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Ti6Al4V diffusion bonding joints using Ag as interlayer
M A TE RI A L S C H A RAC TE RI ZA T ION 5 9 ( 2 00 8 ) 1 4 0 7–1 4 1 1
Characterization of WC–Co/Ti6Al4V diffusion bonding joints using Ag as interlayer M.I. Barrena⁎, J.M. Gómez de Salazar, N. Merino, L. Matesanz Department of Material Science, Faculty of Chemistry, Complutense University of Madrid 28040 Madrid, Spain
AR TIC LE D ATA
ABSTR ACT
Article history:
The present work defines optimal parameters used in a diffusion bonding process. A hard
Received 23 October 2007
metal (WC–15% Co) and a titanium alloy (Ti6Al4V) were joined using a Ticusil (Ag–Cu–Ti)
Accepted 31 December 2007
filler. The identification of the intermetallic phases formed in the joint interfaces provides an explanation of the shear strengths obtained. The temperature and load conditions that
Keywords:
guarantee strong joints are associated with the intermetallic phases formed, ζ(CuTi) and
Diffusion bonding
TiC. These temperature–time conditions are 850 °C, 30–60 min, with loads of 2 MPa and a
Intermetallic alloys and compounds
vacuum between 10− 2–10− 3 Pa.
Electron microscopy
© 2008 Elsevier Inc. All rights reserved.
Mechanical properties
1.
Introduction
Ceramic–metal composites (hard metal) are one of the most important materials for cutting tools and wear resistant materials. At least 70% of the cutting tools and coatings with wear resistance employed at present are cemented carbides [1]. These coatings consist of a metal matrix and reinforcements such as non-metallic solid particles [2,3]. By contrast, Ti6Al4V is an alloy widely used for aeronautical applications, due to its superplasticity, low weight and high mechanical strength [4]. However, Ti6Al4V alloy has poor wear resistance because of its low resistance to plastic shear [5,6]. For these reasons numerous techniques have been used to improve the wear resistance of Ti6Al4V alloy [7]. The joining of ceramic or hard metal to structural alloys has received much attention in recent years because of the potentially attractive combined properties [8]. These materials can be co-joined by a of number different processes [9,10]. The use of an interlayer can further facilitate these welding processes. The activity of Ti in Ag–Cu brazing alloys is increased with a high Ag/Cu ratio [11]. Generally, intermetallic compounds are regarded as brittle phase due to low toughness and poor ductility, so that the formation of the same has been avoided during
processing. However, the possible formation of intermetallic compounds of the Ti–Cu system at room temperature [12–14], is a good option for forming stable interfaces in the joint between the ceramic and the metal. Care, however, must be exercised to minimize the residual stresses originating during the formation of the diffusion joint; these stresses arise due to the different thermal expansion coefficients of cemented carbide (5.5 ° 10− 6 K) and the Ti6Al4V alloy (9° 10− 6 K). This effect can be minimized using a soft interlayer similar to that of a thick film of AgCuTi. However, the mechanical properties of the joint will be influenced by both the interfacial microstructure and the nature of the intermetallic compounds formed during the welding process [15–17]. Also, the lack of continuity in the joint interface will mitigate the formation of voids, which might have a detrimental effect on mechanical properties.
2.
Experimental Procedure
2.1.
Base Materials: WC–Co (Hard metal) and Ti6Al4V
Cemented tungsten carbide (WC–15 wt.% Co) was used in 25mm long, 10-mm diameter bars. The WC in this material is
⁎ Corresponding author. Tel.: +349 13944199; fax: +349 13944357. E-mail address: [email protected] (M.I. Barrena). 1044-5803/$ – see front matter © 2008 Elsevier Inc. All rights reserved. doi:10.1016/j.matchar.2007.12.008
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M A TE RI A L S C H A RAC TE RI ZA T ION 5 9 ( 2 00 8 ) 1 4 0 7–1 4 1 1
with Keller's reagent. The microstructural evaluation was realized (by means) of scanning electron microscopy (JEOL JSM 6400) equipped with an energy dispersive spectroscopy (EDX) microanalyzer (Link Oxford Pentafet). Mechanical properties were evaluated by shear and Vickers microhardness tests. A universal testing machine Servosis MEM-10 was used to perform the shear tests, using the arrangement shown in Fig. 2. Hardness testing was performed on an Akashi MVK-A3 microhardness tester using a 300 g indentation load. All tests were carried out on the polished surfaces, without etching. The microhardness profiles were conducted on a cross-section of transverse specimens. The measurements were made with increments of 3 μm in the area away from the weld interface. Fig. 1 – Thermal diffusion welding cycle used in the joint WC–Co/Ti6Al4V. present with a grain size between 1 μm and 3 μm. The titanium alloy Ti6Al4V (AMS 4911) consists of a two-phase α + β microstructure. The elemental composition of the Ti alloy is 5.5 wt.% Al, 3.5 wt.% V and balance Ti. For use as a weld interlayer, the alloy Ticusil alloy has been used in the form of 50-mm thick layers. This interlayer alloy (68.29 wt.% Ag, 27.19 wt.% Cu and 4.52 wt.% Ti) was used between the base materials, Ti6Al4V and WC–Co. The Ticusil is characterized by a liquidus temperature of 900 °C and a solidus temperature of 780 °C.
2.2.
Surface Preparation
In all cases, the WC–Co and Ti6Al4V were grounded with emery paper to obtain a surface roughness (Ra) between 0.18 and 0.24 μm. These samples were then separated by the interlayer alloy, and introduced in a vacuum furnace (10− 3– 10− 2 Pa). The load used in this investigation was 2 MPa for all joints. The isothermal welding temperatures were: 825 °C at 30 and 60 min, and 850 °C at 15, 30 and 60 min (Fig. 1). These temperatures were chosen to accommodate the solidus and liquidus temperatures of the welding interlayer.
2.3.
3.
Results and Discussion
3.1.
Microstructural Characterization
The bonding temperatures were selected to be between the solidus and liquidus temperatures of the welding interlayer. The bonding temperature is important in understanding the formation of reaction phases associated with elemental diffusion, according to the Ag–Ti–Cu diagram (Fig. 3). As a consequence of the diffusion processes and metallurgical transformations during the welding, the joints in the materials studied occur in either the solid state (low temperature) or a partially transient liquid phase for the higher welding temperature (850 °C). The formation of diffusion layers between WC–Co/Ticusil/Ti6Al4V is evident in the joint interface. The number of these layers, their extent and elementary composition will change as a function of the experimental welding parameters such as time, temperature and the pressure applied during the bonding processes. Four reaction zones can be observed in the joints when the lower temperature and shorter times are used (825 °C, 30 min). The reaction in the surface of the materials begins with the diffusion of material from the Ticusil interlayer to the surface
Joint Characterizations
For metallographic examination, the joints were sectioned transversely to the bondline, grounded, polished and etched
Fig. 2 – Mechanical arrangement for shear tests of the joint WC–Co/Ti6Al4V.
Fig. 3 – Ag–Cu–Ti ternary diagram isotherm (700 °C).
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M A TE RI A L S C H A RAC TE RI ZA T ION 5 9 ( 2 00 8 ) 1 4 0 7–1 4 1 1
of the Ti6Al4V alloy (Fig. 4a). Elemental titanium and copper diffused in a higher quantity in the interface. The phase that appears nearest the Ti6Al4V alloy is a narrow zone which consists of the eutectoid Ti–Cu system, which is formed by the η-Ti2Cu phase and α + β Ti phases. The η phase grows between the eutectoid microstructure in the titanium alloy (α + β Ti) and the intermetallic TiCu (ζ), which grows at both sides of the Ag– Cu–Ti interlayer. The last reaction zone is formed at the WC– Co/TiCu2 (ζ)) interface. The EDX analysis in this reaction zone shows the existence of Co, Cu and W. The diffusion of Co and W from the hard metal to the interlayer is apparently due to the reaction between Ti and WC, which forms the TiC phase, according to the following thermodynamic data: Ti þ C TiC
DH0F ¼ 183:084 kJ=mol
W þ C WC
DH0F ¼ 40:504 kJ=mol
Ti þ WC TiC þ W
DH0F ¼ 142:580 kJ=mol :
When the thermal cycle is applied at 850 °C and the weld time is short (15 min), the welding interface microstructure is very similar to that observed at 825 °C–30 min. However, the diffusion of Cu, Ti, and Ag are higher, and for this reason, the Ag interface tends to disappear. On the other hand, when the
Fig. 5 – Vickers microhardness values of joints WC–Co/ Ti6Al4V, associated with each of the layers. diffusion welding conditions are more energetic (850 °C, 60 min), a more complete reaction process takes place (Fig. 4b). In the WC-interface, the high degradation of the WC takes place due to the reaction with the diffused Ti, which promotes the appearance of TiC in the solid state. The joining processes have allowed the diffusion of all elements. This fact promotes the WC reaction with Ti, forming titanium carbides. The W diffuses to the titanium alloy, where it forms two solid solutions, α-Ti and β-(Ti,W). The chemical joint (in all welding conditions) between WC–Co and Tia6Al4V is achieved when inter-diffusion between the materials is provided without the formation of voids and brittle phases such as intermetallic compounds. The composition, extent, nature and properties of the phases originated during the welds, will control the resulting mechanical properties.
3.2.
Mechanical Characterization
The results obtained from Vickers microhardness carried out on the base materials were 1805 HV and 389 HV for the WC–Co and Ti6Al4V, respectively. The microhardness values obtained in the welding interfaces for the different joints are directly related with the composition, extent and character of the diffusiongenerated layers. Microhardness values associated with each of the layers identified are shown in Fig. 5. From this figure we can observe that the softest areas appear where the silver concentration is the highest. Therefore, this area (D zone) is subject to the largest plastic behaviour and ductility. The A zone (eutectoid) has an intermediate hardness value among those obtained for the α + β Ti and the η phases. The C zone (ζ + Ag) and E zone (ζ + TiC) present higher hardness values than the B zone (η). This implies that the
Fig. 4 – Elemental distributions in the bond line and phases present in each layer of a WC–Co/Ti6Al4V diffusion joint, produced at (a) 825 °C and 30 min. [A: Eutectoid (α + β Ti) + η (Ti2Cu) phases; B: η (Ti2Cu); C: ζ (TiCu) + Ag; D: Ag; E: TiC + ζ (TiCu); F: TiC + WC; G: WC–Co]; and (b) 850 °C and 60 min. [A: Eutectoid (α + β Ti) + η (Ti2Cu) phases; E: TiC + ζ (TiCu); F: TiC + WC; G: WC–Co.]
Table 1 – Values of shear tests obtained for all isothermal conditions in the WC–Co/Ti6Al4V joint Joint isothermal conditions
σ(MPa)
825 850 850 850
314 330 955 879
°C–30 °C–15 °C–30 °C–60
min min min min
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M A TE RI A L S C H A RAC TE RI ZA T ION 5 9 ( 2 00 8 ) 1 4 0 7–1 4 1 1
Fig. 6 – Brittle fracture surface of WC–Co/ζ interface of a joint WC–Co/Ti6Al4V, obtained at 825 °C and 15 min.
phase η will be softer and more ductile than the ζ phase. The E zone has higher hardness values than those obtained for the C area, because the latter zone presents carbides in its composition. The highest hardness is observed in zone F, which consists of a mixture of carbides (TiC and WC). From the shear strength tests the joint bonded at the high temperature exhibited the higher bond strength, as shown in Table 1. The diffusion joints obtained at 850 °C for joint preparation times of 30 and 60 min failed at higher stresses than joints obtained at 825 °C; this generally reflects the higher number of phases formed in the interface at 850 °C. The presence of TiC nanoparticles in the reaction zone formed at 850 °C– 30 and –60 min appears to result in a substantial increase in the resistance of this joint; this may be because it is essentially a composite consisting of a ductile intermetallic compound with TiC particle reinforcement. From the fractographic analysis of surfaces obtained after mechanical testing, a range of fracture mechanisms can be observed, which depend on the microstructural characteristics of each interface formed by diffusion. It can also be observed that crack propagation can cross different areas of the diffusion welding interface, when a high void content exists. The fracture surfaces observed show the various mechanisms as follows: • At low welding energies (825 °C–30 min and 850 °C–15 min), the crack takes places by two different mechanisms: brittle
Fig. 7 – Ductile fracture surface of silver-rich interlayer of a joint WC–Co/Ti6Al4V, obtained at 825 °C and 15 min.
Fig. 8 – Fracture surface of a joint WC–Co/Ti6Al4V at 850 °C and 30 min (A) brittle fracture of the η phase and (B) fibrous brittle fracture of the ζ phase.
fracture of the hard metal (WC–Co/ζ (incipient)) interface (Fig. 6), and ductile fracture through the silver interlayer (Fig. 7). • At higher welding energies (850 °C–30 min and 60 min), the fracture occurs through the η phase by brittle fracture (Fig. 8), and in the ζ phase by fibrous brittle fracture (Fig. 9).
4.
Conclusions
1. Joints were prepared between (WC–15% Co) and a titanium alloy (Ti6Al4V), using a Ticusil (Ag–Cu–Ti) filler as an interlayer. The joints which exhibited the best overall quality and the maximum shear resistance were obtained when the welding temperature was 850 °C for a time of 30 min with a load of 2 MPa. Lower temperatures (825 °C) and lower times do not guaranty strong joints. 2. Both the hardness values and the shear resistance of the joint interfaces depend on the intermetallic compounds formed during the diffusion bonding. The presence of TiC appears to increase the mechanical resistance in the WC–ζ interface.
Fig. 9 – Fibrous brittle fracture of the ζ phase in a joint WC–Co/ Ti6Al4V obtained at 850 °C and 60 min joint condition.
M A TE RI A L S C H A RAC TE RI ZA T ION 5 9 ( 2 00 8 ) 1 4 0 7–1 4 1 1
3. From the fractographic analysis of surfaces obtained after mechanical testing, a range of fracture mechanisms can be observed, which depend on the microstructural characteristics of the various interfaces formed by the diffusion bonding.
Acknowledgement The authors wish to express thanks to the financial support of Project MAT2003-05004-C02-01.
REFERENCES [1] Lu SP, Kwon OY, Guo Y. Wear behaviour of brazed WC/ NiCrBSi(Co) composite coatings. Wear 2003;254:421–8. [2] Cerri, Martinella R, Mor GP, Bianchi P, Angelo D. Laser deposition of carbide-reinforced coatings. Surf Coat Tech 1991;49:40–5. [3] Bhaumik, Upadhayaya GS, Vaidya ML. Full density processing of complex WC-based cemented carbides. J Mater Process Technol 1996;58:45–52. [4] Gil, Ginebra MP, Manero JM, Planell JA. Formation of α-Widmanstätten structure: effects of grain size and cooling rate on the Widmanstätten morphologies and on the mechanical properties in Ti6Al4V alloy. J Alloys Compd 2001;329:142–52. [5] Molinari, Straffelini G, Tesi B, Bacci T. Dry sliding wear mechanisms of the Ti6Al4V alloy. Wear 1997;208:105–12.
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[6] Ye, Liu XY. Microstructure and tensile properties of Ti6Al4V/AM60B magnesium matrix composite. J Alloys Compd 2005;402:162–9. [7] Zhang, Dong H, Bell T, Xu B. The effect of treatment condition on boost diffusion of thermally oxidised titanium alloy. J Alloys Compd 2007;431:93–9. [8] Zhang, Chandel RS, Seow HP. Effects of chromium on the interface and bond strength of metal–ceramic joints. Mater Chem Phys 2002;75:256–9. [9] Pang, Man HC, Yue TM. Laser surface coating of Mo–WC metal matrix composite on Ti6Al4V alloy. Mat Sci Eng, A 2005;390:144–53. [10] Man, Yang YQ, Lee WB. Laser induced reaction synthesis of TiC+WC reinforced metal matrix composites coatings on Al 6061. Surf Coat Technol 2004;185:74–80. [11] Rongti, Wei P, Jian C, Jie L. Thermodynamic properties of Ti in Ag–Cu–Ti alloys. Mat Sci Eng, A 2002;335:21–5. [12] Uzunov, Stojanov SP, Lambov SI. Thin films of intermetallic Cu/Ti compounds and their possible uses. Vacuum 1999;52:321–5. [13] Uzunov, Stojanov SP, Lambov SI. Contact-reactive welding of titanium via a copper layer. Vacuum 1999;52:365–8. [14] Carim, Mohr CH. Brazing of alumina with Ti4Cu2O and Ti3Cu3O interlayers. Mater Lett 1997;33:195–9. [15] Voytovych, Ljungberg LY, Eustathopoulos N. The role of adsorption and reaction in wetting in the CuAg–Ti/alumina system. Scripta Mater 2004;51:431–5. [16] Kliauga, Travessa D, Ferrante M. Al2O3/Ti interlayer/AISI 304 diffusion bonded joint: microstructural characterization of the two interfaces. Mater Charact 2001;46:65–74. [17] Ghosh, Chatterjee S. Effect of interface microstructure on the bond strength of the diffusion welded joints between titanium and stainless steel. Mater Charact 2005;54:27–33.
Characterization of WC–Co/Ti6Al4V diffusion bonding joints using Ag as interlayer M.I. Barrena⁎, J.M. Gómez de Salazar, N. Merino, L. Matesanz Department of Material Science, Faculty of Chemistry, Complutense University of Madrid 28040 Madrid, Spain
AR TIC LE D ATA
ABSTR ACT
Article history:
The present work defines optimal parameters used in a diffusion bonding process. A hard
Received 23 October 2007
metal (WC–15% Co) and a titanium alloy (Ti6Al4V) were joined using a Ticusil (Ag–Cu–Ti)
Accepted 31 December 2007
filler. The identification of the intermetallic phases formed in the joint interfaces provides an explanation of the shear strengths obtained. The temperature and load conditions that
Keywords:
guarantee strong joints are associated with the intermetallic phases formed, ζ(CuTi) and
Diffusion bonding
TiC. These temperature–time conditions are 850 °C, 30–60 min, with loads of 2 MPa and a
Intermetallic alloys and compounds
vacuum between 10− 2–10− 3 Pa.
Electron microscopy
© 2008 Elsevier Inc. All rights reserved.
Mechanical properties
1.
Introduction
Ceramic–metal composites (hard metal) are one of the most important materials for cutting tools and wear resistant materials. At least 70% of the cutting tools and coatings with wear resistance employed at present are cemented carbides [1]. These coatings consist of a metal matrix and reinforcements such as non-metallic solid particles [2,3]. By contrast, Ti6Al4V is an alloy widely used for aeronautical applications, due to its superplasticity, low weight and high mechanical strength [4]. However, Ti6Al4V alloy has poor wear resistance because of its low resistance to plastic shear [5,6]. For these reasons numerous techniques have been used to improve the wear resistance of Ti6Al4V alloy [7]. The joining of ceramic or hard metal to structural alloys has received much attention in recent years because of the potentially attractive combined properties [8]. These materials can be co-joined by a of number different processes [9,10]. The use of an interlayer can further facilitate these welding processes. The activity of Ti in Ag–Cu brazing alloys is increased with a high Ag/Cu ratio [11]. Generally, intermetallic compounds are regarded as brittle phase due to low toughness and poor ductility, so that the formation of the same has been avoided during
processing. However, the possible formation of intermetallic compounds of the Ti–Cu system at room temperature [12–14], is a good option for forming stable interfaces in the joint between the ceramic and the metal. Care, however, must be exercised to minimize the residual stresses originating during the formation of the diffusion joint; these stresses arise due to the different thermal expansion coefficients of cemented carbide (5.5 ° 10− 6 K) and the Ti6Al4V alloy (9° 10− 6 K). This effect can be minimized using a soft interlayer similar to that of a thick film of AgCuTi. However, the mechanical properties of the joint will be influenced by both the interfacial microstructure and the nature of the intermetallic compounds formed during the welding process [15–17]. Also, the lack of continuity in the joint interface will mitigate the formation of voids, which might have a detrimental effect on mechanical properties.
2.
Experimental Procedure
2.1.
Base Materials: WC–Co (Hard metal) and Ti6Al4V
Cemented tungsten carbide (WC–15 wt.% Co) was used in 25mm long, 10-mm diameter bars. The WC in this material is
⁎ Corresponding author. Tel.: +349 13944199; fax: +349 13944357. E-mail address: [email protected] (M.I. Barrena). 1044-5803/$ – see front matter © 2008 Elsevier Inc. All rights reserved. doi:10.1016/j.matchar.2007.12.008
1408
M A TE RI A L S C H A RAC TE RI ZA T ION 5 9 ( 2 00 8 ) 1 4 0 7–1 4 1 1
with Keller's reagent. The microstructural evaluation was realized (by means) of scanning electron microscopy (JEOL JSM 6400) equipped with an energy dispersive spectroscopy (EDX) microanalyzer (Link Oxford Pentafet). Mechanical properties were evaluated by shear and Vickers microhardness tests. A universal testing machine Servosis MEM-10 was used to perform the shear tests, using the arrangement shown in Fig. 2. Hardness testing was performed on an Akashi MVK-A3 microhardness tester using a 300 g indentation load. All tests were carried out on the polished surfaces, without etching. The microhardness profiles were conducted on a cross-section of transverse specimens. The measurements were made with increments of 3 μm in the area away from the weld interface. Fig. 1 – Thermal diffusion welding cycle used in the joint WC–Co/Ti6Al4V. present with a grain size between 1 μm and 3 μm. The titanium alloy Ti6Al4V (AMS 4911) consists of a two-phase α + β microstructure. The elemental composition of the Ti alloy is 5.5 wt.% Al, 3.5 wt.% V and balance Ti. For use as a weld interlayer, the alloy Ticusil alloy has been used in the form of 50-mm thick layers. This interlayer alloy (68.29 wt.% Ag, 27.19 wt.% Cu and 4.52 wt.% Ti) was used between the base materials, Ti6Al4V and WC–Co. The Ticusil is characterized by a liquidus temperature of 900 °C and a solidus temperature of 780 °C.
2.2.
Surface Preparation
In all cases, the WC–Co and Ti6Al4V were grounded with emery paper to obtain a surface roughness (Ra) between 0.18 and 0.24 μm. These samples were then separated by the interlayer alloy, and introduced in a vacuum furnace (10− 3– 10− 2 Pa). The load used in this investigation was 2 MPa for all joints. The isothermal welding temperatures were: 825 °C at 30 and 60 min, and 850 °C at 15, 30 and 60 min (Fig. 1). These temperatures were chosen to accommodate the solidus and liquidus temperatures of the welding interlayer.
2.3.
3.
Results and Discussion
3.1.
Microstructural Characterization
The bonding temperatures were selected to be between the solidus and liquidus temperatures of the welding interlayer. The bonding temperature is important in understanding the formation of reaction phases associated with elemental diffusion, according to the Ag–Ti–Cu diagram (Fig. 3). As a consequence of the diffusion processes and metallurgical transformations during the welding, the joints in the materials studied occur in either the solid state (low temperature) or a partially transient liquid phase for the higher welding temperature (850 °C). The formation of diffusion layers between WC–Co/Ticusil/Ti6Al4V is evident in the joint interface. The number of these layers, their extent and elementary composition will change as a function of the experimental welding parameters such as time, temperature and the pressure applied during the bonding processes. Four reaction zones can be observed in the joints when the lower temperature and shorter times are used (825 °C, 30 min). The reaction in the surface of the materials begins with the diffusion of material from the Ticusil interlayer to the surface
Joint Characterizations
For metallographic examination, the joints were sectioned transversely to the bondline, grounded, polished and etched
Fig. 2 – Mechanical arrangement for shear tests of the joint WC–Co/Ti6Al4V.
Fig. 3 – Ag–Cu–Ti ternary diagram isotherm (700 °C).
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M A TE RI A L S C H A RAC TE RI ZA T ION 5 9 ( 2 00 8 ) 1 4 0 7–1 4 1 1
of the Ti6Al4V alloy (Fig. 4a). Elemental titanium and copper diffused in a higher quantity in the interface. The phase that appears nearest the Ti6Al4V alloy is a narrow zone which consists of the eutectoid Ti–Cu system, which is formed by the η-Ti2Cu phase and α + β Ti phases. The η phase grows between the eutectoid microstructure in the titanium alloy (α + β Ti) and the intermetallic TiCu (ζ), which grows at both sides of the Ag– Cu–Ti interlayer. The last reaction zone is formed at the WC– Co/TiCu2 (ζ)) interface. The EDX analysis in this reaction zone shows the existence of Co, Cu and W. The diffusion of Co and W from the hard metal to the interlayer is apparently due to the reaction between Ti and WC, which forms the TiC phase, according to the following thermodynamic data: Ti þ C TiC
DH0F ¼ 183:084 kJ=mol
W þ C WC
DH0F ¼ 40:504 kJ=mol
Ti þ WC TiC þ W
DH0F ¼ 142:580 kJ=mol :
When the thermal cycle is applied at 850 °C and the weld time is short (15 min), the welding interface microstructure is very similar to that observed at 825 °C–30 min. However, the diffusion of Cu, Ti, and Ag are higher, and for this reason, the Ag interface tends to disappear. On the other hand, when the
Fig. 5 – Vickers microhardness values of joints WC–Co/ Ti6Al4V, associated with each of the layers. diffusion welding conditions are more energetic (850 °C, 60 min), a more complete reaction process takes place (Fig. 4b). In the WC-interface, the high degradation of the WC takes place due to the reaction with the diffused Ti, which promotes the appearance of TiC in the solid state. The joining processes have allowed the diffusion of all elements. This fact promotes the WC reaction with Ti, forming titanium carbides. The W diffuses to the titanium alloy, where it forms two solid solutions, α-Ti and β-(Ti,W). The chemical joint (in all welding conditions) between WC–Co and Tia6Al4V is achieved when inter-diffusion between the materials is provided without the formation of voids and brittle phases such as intermetallic compounds. The composition, extent, nature and properties of the phases originated during the welds, will control the resulting mechanical properties.
3.2.
Mechanical Characterization
The results obtained from Vickers microhardness carried out on the base materials were 1805 HV and 389 HV for the WC–Co and Ti6Al4V, respectively. The microhardness values obtained in the welding interfaces for the different joints are directly related with the composition, extent and character of the diffusiongenerated layers. Microhardness values associated with each of the layers identified are shown in Fig. 5. From this figure we can observe that the softest areas appear where the silver concentration is the highest. Therefore, this area (D zone) is subject to the largest plastic behaviour and ductility. The A zone (eutectoid) has an intermediate hardness value among those obtained for the α + β Ti and the η phases. The C zone (ζ + Ag) and E zone (ζ + TiC) present higher hardness values than the B zone (η). This implies that the
Fig. 4 – Elemental distributions in the bond line and phases present in each layer of a WC–Co/Ti6Al4V diffusion joint, produced at (a) 825 °C and 30 min. [A: Eutectoid (α + β Ti) + η (Ti2Cu) phases; B: η (Ti2Cu); C: ζ (TiCu) + Ag; D: Ag; E: TiC + ζ (TiCu); F: TiC + WC; G: WC–Co]; and (b) 850 °C and 60 min. [A: Eutectoid (α + β Ti) + η (Ti2Cu) phases; E: TiC + ζ (TiCu); F: TiC + WC; G: WC–Co.]
Table 1 – Values of shear tests obtained for all isothermal conditions in the WC–Co/Ti6Al4V joint Joint isothermal conditions
σ(MPa)
825 850 850 850
314 330 955 879
°C–30 °C–15 °C–30 °C–60
min min min min
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M A TE RI A L S C H A RAC TE RI ZA T ION 5 9 ( 2 00 8 ) 1 4 0 7–1 4 1 1
Fig. 6 – Brittle fracture surface of WC–Co/ζ interface of a joint WC–Co/Ti6Al4V, obtained at 825 °C and 15 min.
phase η will be softer and more ductile than the ζ phase. The E zone has higher hardness values than those obtained for the C area, because the latter zone presents carbides in its composition. The highest hardness is observed in zone F, which consists of a mixture of carbides (TiC and WC). From the shear strength tests the joint bonded at the high temperature exhibited the higher bond strength, as shown in Table 1. The diffusion joints obtained at 850 °C for joint preparation times of 30 and 60 min failed at higher stresses than joints obtained at 825 °C; this generally reflects the higher number of phases formed in the interface at 850 °C. The presence of TiC nanoparticles in the reaction zone formed at 850 °C– 30 and –60 min appears to result in a substantial increase in the resistance of this joint; this may be because it is essentially a composite consisting of a ductile intermetallic compound with TiC particle reinforcement. From the fractographic analysis of surfaces obtained after mechanical testing, a range of fracture mechanisms can be observed, which depend on the microstructural characteristics of each interface formed by diffusion. It can also be observed that crack propagation can cross different areas of the diffusion welding interface, when a high void content exists. The fracture surfaces observed show the various mechanisms as follows: • At low welding energies (825 °C–30 min and 850 °C–15 min), the crack takes places by two different mechanisms: brittle
Fig. 7 – Ductile fracture surface of silver-rich interlayer of a joint WC–Co/Ti6Al4V, obtained at 825 °C and 15 min.
Fig. 8 – Fracture surface of a joint WC–Co/Ti6Al4V at 850 °C and 30 min (A) brittle fracture of the η phase and (B) fibrous brittle fracture of the ζ phase.
fracture of the hard metal (WC–Co/ζ (incipient)) interface (Fig. 6), and ductile fracture through the silver interlayer (Fig. 7). • At higher welding energies (850 °C–30 min and 60 min), the fracture occurs through the η phase by brittle fracture (Fig. 8), and in the ζ phase by fibrous brittle fracture (Fig. 9).
4.
Conclusions
1. Joints were prepared between (WC–15% Co) and a titanium alloy (Ti6Al4V), using a Ticusil (Ag–Cu–Ti) filler as an interlayer. The joints which exhibited the best overall quality and the maximum shear resistance were obtained when the welding temperature was 850 °C for a time of 30 min with a load of 2 MPa. Lower temperatures (825 °C) and lower times do not guaranty strong joints. 2. Both the hardness values and the shear resistance of the joint interfaces depend on the intermetallic compounds formed during the diffusion bonding. The presence of TiC appears to increase the mechanical resistance in the WC–ζ interface.
Fig. 9 – Fibrous brittle fracture of the ζ phase in a joint WC–Co/ Ti6Al4V obtained at 850 °C and 60 min joint condition.
M A TE RI A L S C H A RAC TE RI ZA T ION 5 9 ( 2 00 8 ) 1 4 0 7–1 4 1 1
3. From the fractographic analysis of surfaces obtained after mechanical testing, a range of fracture mechanisms can be observed, which depend on the microstructural characteristics of the various interfaces formed by the diffusion bonding.
Acknowledgement The authors wish to express thanks to the financial support of Project MAT2003-05004-C02-01.
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