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Ti joints with Al element from filler
Materials Letters 167 (2016) 38–42
Contents lists available at ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/matlet
Enhanced interfacial reaction and mechanical properties of laser welded-brazed Mg/Ti joints with Al element from filler Caiwang Tan a,b, Xiaoguo Song a,n, Bo Chen a, Liqun Li b, Jicai Feng a,b a b
Shandong Provincial Key Laboratory of Special Welding Technology, Harbin Institute of Technology at Weihai, Weihai 264209, China State Key Laboratory of Advanced Welding and Joining, Harbin Institute of Technology, Harbin 150001, China
art ic l e i nf o
a b s t r a c t
Article history: Received 4 September 2015 Received in revised form 18 November 2015 Accepted 27 December 2015 Available online 29 December 2015
Dissimilar metal joining between magnesium and titanium alloys was performed using laser weldingbrazing process. Metallurgical bonding of immiscible AZ31 magnesium and TC4 titanium was successfully achieved by adding Al element into Mg based filler, compared to mechanical bonding when direct joining. An ultra-thin serrated-shape reaction layer was evidently observed at the interface of AZ91 (9% Al) fusion zone/Ti and identified as Ti3Al phase by transmission electron microscopy (TEM), which could prevent crack propagation effectively and thus improved the joint strength. The average fracture load of joint with AZ91 filler could reach 1947 761 N representing 47.371.5% joint efficiency relative to the magnesium base metal, which was 85% higher than that with AZ31filler. & 2015 Elsevier B.V. All rights reserved.
Keywords: Dissimilar joint Laser processing Welding Microstructure Mechanical properties
1. Introduction Recently, fabrication of lightweight structural materials has received increasingly attention [1–3]. It offers significant advantages over weight reduction in automobile and aerospace industries, thereby improving fuel efficiency and load capacity. Magnesium (Mg) and titanium (Ti) dissimilar joint has been of particular interest since both metals have some good features, such as low density, high specific strength and good formability [4–6]. Therefore, reliable joining of dissimilar metals Mg alloy and Ti alloy will expand the engineering application of both metals in advanced manufacturing industry, which in turn provides the production engineer with greater flexibility by using different metals and alloys in one product. However, joining of Mg and Ti faces a huge challenge because of their great differences in their physical and metallurgical properties. The melting points of Mg and Ti are 649 °C and 1678 °C, respectively. The conventional fusion welding is thus not applicable for reliable joining of Mg to Ti. The immiscibility of Mg and Ti indicates that no reaction layer or atomic diffusion occurs after solidification. Therefore, an intermediate element which can react with or possess substantial solid solubility in Mg and Ti must be employed. Transient liquid phase (TLP) bonding and friction stir welding (FSW) have been adopted to join Mg and Ti in previous n
Corresponding author. E-mail address: [email protected] (X. Song).
http://dx.doi.org/10.1016/j.matlet.2015.12.119 0167-577X/& 2015 Elsevier B.V. All rights reserved.
studies [7,8]. Eutectic structure formed at the Mg/Ni alloy interface and a solid-state diffusion bonding was produced at the Ni/Ti alloy interface. The formation of reaction layer with Ni or Cu addition cannot be controlled precisely in the TLP welding. In the case of FSW process, Ti–Al intermetallic compound layers formed at the interface due to external force and stirring effect, while magnesium alloy with higher Al content could result in the thick reaction layer reducing the tensile strength. Laser welding technique has the great potential for increased flexibility and adaptability when joining of dissimilar metals such as Mg/Ti [9,10], Mg/steel [11], Al/ steel [12]. Intermixing of molten Ti–6Al–4V with the liquid AZ31B caused the formation of lamellar and granular mixtures in the fusion zone, which yielded acceptable joints with high tensile strength [9,10] . In this work, Al element was selected as the intermediate element to bond Mg and Ti based on the Mg–Al and Al–Ti binary diagrams. It was added through Mg based filler, which could contain maximum content of 9 wt% Al without any crack after wire drawing. Metallurgical bonding of Mg and Ti was expected to achieve while thickness of reaction layer needed to be controlled below 10 mm, which was considered to be favorable for the joint strength [13]. The interfacial microstructure and mechanical properties of joints with and without Al addition were comparatively studied.
C. Tan et al. / Materials Letters 167 (2016) 38–42
2. Experimental The materials used in the present study were AZ31B magnesium alloy (Mg–3%Al–1%Zn) and Ti–6Al–4V (TC4) sheets, both with a thickness of 1.5 mm. Both sheets were machined to rectangular strips of 100 30 mm. AZ91 filler (Mg–9%Al–1%Zn) was fabricated and used. For comparison, AZ31 Mg filler (Mg–3%Al–1% Zn) having the same composition as the AZ31 Mg base metal was used. A fiber laser (IPG YLR-10000) with a maximum power of 10 kW was used. The diameter of focused laser beam was 0.2 mm. The assembly was fixed in a lap configuration by placing Mg sheet on top of Ti sheet. The laser beam was irradiated on the surface of the assembly. The filler wire was fed in front of the laser beam. Argon shielding gas was provided to prevent oxidation. The angle of the filler wire and the workpiece were adjusted for smooth wire feed. To completely irradiate the filler metal and promote brazing between molten filler metal and titanium, the laser beam was defocused. After laser welding-brazing process, test specimens were cut transverse to the weldment. Standard metallographic preparation procedures were utilized. The reaction layer along the interface was observed by scanning electron microscope (SEM). Transmission electron microscopy (TEM) with a Tecnai-G2 F30, operating at a nominal voltage of 300 kV, was used to characterize the microstructure in detail. Vickers hardness measurement was performed
39
across the Mg fusion zone/Ti interface under a test load of 100 g and a dwell time of 10 s. The 10-mm-wide, rectangular-shaped specimens were cut and subjected to tensile-shear test which was evaluated by a testing machine (INSTRON-5569) at a cross-head speed of 1 mm/min.
3. Results and discussion Fig. 1 shows the interfacial microstructure and elemental distribution of Mg/Ti joint with AZ31 and AZ91 filler. In Fig. 1a, no distinct interfacial layer could be observed at the interface of AZ31 fusion zone/Ti, even at higher magnification. It indicated that the joining mechanism of Mg and Ti was mainly mechanical bonding, which was good consistent with previous studies [9,10]. In contrast, an ultra-thin reaction layer formed at the interface of the AZ91 fusion zone/Ti indicated in Fig. 1b. Its thickness was less than 1 μm. Therefore, metallurgical bonding rather than mechanical bonding was achieved at the Mg/Ti interface when with more Al element from filler. The concentration profiles of the alloying elements across the interface obtained from line scanning analysis are shown in Fig. 1c and Fig. 1d. No obvious atomic diffusion was observed to occur at the interface of AZ31 fusion zone/Ti (Fig. 1c). However, an apparent Al segregation was observed at the interface of AZ91 fusion zone/Ti shown in Fig. 1d, indicating the presence of atomic diffusion or dissolution when
Fig. 1. Microstructures and line scanning results of AZ31/Ti and AZ91/Ti: (a) AZ31/Ti interface, (b) AZ91/Ti interface, (c) line scanning result of (a) and (d) line scanning result of (b).
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C. Tan et al. / Materials Letters 167 (2016) 38–42
Fig. 2. TEM micrograph and EDS analysis showing reaction layer at the AZ91/Ti interface: (a) TEM micrograph and (b) TEM-EDS result.
increasing the Al content. Fig. 2 shows a TEM micrograph taken at the interface of Mg/Ti joint made with AZ91 filler. An ultra-thin interfacial reaction layer differing from Ti substrate was observed, which further confirmed the SEM observation in Fig. 1b. The reaction phase at the interfacial layer was identified as Ti3Al by selected area electron diffraction (SAED) combined with TEM-EDS result shown in Fig. 2b. Based on the observation of Fig. 1 and Fig. 2, the presence of Ti3Al phase at the Mg/Ti interface with AZ91 filler proved metallurgical bonding could be produced by Al element diffusing from filler to Ti side during laser welding-brazing process. The diffusion-controlled
growth of Ti–Al phase was limited due to fast heating and cooling rates of laser welding, which was beneficial to the joint strength. The standard molar enthalpies for formation of binary and Mg– Al–Ti ternary systems were calculated based on the Miedema model. The results are shown in Fig. 3. The standard molar enthalpy of Mg–Ti in Fig. 3a was positive, since they did not react with each other and thus mutual reaction did not occur [14]. Note that the standard molar enthalpy of Al–Ti binary system was lower than that of Al–Mg binary system, indicating there was a driving force for formation of firstly precipitated Al–Ti compound. The result was confirmed in ternary system as shown in Fig. 3b. Fig. 3c
Fig. 3. Formation enthalpies of the Mg–Al–Ti system and Al chemical potential with variation of Al content: (a) binary system, (b) ternary system and (c) influence of Al molar fraction on Al chemical potential.
C. Tan et al. / Materials Letters 167 (2016) 38–42
41
Fig. 4. Mechanical properties of Mg/Ti joints with AZ31 filler and AZ91 filler: (a) hardness profile, (b) tensile-shear fracture load, (c) fracture surface of AZ31/Ti and (d) fracture surfaces of AZ91/Ti.
shows the relationship of Al chemical potential with Al content under the condition of the same Ti molar fraction. Al chemical potential decreased with the reduction of Al molar fraction for the same Ti content. For the same Al content, Al chemical potential decreased with increasing Ti molar fraction. Therefore, Al atom tended to diffuse from molten pool to the zone with low Al and high Ti molar fraction in the Mg–Al–Ti ternary system [15]. Microhardness and tensile testing were performed to evaluate the effect of Al content from filler on the mechanical properties of the joint. The results are shown in Fig. 4. The hardness in AZ91 weld was higher than that in AZ31 weld, since more eutectic structure (α-Mgþ β-Mg17Al12) were precipitated due to more Al content in AZ91 filler metal. It was worth noticing that the hardness of AZ91 weld/Ti near the interface increased sharply, which was higher than the weld and close to the neighboring Ti substrate. The increased hardness was associated with the formation of an intermetallic compound and diffusion of Al atoms to the Ti side. In contrast, no distinct change was observed in the AZ31 weld/Ti due to no atomic diffusion at the interface. Results of tensile testing show that joint strength of AZ91/Ti was higher than that of AZ31/Ti. The average fracture load of AZ31/Ti joint was 1049 7227 N, representing a 25.5 75.5% joint efficiency with respect to the fracture load of the magnesium alloy for the same size tensile-shear specimen. While the fracture load of joint using AZ91 filler was 1947 761 N, reaching 47.37 1.5% joint efficiency relative to the magnesium base metal. Serration-shaped reaction layer formed at the AZ91 fusion zone/Ti interface could prevent crack propagation, which was responsible for the improvement of tensile strength [13].
Fig. 4c and Fig. 4d shows the fracture morphologies of the lap joints at both cases. Note that there were differences in fracture behavior despite similar interfacial failures. For joint made with AZ31 filler, the fracture surface exhibited smooth characteristics without adequate plastic deformation due to low strength caused by mechanical bonding of Mg/Ti. In the case of joint made with AZ91 filler, a great number of particles were observed to exist on the fracture surface, which was confirmed as Ti–Al reaction layer at a higher magnification seen in the inset of Fig. 4d. Therefore, serrate-shaped microstructure formed at the interface played an important role in high resistance to the crack propagation by precipitation strengthening. Therefore, it was feasible to employ Al from filler metal to join Mg and Ti alloy based on the analyses of microstructure and mechanical properties.
4. Conclusions
(1) Immiscible Mg/Ti dissimilar joint was successfully joined by laser welding-brazing process with Al element from Mg based filler. Metallurgical bonding of magnesium and titanium was achieved by diffusion of Al atoms to the Ti side, while Mechanical bonding occurred when direct joining of Mg to Ti. (2) A Ti–Al reaction layer with thickness of less than 1 μm formed at the interface of Mg/Ti joint when using AZ91 filler, which was identified as Ti3Al phase by TEM analysis. Al atom was believed to diffuse from molten filler to the interface with low Al and high Ti molar fraction driven by chemical potential.
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C. Tan et al. / Materials Letters 167 (2016) 38–42
(3) The tensile-shear fracture load of joint made with AZ91 filler was 85% higher than that of joint with AZ31 filler. The joint efficiency was enhanced from 25.5 75.5% to 47.37 1.5%, relative to the magnesium alloy base metal. A significant amount of Ti–Al particles were observed at the fracture surface of AZ91 filler acting as precipitation strengthening, which finally resulted in high tensile joint.
References [1] [2] [3] [4] [5] [6] [7] [8] [9]
Acknowledgments The research was financially supported by Natural Science Foundation of Shandong Province, China (Grant no. BS2015ZZ008), National Natural Science Foundation of China (Grant no. 51504074) and National Science and Technology Major Project (No.2014ZX04001131).
[10] [11] [12] [13] [14] [15]
H.T. Zhang, X.Y. Dai, J.C. Feng, Mater. Lett. 122 (2014) 49–51. Y. Liu, Q. Sun, J. Liu, S. Wang, J. Feng, Mater. Lett. 152 (2015) 29–31. J. Zhang, Y. Xiao, G.Q. Luo, Q. Shen, L.M. Zhang, Mater. Lett. 66 (2012) 113–116. H. Friedrich, S.J. Schumann, Mater. Process. Technol. 117 (2001) 276–281. K.K. Mustafa, Int. J. Adv. Manuf. Technol. 39 (2008) 851–865. R.R. Boyer, Mater. Sci. Eng. A 213 (1996) 103–114. M. Aonuma, K. Nataka, Mater. Sci. Eng. B 161 (2009) 46–49. A.M. Atieh, T.I. Kan, J. Mater. Sci. 49 (2014) 7648–7658. M. Gao, Z.M. Wang, J. Yan, X.Y. Zeng, Sci. Technol. Weld. Join. 16 (2011) 488–496. M. Gao, Z.M. Wang, X.Y. Li, X.Y. Zeng, Met. Mater. Trans. A 43 (2011) 163–172. L.M. Liu, X.D. Qi, Z.H. Wu, Mater. Lett. 64 (2010) 89–92. S. Chen, J. Huang, K. Ma, H. Zhang, X. Zhao, Mater. Lett. 79 (2012) 296–299. Y. Chen, S. Chen, L. Li, Mater. Des. 31 (2010) 227–233. M. Gao, S. Mei, X. Li, X. Zeng, Scr. Mater. 67 (2012) 193–196. S.H. Chen, L.Q. Li, Y.B. Chen, D.J. Liu, Trans. Nonferrous Met. Soc. China 20 (2010) 64–70.
Contents lists available at ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/matlet
Enhanced interfacial reaction and mechanical properties of laser welded-brazed Mg/Ti joints with Al element from filler Caiwang Tan a,b, Xiaoguo Song a,n, Bo Chen a, Liqun Li b, Jicai Feng a,b a b
Shandong Provincial Key Laboratory of Special Welding Technology, Harbin Institute of Technology at Weihai, Weihai 264209, China State Key Laboratory of Advanced Welding and Joining, Harbin Institute of Technology, Harbin 150001, China
art ic l e i nf o
a b s t r a c t
Article history: Received 4 September 2015 Received in revised form 18 November 2015 Accepted 27 December 2015 Available online 29 December 2015
Dissimilar metal joining between magnesium and titanium alloys was performed using laser weldingbrazing process. Metallurgical bonding of immiscible AZ31 magnesium and TC4 titanium was successfully achieved by adding Al element into Mg based filler, compared to mechanical bonding when direct joining. An ultra-thin serrated-shape reaction layer was evidently observed at the interface of AZ91 (9% Al) fusion zone/Ti and identified as Ti3Al phase by transmission electron microscopy (TEM), which could prevent crack propagation effectively and thus improved the joint strength. The average fracture load of joint with AZ91 filler could reach 1947 761 N representing 47.371.5% joint efficiency relative to the magnesium base metal, which was 85% higher than that with AZ31filler. & 2015 Elsevier B.V. All rights reserved.
Keywords: Dissimilar joint Laser processing Welding Microstructure Mechanical properties
1. Introduction Recently, fabrication of lightweight structural materials has received increasingly attention [1–3]. It offers significant advantages over weight reduction in automobile and aerospace industries, thereby improving fuel efficiency and load capacity. Magnesium (Mg) and titanium (Ti) dissimilar joint has been of particular interest since both metals have some good features, such as low density, high specific strength and good formability [4–6]. Therefore, reliable joining of dissimilar metals Mg alloy and Ti alloy will expand the engineering application of both metals in advanced manufacturing industry, which in turn provides the production engineer with greater flexibility by using different metals and alloys in one product. However, joining of Mg and Ti faces a huge challenge because of their great differences in their physical and metallurgical properties. The melting points of Mg and Ti are 649 °C and 1678 °C, respectively. The conventional fusion welding is thus not applicable for reliable joining of Mg to Ti. The immiscibility of Mg and Ti indicates that no reaction layer or atomic diffusion occurs after solidification. Therefore, an intermediate element which can react with or possess substantial solid solubility in Mg and Ti must be employed. Transient liquid phase (TLP) bonding and friction stir welding (FSW) have been adopted to join Mg and Ti in previous n
Corresponding author. E-mail address: [email protected] (X. Song).
http://dx.doi.org/10.1016/j.matlet.2015.12.119 0167-577X/& 2015 Elsevier B.V. All rights reserved.
studies [7,8]. Eutectic structure formed at the Mg/Ni alloy interface and a solid-state diffusion bonding was produced at the Ni/Ti alloy interface. The formation of reaction layer with Ni or Cu addition cannot be controlled precisely in the TLP welding. In the case of FSW process, Ti–Al intermetallic compound layers formed at the interface due to external force and stirring effect, while magnesium alloy with higher Al content could result in the thick reaction layer reducing the tensile strength. Laser welding technique has the great potential for increased flexibility and adaptability when joining of dissimilar metals such as Mg/Ti [9,10], Mg/steel [11], Al/ steel [12]. Intermixing of molten Ti–6Al–4V with the liquid AZ31B caused the formation of lamellar and granular mixtures in the fusion zone, which yielded acceptable joints with high tensile strength [9,10] . In this work, Al element was selected as the intermediate element to bond Mg and Ti based on the Mg–Al and Al–Ti binary diagrams. It was added through Mg based filler, which could contain maximum content of 9 wt% Al without any crack after wire drawing. Metallurgical bonding of Mg and Ti was expected to achieve while thickness of reaction layer needed to be controlled below 10 mm, which was considered to be favorable for the joint strength [13]. The interfacial microstructure and mechanical properties of joints with and without Al addition were comparatively studied.
C. Tan et al. / Materials Letters 167 (2016) 38–42
2. Experimental The materials used in the present study were AZ31B magnesium alloy (Mg–3%Al–1%Zn) and Ti–6Al–4V (TC4) sheets, both with a thickness of 1.5 mm. Both sheets were machined to rectangular strips of 100 30 mm. AZ91 filler (Mg–9%Al–1%Zn) was fabricated and used. For comparison, AZ31 Mg filler (Mg–3%Al–1% Zn) having the same composition as the AZ31 Mg base metal was used. A fiber laser (IPG YLR-10000) with a maximum power of 10 kW was used. The diameter of focused laser beam was 0.2 mm. The assembly was fixed in a lap configuration by placing Mg sheet on top of Ti sheet. The laser beam was irradiated on the surface of the assembly. The filler wire was fed in front of the laser beam. Argon shielding gas was provided to prevent oxidation. The angle of the filler wire and the workpiece were adjusted for smooth wire feed. To completely irradiate the filler metal and promote brazing between molten filler metal and titanium, the laser beam was defocused. After laser welding-brazing process, test specimens were cut transverse to the weldment. Standard metallographic preparation procedures were utilized. The reaction layer along the interface was observed by scanning electron microscope (SEM). Transmission electron microscopy (TEM) with a Tecnai-G2 F30, operating at a nominal voltage of 300 kV, was used to characterize the microstructure in detail. Vickers hardness measurement was performed
39
across the Mg fusion zone/Ti interface under a test load of 100 g and a dwell time of 10 s. The 10-mm-wide, rectangular-shaped specimens were cut and subjected to tensile-shear test which was evaluated by a testing machine (INSTRON-5569) at a cross-head speed of 1 mm/min.
3. Results and discussion Fig. 1 shows the interfacial microstructure and elemental distribution of Mg/Ti joint with AZ31 and AZ91 filler. In Fig. 1a, no distinct interfacial layer could be observed at the interface of AZ31 fusion zone/Ti, even at higher magnification. It indicated that the joining mechanism of Mg and Ti was mainly mechanical bonding, which was good consistent with previous studies [9,10]. In contrast, an ultra-thin reaction layer formed at the interface of the AZ91 fusion zone/Ti indicated in Fig. 1b. Its thickness was less than 1 μm. Therefore, metallurgical bonding rather than mechanical bonding was achieved at the Mg/Ti interface when with more Al element from filler. The concentration profiles of the alloying elements across the interface obtained from line scanning analysis are shown in Fig. 1c and Fig. 1d. No obvious atomic diffusion was observed to occur at the interface of AZ31 fusion zone/Ti (Fig. 1c). However, an apparent Al segregation was observed at the interface of AZ91 fusion zone/Ti shown in Fig. 1d, indicating the presence of atomic diffusion or dissolution when
Fig. 1. Microstructures and line scanning results of AZ31/Ti and AZ91/Ti: (a) AZ31/Ti interface, (b) AZ91/Ti interface, (c) line scanning result of (a) and (d) line scanning result of (b).
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Fig. 2. TEM micrograph and EDS analysis showing reaction layer at the AZ91/Ti interface: (a) TEM micrograph and (b) TEM-EDS result.
increasing the Al content. Fig. 2 shows a TEM micrograph taken at the interface of Mg/Ti joint made with AZ91 filler. An ultra-thin interfacial reaction layer differing from Ti substrate was observed, which further confirmed the SEM observation in Fig. 1b. The reaction phase at the interfacial layer was identified as Ti3Al by selected area electron diffraction (SAED) combined with TEM-EDS result shown in Fig. 2b. Based on the observation of Fig. 1 and Fig. 2, the presence of Ti3Al phase at the Mg/Ti interface with AZ91 filler proved metallurgical bonding could be produced by Al element diffusing from filler to Ti side during laser welding-brazing process. The diffusion-controlled
growth of Ti–Al phase was limited due to fast heating and cooling rates of laser welding, which was beneficial to the joint strength. The standard molar enthalpies for formation of binary and Mg– Al–Ti ternary systems were calculated based on the Miedema model. The results are shown in Fig. 3. The standard molar enthalpy of Mg–Ti in Fig. 3a was positive, since they did not react with each other and thus mutual reaction did not occur [14]. Note that the standard molar enthalpy of Al–Ti binary system was lower than that of Al–Mg binary system, indicating there was a driving force for formation of firstly precipitated Al–Ti compound. The result was confirmed in ternary system as shown in Fig. 3b. Fig. 3c
Fig. 3. Formation enthalpies of the Mg–Al–Ti system and Al chemical potential with variation of Al content: (a) binary system, (b) ternary system and (c) influence of Al molar fraction on Al chemical potential.
C. Tan et al. / Materials Letters 167 (2016) 38–42
41
Fig. 4. Mechanical properties of Mg/Ti joints with AZ31 filler and AZ91 filler: (a) hardness profile, (b) tensile-shear fracture load, (c) fracture surface of AZ31/Ti and (d) fracture surfaces of AZ91/Ti.
shows the relationship of Al chemical potential with Al content under the condition of the same Ti molar fraction. Al chemical potential decreased with the reduction of Al molar fraction for the same Ti content. For the same Al content, Al chemical potential decreased with increasing Ti molar fraction. Therefore, Al atom tended to diffuse from molten pool to the zone with low Al and high Ti molar fraction in the Mg–Al–Ti ternary system [15]. Microhardness and tensile testing were performed to evaluate the effect of Al content from filler on the mechanical properties of the joint. The results are shown in Fig. 4. The hardness in AZ91 weld was higher than that in AZ31 weld, since more eutectic structure (α-Mgþ β-Mg17Al12) were precipitated due to more Al content in AZ91 filler metal. It was worth noticing that the hardness of AZ91 weld/Ti near the interface increased sharply, which was higher than the weld and close to the neighboring Ti substrate. The increased hardness was associated with the formation of an intermetallic compound and diffusion of Al atoms to the Ti side. In contrast, no distinct change was observed in the AZ31 weld/Ti due to no atomic diffusion at the interface. Results of tensile testing show that joint strength of AZ91/Ti was higher than that of AZ31/Ti. The average fracture load of AZ31/Ti joint was 1049 7227 N, representing a 25.5 75.5% joint efficiency with respect to the fracture load of the magnesium alloy for the same size tensile-shear specimen. While the fracture load of joint using AZ91 filler was 1947 761 N, reaching 47.37 1.5% joint efficiency relative to the magnesium base metal. Serration-shaped reaction layer formed at the AZ91 fusion zone/Ti interface could prevent crack propagation, which was responsible for the improvement of tensile strength [13].
Fig. 4c and Fig. 4d shows the fracture morphologies of the lap joints at both cases. Note that there were differences in fracture behavior despite similar interfacial failures. For joint made with AZ31 filler, the fracture surface exhibited smooth characteristics without adequate plastic deformation due to low strength caused by mechanical bonding of Mg/Ti. In the case of joint made with AZ91 filler, a great number of particles were observed to exist on the fracture surface, which was confirmed as Ti–Al reaction layer at a higher magnification seen in the inset of Fig. 4d. Therefore, serrate-shaped microstructure formed at the interface played an important role in high resistance to the crack propagation by precipitation strengthening. Therefore, it was feasible to employ Al from filler metal to join Mg and Ti alloy based on the analyses of microstructure and mechanical properties.
4. Conclusions
(1) Immiscible Mg/Ti dissimilar joint was successfully joined by laser welding-brazing process with Al element from Mg based filler. Metallurgical bonding of magnesium and titanium was achieved by diffusion of Al atoms to the Ti side, while Mechanical bonding occurred when direct joining of Mg to Ti. (2) A Ti–Al reaction layer with thickness of less than 1 μm formed at the interface of Mg/Ti joint when using AZ91 filler, which was identified as Ti3Al phase by TEM analysis. Al atom was believed to diffuse from molten filler to the interface with low Al and high Ti molar fraction driven by chemical potential.
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C. Tan et al. / Materials Letters 167 (2016) 38–42
(3) The tensile-shear fracture load of joint made with AZ91 filler was 85% higher than that of joint with AZ31 filler. The joint efficiency was enhanced from 25.5 75.5% to 47.37 1.5%, relative to the magnesium alloy base metal. A significant amount of Ti–Al particles were observed at the fracture surface of AZ91 filler acting as precipitation strengthening, which finally resulted in high tensile joint.
References [1] [2] [3] [4] [5] [6] [7] [8] [9]
Acknowledgments The research was financially supported by Natural Science Foundation of Shandong Province, China (Grant no. BS2015ZZ008), National Natural Science Foundation of China (Grant no. 51504074) and National Science and Technology Major Project (No.2014ZX04001131).
[10] [11] [12] [13] [14] [15]
H.T. Zhang, X.Y. Dai, J.C. Feng, Mater. Lett. 122 (2014) 49–51. Y. Liu, Q. Sun, J. Liu, S. Wang, J. Feng, Mater. Lett. 152 (2015) 29–31. J. Zhang, Y. Xiao, G.Q. Luo, Q. Shen, L.M. Zhang, Mater. Lett. 66 (2012) 113–116. H. Friedrich, S.J. Schumann, Mater. Process. Technol. 117 (2001) 276–281. K.K. Mustafa, Int. J. Adv. Manuf. Technol. 39 (2008) 851–865. R.R. Boyer, Mater. Sci. Eng. A 213 (1996) 103–114. M. Aonuma, K. Nataka, Mater. Sci. Eng. B 161 (2009) 46–49. A.M. Atieh, T.I. Kan, J. Mater. Sci. 49 (2014) 7648–7658. M. Gao, Z.M. Wang, J. Yan, X.Y. Zeng, Sci. Technol. Weld. Join. 16 (2011) 488–496. M. Gao, Z.M. Wang, X.Y. Li, X.Y. Zeng, Met. Mater. Trans. A 43 (2011) 163–172. L.M. Liu, X.D. Qi, Z.H. Wu, Mater. Lett. 64 (2010) 89–92. S. Chen, J. Huang, K. Ma, H. Zhang, X. Zhao, Mater. Lett. 79 (2012) 296–299. Y. Chen, S. Chen, L. Li, Mater. Des. 31 (2010) 227–233. M. Gao, S. Mei, X. Li, X. Zeng, Scr. Mater. 67 (2012) 193–196. S.H. Chen, L.Q. Li, Y.B. Chen, D.J. Liu, Trans. Nonferrous Met. Soc. China 20 (2010) 64–70.