Effect of bonding time on microstructure and mechanical properties of transient liquid phase bonded magnesium AZ31 alloy

The transient liquid phase (TLP) bonding of a magnesium alloy AZ31 was undertaken using pure nickel interlayers. The formation of a eutectic between the magnesium and nickel was obtained by bonding at 515 °C and the microstructural developments across the joint region were examined as a function of hold time from 5 to 120 min. Reaction zones were identified within the joint and the changes of width of the reaction zones were examined with respect to changes in the joint shear strength and hardness. The results showed that as the bonding time was increased to 60 min, the width of the eutectic zone was completely removed and the joint solidified isothermally. Under these conditions a maximum hardness value of 179 VHN was recorded and the highest joint shear strength of 36 MPa was obtained. However, when the bonding time was increased to 120 min, the shear strength of the interface decreased and this was attributed to the formation and segregation of brittle Mg–Ni intermetallics within the joint.     

Laser-tungsten inert gas hybrid welding of dissimilar metals AZ31B Mg alloys to Zn coated steel

Laser-tungsten inert gas (TIG) hybrid welding has been developed for joining Mg alloys to Zn coated steel in a lap joint configuration. The joint could not be produced in laser or arc welding only, while acceptable joints without obvious defects were obtained with a relatively wide processing window in the hybrid process. Two reaction layers were observed to form at the interface and were identified as Mg–Zn eutectic structure (α-Mg + MgZn) and Fe₃Al phase by TEM analysis. In some cases, Al₆Mn phase also formed adjacent to the Fe–Al reaction layer. The tensile-shear strength attained the maximum value of 68 MPa, representing 52.3% joint efficiency relative to Mg base metal. The element Al from AZ31B Mg alloys diffused to the liquid/solid interface and then reacted with the elements from steel, such as Fe and Mn, contributing to the metallurgical bonding at the interface. The weak bonding between Mg–Zn reaction layer and newly formed Fe–Al layer resulted in the interfacial failure.     

Diffusion bonding of titanium alloy to micro-duplex stainless steel using a nickel alloy interlayer: Interface microstructure and strength properties

In the present study, diffusion bonding of titanium alloy and micro-duplex stainless steel with a nickel alloy interlayer was carried out in the temperature range of 800–950 °C for 45 min under the compressive stress of 4 MPa in a vacuum. The bond interfaces were characterised by scanning electron microscopy, electron probe microanalyzer and X-ray diffraction analysis. The layer wise Ni₃Ti, NiTi and NiTi₂ intermetallics were observed at the nickel alloy/titanium alloy interface and irregular shaped particles of Fe₂₂Mo₂₀Ni₄₅Ti₁₃ was observed in the Ni₃Ti intermetallic layer. At 950 °C processing temperature, black island of β-Ti phase has been observed in the NiTi₂ intermetallics. However, the stainless steel/nickel alloy interface indicates the free of intermetallics phase. Fracture surface observed that, failure takes place through the NiTi₂ phase at the NiA–TiA interface when bonding was processed up to 900 °C, however, failure takes place through NiTi₂ and β-Ti phase mixture for the diffusion joints processed at 950 °C. Joint strength was evaluated and maximum tensile strength of ∼560 MPa and shear strength of ∼415 MPa along with ∼8.3% ductility were obtained for the diffusion couple processed at 900 °C for 45 min.     

Laser welding of AZ31B magnesium alloy to Zn-coated steel

The characteristics of laser lap welding of AZ31B magnesium alloy to Zn-coated steel were investigated. Welding was difficult when the laser beam was irradiated onto the AZ31B alloy and the processing parameters were set to obtain a keyhole welding mode. The difference in the physical properties between the two materials resulted in unstable welding process particularly when the laser beam penetrated into the steel specimen and a keyhole was formed therein. By switching to a conduction mode, the process stability was improved and successful welding could be achieved because the liquid metal film remained unbroken and the laser beam did not penetrate into the material. A 25 mm wide joint failed in tensile shear testing at loads exceeding 6000 N. This high joint strength was attributed to the formation of a 450 nm thick layer of Fe₃Al intermetallic compound on the steel surface as a result of the interaction between Al from the AZ31B alloy and Fe. The presence of Zn-coating layer was essential to eliminate the negative effects of oxides on the joining process.     

Investigation of laser welding on butt joints of Al/steel dissimilar materials

Dissimilar metals of AA6013 aluminum alloy and Q235 low-carbon steel of 2.5 mm thickness were butt joined using a 10 kW fiber laser welding system with ER4043 filler metal. The study indicates that it is feasible to join aluminum alloy to steel by butt joints when zinc layer was hot-dip galvanized at the steel’s groove face in advance, and better weld appearance can be obtained at appropriate welding parameters. The joints had dual characteristics of a welding joint on the aluminum side and a brazing joint on the steel side. The smooth Fe₂Al₅ layer adjacent to the steel matrix and the serrated-shape FeAl₃ layer close to the weld metal were formed at the brazing interface. The overall thickness of Fe–Al intermetallic compounds layers produced in this experiment were varied from 1.8 μm to 6.2 μm at various welding parameters with laser power of 2.85–3.05 kW and wire feed speed of 5–7 m/min. The Al/steel butt joints were failed at the brazing interface during the tensile test and reached the maximum tensile strength of 120 MPa.     

An investigation on microstructure and mechanical properties of Al7075 to Ti–6Al–4V Transient Liquid Phase (TLP) bonded joint

Transient Liquid Phase (TLP) bonding of two dissimilar alloys Al7075 and Ti–6Al–4V has been done at 500 °C under 5 × 10⁻⁴ torr. Cu was electrodeposited on Al7075 and Ti–6Al–4V surfaces, 50 μm thick Sn–4Ag–3.5Bi film was used as interlayer and bonding process was carried out at several bonding times. The microstructure of the diffusion bonded joints was evaluated by Light Optical Microscopy (LOM), Scanning Electron Microscopy (SEM) and Energy Dispersive Spectroscopy (EDS). The eutectic and intermetallic compounds formation along Al7075 grain boundaries and Ti/Al interface such as θ(Al₂Cu), TiAl and Ti₃Al were responsible for joint formation at the aluminum and titanium interfaces. Microhardness and shear strength tests were used to investigate the mechanical properties of the bonds. Hardness of the joints increased with increasing bonding time which can be attributed to the intermetallics formation at the interface. The study showed that the highest bond strength was 36 MPa which was obtained for the samples joined for 60 min.     

Comparative study on microstructure and mechanical properties of laser welded–brazed Mg/mild steel and Mg/stainless steel joints

A laser welding–brazing (LWB) technology using Mg based filler has been developed for joining Mg alloy to mild steel and Mg alloy to stainless steel in a lap configuration. Microstructure and mechanical properties of laser welded–brazed lap joints in both cases were comparatively studied. The results indicated that no distinct reaction layer was observed at the interface of Mg/mild steel and subsequently the interface was confirmed as mechanical bonding, whereas an ultra thin reaction layer with a continuous and uniform morphology was evidenced at the Mg/stainless steel interface, which was indicative of metallurgical bonding. The newly formed interfacial layer was indexed as FeAl phase by transmission electron microscopy (TEM) combined with energy dispersive spectroscopy (EDS). The average tensile–shear strength of Mg/mild steel joint was only 142 N/mm with typical interfacial failure, while that of Mg/stainless steel joint could reach 270 N/mm, representing 82.4% joint efficiency relative to the Mg alloy base metal. The fracture location of Mg/stainless steel joint was at Mg fusion welding side, suggesting the interface was not weak point due to the formation of ultra thin interfacial layer. The role of alloying elements in base metal and bonding mechanism of the interfacial layer were discussed, respectively.     

Constitutive analysis of Mg–Al–Zn magnesium alloys during hot deformation

The constitutive behaviors of Mg–Al–Zn magnesium alloys during hot deformation were studied over a wide range of Zener–Hollomon parameters by consideration of physically-based material’s parameters. It was demonstrated that the theoretical exponent of 5 and the lattice self-diffusion activation energy of magnesium (135 kJ/mol) can be used in the hyperbolic sine law to describe the flow stress of AZ31, AZ61, AZ80, and AZ91 alloys. The apparent hyperbolic sine exponents of 5.18, 5.06, 5.17, and 5.12, respectively for the AZ31, AZ61, AZ80, and AZ91 alloys by consideration of deformation activation energy of 135 kJ/mol were consistent with the considered theoretical exponent of 5. The influence of Al upon the hot flow stress of Mg–Al–Zn alloys was characterized by the proposed approach, which can be considered as a versatile tool in comparative hot working and alloy development studies. It was also shown that while the consideration of the apparent material’s parameters may result in a better fit to experimental data, but the possibility of elucidating the effects of alloying elements on the hot working behavior based on the constitutive equations will be lost.     

Effect of bonding temperature on microstructure and intermetallic compound formation of diffusion bonded magnesium/aluminum joints

The diffusion bonding of two dissimilar alloys Aluminum 5083 and Magnesium AZ31 was carried out at 420 °C, 430 °C,440 °C and 450 °C for bonding time of 60 min. In order to characterize the microstructure evolution in the joint zone, scanning electron microscopy, energy dispersive spectroscopy and x-ray diffraction were applied. The results show that joint formation is attributed to the solid-state diffusion of magnesium and aluminum into Aluminum 5083 and Magnesium AZ31 alloys followed by eutectic formation and constitutional liquation along the interface. At bonding temperature of 430 °C diffusion induced grain coarsening was observed at the interface. With increase in bonding temperature, the atomic diffusivity increases, results in easier and speeder chemical bonding. In bonding temperature of 440 °C the weld had an irregular shaped region in the weld center, having a different microstructure from the two base materials. The irregular shaped region contained a large volume of intermetallic compound Al₁₂Mg₁₇ and showed significantly higher hardness in the weld center. The present study suggests that constitutional liquation resulted in the intermetallic compound Al₁₂Mg₁₇ in the weld center.     

Microstructure and mechanical properties of MoSi2–MoSi2 joints brazed by Ag–Cu–Zr interlayer

The present work investigates joining of two MoSi₂ parts through Cusil/Zr/Cusil interlayer with Cusil being a commercial eutectic of Cu–Ag alloy. The joining operation was implemented in an inert gas tube furnace by brazing. The brazing temperature ranged from 800 to 930 °C while the operation lasted for 60 min. Evaluation of joints strength through shear loading identified the maximum strength 60.31 MPa for the brazed sample at 830 °C. Interfacial microstructure was studied by Scanning Electron Microscopy (SEM), Energy Dispersive Spectroscopy (EDS) and X-ray Diffraction (XRD) techniques. Applying the temperature of 830 °C was led to a uniform dense joint consisting of various phases with excellent bonding within the interfaces. XRD and EDS results revealed different phases such as Mo₅Si₃, Ag-rich solid solution and Cu₁₀Zr₇ at the interface. At higher brazing temperatures the amount of intemetallic compounds and residual stresses increased and therefore, mechanical properties of the joint degraded. The fracture analysis by SEM revealed various fracture path and morphology for different brazing temperatures.     

An investigation on the tribological behavior of AZ91 and AZ91 + 3 wt% RE magnesium alloys at elevated temperatures

The wear behavior of AZ91 and AZ91 + 3 wt% RE magnesium alloys was investigated under a normal load of 20 N at the wear testing temperatures of 25–250 °C and sliding speeds of 0.4 and 1 m s⁻¹. As the sliding speed increased from 0.4 to 1 m s⁻¹ at the wear temperature of 25 °C, the wear rates of AZ91 and AZ91 + 3 wt% RE alloys decreased by about 8% and 60%, respectively. With an increase in the wear temperature to 100 °C, the wear rate of AZ91 alloy was reduced by 58% at a sliding speed of 0.4 m s⁻¹, while the wear rate was sharply increased at a sliding speed of 1 m s⁻¹. At higher wear temperatures, the wear of the AZ91 alloy at both sliding speeds soared as a result of the softening of β-Mg₁₇Al₁₂ phase. However, the wear rate of AZ91 + 3 wt% RE alloy showed a minimum at the wear temperatures of 100 and 200 °C at sliding speeds of 1 and 0.4 m s⁻¹, respectively. Superior wear behavior of AZ91 + 3 wt% RE at the elevated temperatures could be attributed to its higher thermal stability and strength. Furthermore, a rise in sliding speed led to a 55% reduction in the wear rate of AZ91 + 3 wt% RE alloy at the wear temperature of 100 °C due to the formation of stable oxide layers on the wear surface.     

The effect of thermal cycling in superplastic diffusion bonding of 2205 duplex stainless steel

In view of the requirement of large cold rolling deformation and bonding pressure in the conventional superplastic diffusion bonding of 2205 duplex stainless steel, a novel method of introducing thermal cycling into the process was proposed. During the thermal cycling process, due to the change of temperature, surface chemical activity of 2205 duplex stainless steel was improved, activity of atoms and grain boundaries were improved, and the recrystallized grains were refined. The shear bond strength of joint prepared in the mode of thermal cycling using specimens with the cold roll reduction of 60% was 15 MPa higher than that of conventional bonding using specimens with the cold roll reduction of 85%. Compared to the shear bond strength of 430 MPa under the specific pressure of 10 MPa after conventional bonding, shear bond strength of 623 MPa was obtained under the condition of T_max = 1000 °C, T_min = 900 °C, cycle number of heating and cooling N = 3, and specific pressure P = 5 MPa.     

Liquid Phase Bonding of 316L Stainless Steel to AZ31 Magnesium Alloy

The excellent corrosion resistance,formability and strength make stainless steels versatile for diverse applications.However,its high specific strength and good crashworthiness make it suitable for transportation and aerospace industry.On the other hand,the need to reduce the weight of vehicle and aerospace components has created renewed interest in the use of magnesium alloys.Due to their differences in physical and metallurgical properties,bonding of the 316L steel and AZ31 magnesium alloy using conventional fusion welding methods encountered many limitations.Therefore,the use of liquid phase forming interlayers is required to overcome the differences in their properties,eliminates the need for a high bonding pressure to achieve intimate contact between the bonded surfaces.Both Cu and Ni interlayers successively formed a eutectic phase with magnesium.The formation of intermetallics and Mg diffusion caused the eutectic phase to isothermally solidify with increasing bonding time.The formation of ternary intermetallic phases(λ1 and B2) impaired the bond shear strength particularly at the end of the isothermal solidification stage where no eutectic phase was observed.However,the joints showed a higher shear strength value of 57 MPa when bonding with Cu interlayer at 530℃ for 30 min compared to 32 MPa when Ni interlayer was used at 510℃ for 20 min.     

Interface microstructure and mechanical properties of zinc–aluminum thermal diffusion coating on AZ31 magnesium alloy

The zinc–aluminum (Zn–Al) alloy coating with excellent wear and corrosion resistance was fabricated on the surface of magnesium substrate (AZ31) using thermal diffusion technique. The microstructure, phase constitution and chemical composition were investigated. The experimental observation exhibited that the interfacial microstructures were composed of network eutectic structures and lamellar eutectoid structures at heating temperature of 350 °C for holding time of 30 min under 0.1 MPa in a vacuum of 10⁻³ Pa. X-ray diffraction (XRD) pattern analysis identified that α-Mg, Mg₇Zn₃ and MgZn phases were formed in the diffusion layer. The interdiffusion of Mg and Al atoms were restricted by Mg–Zn intermetallic compounds (IMCs). The value of microhardness at the diffusion layer increased due to the formation of Mg–Zn eutectic phases. This technique is beneficial to improving poor wear and corrosion resistance of magnesium alloy.     

Partial transient liquid phase diffusion bonding of Zircaloy-4 to stabilized austenitic stainless steel 321

An innovative method was applied for bonding Zircaloy-4 to stabilized austenitic stainless steel 321 using an active titanium interlayer. Specimens were joined by a partial transient liquid phase diffusion bonding method in a vacuum furnace at different temperatures under 1 MPa dynamic pressure of contact. The influence of different bonding temperatures on the microstructure, microindentation hardness, joint strength and interlayer thickness has been studied. The diffusion of Fe, Cr, Ni and Zr has been investigated by scanning electron microscopy and energy dispersive spectroscopy elemental analyses. Results showed that control of the heating and cooling rate and 20 min soaking at 1223 K produces a perfect joint. However, solid-state diffusion of the melting point depressant elements into the joint metal causes the solid/liquid interface to advance until the joint is solidified. The tensile strength of all the bonded specimens was found around 480–670 MPa. Energy dispersive spectroscopy studies indicated that the melting occurred along the interface of the bonded specimens as a result of the transfer of atoms between the interlayer and the matrix during bonding. This technique provides a reliable method of bonding zirconium alloy to stainless steel.     

Diffusion bonding of 410 stainless steel to copper using a nickel interlayer

In the present work, plates of stainless steel (grade 410) were joined to copper ones through a diffusion bonding process using a nickel interlayer at a temperature range of 800–950 °C. The bonding was performed through pressing the specimens under a 12-MPa compression load and a vacuum of 10^? 4 torr for 60 min. The results indicated the formation of distinct diffusion zones at both Cu/Ni and Ni/SS interfaces during the diffusion bonding process. The thickness of the reaction layer in both interfaces was increased by raising the processing temperature. The phase constitutions and their related microstructure at the Cu/Ni and Ni/SS diffusion bonding interfaces were studied using optical microscopy, scanning electron microscopy, X-ray diffraction and elemental analyses through energy dispersive spectrometry. The resulted penetration profiles were examined using a calibrated electron probe micro-analyzer. The diffusion transition regions near the Cu/Ni and Ni/SS interfaces consist of a complete solid solution zone and of various phases based on (Fe, Ni), (Fe, Cr, Ni) and (Fe, Cr) chemical systems, respectively. The diffusion-bonded joint processed at 900 °C showed the maximum shear strength of about 145 MPa. The maximum hardness was obtained at the SS–Ni interface with a value of about 432 HV.     

Oxidation characterization of FeAl coated 316 stainless steel interconnects by high-energy micro-arc alloying technique for SOFC

Fe-based alloys have been extensively evaluated and considered as outstanding metallic interconnect materials for fuel cell. High-energy micro-arc alloying technique has been determined to be a feasible method of producing a consistent and dense FeAl intermetallic coating on 316 stainless steel substrate. The coating had an average thickness of about 50 μm and grain size was significantly refined. When exposed at 800 °C, 900 °C and 1000 °C in air after 100 h, FeAl coating on 316 SS substrate exhibited better high temperature oxidation resistance than electrode materials due to the conversion of non-protective Fe-rich scale into protective Al-rich one. FeAl intermetallic coating deposited by HEMAA will be available as interconnects for SOFC at very low costs.     

A novel weld-bonding hybrid process for joining Mg alloy and Al alloy

A novel weld-bonding hybrid process is carried out to join Mg alloy and Al alloy, and the technology combines a modified metal inert gas (MIG) spot welding process with adhesive bonding. The Mg base metal and the fusion zone are metallurgical connected by an Al–Mg transition layer with the thickness of 30–60 μm. Single nugget of spot welded joint can offer high shear strength of 130 MPa, which reach 81% of that of Mg base metal. The increased strength is due to the intermetallic layer being formed at the region with low stress, so the joint fractures in an Al-rich dendritic region. Superior mechanical properties can be obtained by weld bonded joint, benefiting from the advantages of both welding and adhesive bonding.     

Transient liquid phase (TLP) bonding of Al7075 to Ti–6Al–4V alloy

TLP diffusion bonding of two dissimilar aerospace alloys, Ti–6Al–4V and Al7075, was carried out at 500 °C using 22 μm thick Cu interlayers for various bonding times. Joint formation was attributed to the solid-state diffusion of Cu into the Ti alloy and Al7075 alloy followed by eutectic formation and isothermal solidification along the Cu/Al7075 interface. Examination of the joint region using SEM, EDS and XPS showed the formation of eutectic phases such as, ?(Al₂Cu), T(Al₂Mg₃Zn₃) and Al₁₃Fe along grain boundaries within the Al7075 matrix. At the Cu/Ti alloy bond interface a solid-state bond formed resulting in a Cu₃Ti₂ phase formation along this interface. The joint region homogenized with increasing bonding time and gave the highest bond strength of 19.5 MPa after a bonding time of 30 min.     

Diffusion bonding of TiAl intermetallic and Ti3AlC2 ceramic: Interfacial microstructure and joining properties

TiAl intermetallics and Ti₃AlC₂ ceramics were jointed through diffusion bonding using Ti/Ni interlayer. The effect of bonding temperature and holding time on interfacial microstructure and mechanical properties of the bonded joints were investigated. The typical interfacial microstructure of the joint from TiAl to Ti₃AlC₂ side could be divided into τ₃-Al₃NiTi₂, α₂-Ti₃Al, α-Ti + δ-Ti₂Ni, δ-Ti₂Ni, β₂-TiNi, η-TiNi₃, γ-(Ni)_ss, γ′-Ni₃(Al, Ti), γ′-Ni₃(Al, Ti) + Ti₃AlC₂, respectively. The value of the microhardness in the reactive zones increased due to the formation of intermetallcs. Lower or higher bonding temperature and longer or shorter holding time both resulted in low strength owing to the insufficient diffusion of atoms or excessive formation of intermetallics. A high bonding strength can be obtained when bonding at 920 °C for 60 min. Fracture occurred through the intermetallic layer adjacent to the Ti₃AlC₂ substrate during shear test, showing brittle intergranular and transgranular characteristic.