the cracking for low resistant joints. The cracking always occurs in the hard metal. 3.5. Hardness tests Hardness tests were carried out in samples welded with tw ¼ 0 and 0.2 mm, due to the fact that their bending strengths were considerably different. Also tw ¼ 0:4 mm was observed for samples welded with CO2 laser. Figs. 16–19 show the Vickers hardness test results. In general the hardness profiles show an expected high hardness in the hard metal (base material and HAZ). K10 samples have considerably higher hardness than the K40 samples (due to different Cobalt content) and did not show any differences between the base material and the HAZ (despite the presence of some iron in this zone this did not influence the hard metal hardness). Steel presents hardness around 400 HV, and the steel HAZ shows a little hardness increase.

3.5.1. (cw) Nd:YAG laser welds Significant hardness differences (bead and steel HAZ) between specimens welded with tw ¼ 0 and 0.2 mm were not detected. K10 samples welded with tw ¼ 0:2 mm exhibit some cracks in the hard metal near the bead boundary, inducing high indentations and as a result hardness decreased in this area. The weld bead and steel HAZ hardness were similar for K10 and K40 samples. The weld bead hardnesses are slightly higher than the ones of the steel HAZ, probably due to the presence of multiple tungsten carbides in the weld bead, leading to hardness increase. 3.5.2. (pw) Nd:YAG laser welds In spite of the different appearance in between the samples welded with tw ¼ 0 and 0.2 mm, K40 samples did not show any hardness differences. K10 samples welded with tw ¼ 0:3 mm, show a lower weld bead hardness. Globally the weld bead hardness and the steel HAZ hardness, for K10 and K40 samples, are similar. However the weld bead hardness is higher than the steel HAZ hardness, although

170

A.P. Costa et al. / Journal of Materials Processing Technology 141 (2003) 163–173

Fig. 12. Bending tests results for specimens welded by (cw) CO2 laser: (a) vertical f.p.p. influence on maximum bending load; (b) horizontal f.p.p. influence on maximum bending load; (c) power influence on maximum bending load; (d) speed influence on maximum bending load.

both the zones are constituted by martensite. This increase in hardness is probably due to the presence of multiple tungsten carbide particles (very hard) in the weld bead. The sample welded with tw ¼ 0:3 mm show a weld bead hardness, lower than the remaining samples, probably due to the fact that it

Fig. 13. The three lasers comparison: tw influence on weldability of K40 hard metals to steel.

has a particles amount in the bead lower than the other samples analysed. 3.5.3. (cw) CO2 laser welds As shown in Fig. 18, there are no hardness differences between the samples welded with tw ¼ 0, 0.2 and 0.4 mm. Weld bead hardness and the steel HAZ hardness are similar for both compositions (K40 and K10). The weld bead hardness is a little bit higher than the steel HAZ hardness,

Fig. 14. Cracking of high resistant joints submitted to bending.

A.P. Costa et al. / Journal of Materials Processing Technology 141 (2003) 163–173

171

Fig. 15. Cracking of low resistant joints submitted to bending. Fig. 16. Hardness tests done in samples welded with (cw) Nd:YAG laser.

Fig. 17. Hardness tests done in samples welded with (pw) Nd:YAG laser.

probably also due to some amount of multiple tungsten carbides present in the weld bead. Fig. 19 shows the hardness comparison for the three lasers systems used, for samples with hard metals (HM) tips K40, welded with tw ¼ 0:2 mm. It can be observed that there are no hardness differences between the three lasers, in the base materials (hard metal and steel) neither in the HAZ. Just in

the weld bead some differences can be noted, due to the multiple carbides content presented in the weld bead. The (cw) CO2 and the (cw) Nd:YAG lasers present very similar hardness values; however (pw) Nd:YAG laser present significant higher hardness. This fact can be due to the large amount of multiple carbide particles spread over the weld bead (Figs. 6 and 7).

Fig. 18. Hardness tests done in samples welded with (cw) CO2 laser.

172

A.P. Costa et al. / Journal of Materials Processing Technology 141 (2003) 163–173

Fig. 19. Hardness tests: comparison of samples (K40) welded by three different laser systems, with tw ¼ 0:2 mm.

4. Conclusions Hard metals are brittle, have low ductility and are thermal shock sensitive. Due to its great hardness and good wear properties they have been used mostly as cutting tools, where high strength at high temperatures is required. Brazing is the most common joining technique used but it presents some limitations, namely at high temperatures, leading to tip detachment as a result of degradation of the brazing alloy. Mechanical clamping (another commonly used technique) also presents limitations, namely regarding geometric aspects. In this paper the weldability of hard metals (K10 and K40) to steel was examined with high power (cw) CO2 laser, (cw) Nd:YAG laser and (pw) Nd:YAG laser, in specimens 2.5 mm thick. The welding parameters influence was studied in order to obtain full penetration and high resistant joints. The experiments undertaken with the different laser sources, allow to conclude the following. Continuous lasers leads to full penetration, high resistant and low cracking weld joints. K40 (88% WC, 12% Co) quality tips have higher resistance after welding, comparing with K10 samples. K10 (94.4% WC, 5.6% Co) quality tips commonly present cracks after welding. The horizontal f.p.p. (tw) is an important parameter to prevent overheating of the hard metal and the risk of crack formation. The vertical f.p.p. is also an important parameter to achieve full penetration and high resistant joints. There is tungsten diffusion to the bead, inducing multiple W carbide formation in the martensite structure and contributing for higher bead hardness. The amount of carbide formed in the bead depends on tw (horizontal f.p.p.), laser spot size and heat input. Multiple carbide appear in the beads, as scattered particles or like thin filaments (pulsed laser or continuos mode laser). In spite of the eutectic structure and multiple carbide formation in the weld bead (brittle structures) the cracking never occurred in

this area, but always in the hard metal, probably due to the high thermal stresses developed in this material. These results show that laser welding is an alternative joining technique for hard metals. It proved its effectiveness for the production of cutting tools, joining with success hard metals and steel. In fact this process has the overall advantage of producing small beads and HAZ and minimising residual stresses. Continuous Nd:YAG laser was found to present the best results. This technique has a good potential for application in the welding of cutting tool tips, used in many different industrial sectors, with the advantage of higher duration than brazing and less shape problems than mechanical clamping.

Acknowledgements This work was done at Instituto Superior Te´ cnico, Technical University of Lisbon and at the State Material Testing Institute (MPA), Stuttgart University, with the support of PRAXIS XXI. The authors would like to acknowledge to CERAMETAL S.A.R.L. for the hard metal tips supply, to Schweisstechnische Lehr und Versuchsanstalt (SLV) Fellbach, Instituto de Desenvolvimento e Inovac¸a˜ o Te´ cnolo´ gica (IDIT) and to Instituto de Soldadura e Qualidade (ISQ) for the use of laser equipment.

Appendix A. Experimental details After welding, the samples were metallographic prepared according to ISO Standard 4499, after the specimens were cut by electro-erosion and resin mounted. This procedure allows the weld beads measurements, hardness tests, the optical and

A.P. Costa et al. / Journal of Materials Processing Technology 141 (2003) 163–173

electronic microscope (AES and SEM) evaluation. The chemical etching was made with Murakami’s solution: 10 g K3 FeðCNÞ6 þ 10 g NaOH þ 100 ml H2 O This etching was done in two phases, the first, of only a few seconds, for identification of Z phase (multiple W carbides) and the second, with a few minutes (ffi5 min) for identification of a phase (WC). SEM was performed with a Hitachi equipment, model S2400, with a solid state back scattered electron detector (BSE) from KE Developments. A 25 kVacceleration voltage was used. For AES analysis a VG Scientific equipment was used, model Microlab 310 F. The samples, after cut with a proper dimension for the vacuum camera, were only polished in the area to analyse.

References [1] K. Brookes, World Directory and Handbook of Hard Metals and Hard Materials, International Carbide Data, 5th ed., 1992.

173

[2] H. Kolaska, Pulvermetallurgie der Hartmetalle, Germany, 1992. [3] P. Schwarzkopf, R. Kieffer, Cemented Carbide, Macmillan, New York, 1960. [4] ASM Handbook Committee, Powder Metallurgy, Metals Handbook, vol. 7, 9th ed. [5] H. Maruo, I. Miyamoto, Y. Arata, CO2 laser welding of ceramics, in: Proceedings of the International Laser Processing Conference, Anaheim, November 1981. [6] M. Tomie, N. Abe, S. Naguchi, T. Oda, Y. Arata, High power CO2 laser cutting and welding of ceramics, Trans. JWRI (1989). [7] M. Tomie, N. Abe, S. Noguchi, Y. Arata, Bending strength of CO2 laser welded joints of 87% Al2O3 ceramics, Trans. JWRI 22 (2) (1993). [8] A. Paris, M. Robin, G. Fantozzi, Soldagem de Ceraˆ micas por laser de CO2, Soldagem e Materiais, Brasil, January/March 1992. [9] G. Weber, Laser welding of diamond tools, Ind. Diam. Rev. (1991). [10] G. Clauser, A. Valle, Laser welding explained, Ind. Diam. Rev. (1987). [11] S. Sandig, P. Wiesner, M. Greitmann, G. Deutschmann, Laser Welding of Hard Metal Components onto Steel, DVS Berichte 163, Germany, 1994. [12] S. Sandig, P. Wiesner, M. Greitmann, G. Deutschmann, Technological Aspects of Laser Joining of Steel and Hard Metal Components, ICALEO 1995. [13] A. Costa, L. Quintino, Hard Metals Irradiation by a (cw) CO2 Laser Beam, Joining of Materials, 2001.