Dissimilar Friction-Stir Lap Joining of 5083 Aluminum Alloy to CuZn34 Brass

In this research, lap joining of Al-Mg aluminum alloy and CuZn34 brass was produced by friction-stir welding during which the aluminum alloy sheet was placed on the CuZn34. Optical microscopy, scanning electron microscopy (SEM), X-ray diffraction analysis, and energy-dispersive X-ray spectroscopy (EDS) analysis were used to probe the microstructures and chemical compositions. In addition, the mechanical properties of each sample are characterized using both shear and hardness tests. The optimum parameters resulted in no visible welding cracks and defects. A dark area in the Al/CuZn34 interface contained intermetallic compounds Al₂Cu, Al₄Cu₉, and CuZn. In addition, the results show that using high rotational speeds or low traverse speeds causes the growth of the interfacial intermetallic area.     

Formation of Liquid and Intermetallics in Al-to-Mg Friction Stir Welding

In dissimilar-metal friction stir welding (FSW), intermetallic compounds can form in the stir zone and significantly reduce the joint strength. The formation of intermetallic compounds in Al-to-Mg FSW was investigated in lap and butt FSW of the widely used 6061 Al and AZ31B Mg and discussed using the binary Al-Mg phase diagram as an approximation. Temperature measurements during lap FSW indicated a 703 K (430 °C) peak temperature, slightly below the eutectic reaction (Mg) + Al₁₂Mg₁₇ → L at 710 K (437 °C), because the thermocouples were pushed downward during welding. The intermetallic compounds in the stir zone were revealed by color etching and identified by X-ray diffraction (XRD), electron probe microanalysis (EPMA), and transmission electron microscopy (TEM) as Al₃Mg₂ and Al₁₂Mg₁₇. Additional FSW was conducted near the edge of the upper sheet, and the liquid droplets squeezed out during welding solidified along the edge. Optical microscopy of the solidified droplets and EPMA revealed dendrites of Al₃Mg₂ and Al₁₂Mg₁₇ and interdendritic eutectics, thus indicating eutectic reactions (Mg) + Al₁₂Mg₁₇ → L (710 K (437 °C)) and (Al) + Al₃Mg₂ → L (723 K (450 °C)). Differential scanning calorimetry (DSC) confirmed that the solidified droplets melted at 709 K (436 °C) and 722 K (449 °C), nearly identical to the eutectic temperatures. Formation of intermetallic compounds on the order of 1 mm in size suggests they form upon solidification of the liquated material instead of solid-state diffusion.     

Al-to-Mg Friction Stir Welding: Effect of Material Position,Travel Speed,and Rotation Speed

Because joining dissimilar metals is often difficult by fusion joining, interest has been growing rapidly in using friction stir welding (FSW), which is considered a revolutionary solid-state welding process, as a new way to join dissimilar metals such as Al alloys to Mg alloys, Cu, and steels. Butt FSW of Al to Mg alloys has been studied frequently recently, but the basic issue of how the welding conditions affect the resultant joint strength still is not well understood. Using the widely used alloys 6061 Al and AZ31 Mg, the current study investigated the effect of the welding conditions, including the positions of Al and Mg with respect to the welding tool, the tool travel speed, and the tool rotation speed on the weld strength. Unlike previous studies, the current study (1) determined the heat input by both torque and temperature measurements during FSW, (2) used color metallography with Al, Mg, Al₃Mg₂, and Al₁₂Mg₁₇ all shown in different colors to reveal clearly the formation of intermetallic compounds and material flow in the stir zone, which are known to affect the joint strength significantly, and (3) determined the windows for travel and rotation speeds to optimize the joint strength for various material positions. The current study demonstrated clearly that the welding conditions affect the heat input, which in turn affects (1) the formation of intermetallics and even liquid and (2) material flow. Thus, the effect of welding conditions in Al-to-Mg butt FSW on the joint strength now can be explained.     

Analysis of Nugget Formation During Resistance Spot Welding on Dissimilar Metal Sheets of Aluminum and Magnesium Alloys

The nugget formation of resistance spot welding (RSW) on dissimilar material sheets of aluminum and magnesium alloys was studied, and the element distribution, microstructure, and microhardness distribution near the joint interface were analyzed. It was found that the staggered high regions at the contact interface of aluminum and magnesium alloy sheets, where the dissimilar metal melted together, tended to be the preferred nucleation regions of nugget. The main technical problem of RSW on dissimilar metal sheets of aluminum and magnesium alloys was the brittle-hard Al₁₂Mg₁₇ intermetallic compounds distributed in the nugget, with hardness much higher than either side of the base materials. Microcracks tended to generate at the interface of the nugget and base materials, which affected weld quality and strength.     

The Effectiveness of Surface Coatings on Preventing Interfacial Reaction During Ultrasonic Welding of Aluminum to Magnesium

High power ultrasonic spot welding (USW) is a solid-state joining process that is advantageous for welding difficult dissimilar material couples, like magnesium to aluminum. USW is also a useful technique for testing methods of controlling interfacial reaction in welding as the interface is not greatly displaced by the process. However, the high strain rate deformation in USW has been found to accelerate intermetallic compound (IMC) formation and a thick Al₁₂Mg₁₇ and Al₃Mg₂ reaction layer forms after relatively short welding times. In this work, we have investigated the potential of two approaches for reducing the IMC reaction rate in dissimilar Al-Mg ultrasonic welds, both involving coatings on the Mg sheet surface to (i) separate the join line from the weld interface, using a 100-μm-thick Al cold spray coating, and (ii) provide a diffusion barrier layer, using a thin manganese physical vapor deposition (PVD) coating. Both methods were found to reduce the level of reaction and increase the failure energy of the welds, but their effectiveness was limited due to issues with coating attachment and survivability during the welding cycle. The effect of the coatings on the joint’s interface microstructure, and the fracture behavior have been investigated in detail. Kinetic modeling has been used to show that the benefit of the cold spray coating can be attributed to the reaction rate reverting to that expected under static conditions. This reduces the IMC growth rate by over 50 pct because at the weld line, the high strain rate dynamic deformation in USW normally enhances diffusion through the IMC layer. In comparison, the thin PVD barrier coating was found to rapidly break up early in USW and become dispersed throughout the deformation layer reducing its effectiveness.     

A Comprehensive Study of Diffusion Bonding of Mg AZ31 to Al 5754, Al 6061 and Al 7039 Alloys

In the present study, microstructural and mechanical properties of diffusion bonding of AZ31–Mg with Al 5754, Al 6061, and Al 7039 alloys were compared under same conditions. The vacuum diffusion processes were performed at a temperature of 440 °C, the pressure of 29 MPa, and a vacuum of 1?×?10^?4 torr for 60 min. The microstructural characterizations were investigated using optical microscopy and scanning electron microscopy equipped with EDS analysis and linear scanner. The XRD analysis was performed to study phase figures near the interface zone. The results revealed the formation of brittle intermetallic compounds like Al₁₂Mg₁₇, Al₃Mg₂, and their other combinations at bonding interfaces of all samples. Additionally, the hardness of Al alloys seemed to play a key role in increasing diffusion rate of magnesium atoms toward the aluminum atoms, with Al 6061 alloy having the highest diffusion rate. It consequently led to an increase in diffusion rate and thus formation of a strong diffusion bonding between magnesium and aluminum alloys. The highest strength was about 42 MPa for the diffusion bonding between Mg AZ31 and Al 6061. Further investigations on surfaces indicated that the brittle phases especially Al₃Mg₂ caused brittle fracturing.     

The Effectiveness of Al-Si Coatings for Preventing Interfacial Reaction in Al-Mg Dissimilar Metal Welding

The dissimilar welding of aluminum to magnesium is challenging because of the rapid formation of brittle intermetallic compounds (IMC) at the weld interface. An Al-Si coating interlayer was selected to address this problem, based on thermodynamic calculations which predicted that silicon would change the reaction path to avoid formation of the normally observed binary Al-Mg IMC phases (β-Al₃Mg₂ and γ-Al₁₂Mg₁₇). Long-term static heat treatments confirmed that a Si-rich coating will preferentially produce the Mg₂Si phase in competition with the less stable, β-Al₃Mg₂ and γ-Al₁₂Mg₁₇ binary IMC phases, and this reduced the overall reaction layer thickness. However, when an Al-Si clad sheet was tested in a real welding scenario, using the Refill^? friction stir spot welding (FSSW) technique, Mg₂Si was only produced in very small amounts owing to the much shorter reaction time. Surprisingly, the coating still led to a significant reduction in the IMC reaction layer thickness and the welds exhibited enhanced mechanical performance, with improved strength and fracture energy. This beneficial behavior has been attributed to the softer coating material both reducing the welding temperature and giving rise to the incorporation of Si particles into the reaction layer, which toughened the brittle interfacial IMC phases during crack propagation.     

铝合金-镁合金间搅拌摩擦异质焊接研究现状

综合了近几年铝-镁搅拌摩擦异质焊的研完成果,分析和讨论以下几个方面的内容:(1)铝.镁异质焊的焊接性能及其影响因素;(2)焊接过程中的热输入情况及焊缝处的温度分布;(3)焊缝的力学性能及其影响因素;(4)焊缝的宏观、微观组织及其在局部升温和塑性变形下的演化过程,着重分析了金属间化合物的生成、形貌和分布、及其对焊缝力学性...     

Microstructure Characteristics and Mechanical Properties of Al 413/Mg Joint in Compound Casting Process

Al 413/Mg couples were prepared by the compound casting process. Characterization of the interface by an optical microscope and scanning electron microscope (SEM) showed that a relatively uniform interface composed of three different layers is formed at the interface. The thickness of the interface depended on the melt/insert volume ratio (VR) significantly and was 80?and 470? ??m? in 1.25?and 3?VRs, respectively. The results of the energy dispersive X-ray spectroscopy (EDS), wavelength dispersive X-ray spectroscopy (WDS), and X-ray diffraction analysis showed that the interface layers are mainly composed of Al₃Mg₂, Al₁₂Mg₁₇, and Mg₂Si intermetallic compounds. An accumulation of magnesium oxide films was detected within the (Al₁₂Mg₁₇?+???) eutectic structure of the interface next to the magnesium base metal. Despite different thicknesses of the interface, shear strengths of the Al 413/Mg couples prepared in 1.25?and 3?VRs were almost same. The study of the fracture surfaces of the Al 413/Mg couples revealed that the accumulated magnesium oxide films act as a weak point for initiation of longitudinal cracks and failure of the joint.     

Microstructure and Properties Analysis of Laser Welding and Laser Weld Bonding Mg to Al Joints

Laser welding and laser weld bonding (LWB) Mg to Al joints were obtained in different welding parameters. The penetrations and microstructures of these kinds of joints changed with the increasing of pulse laser power density. Both laser welding and LWB Mg to Al joints with the best properties were obtained in conductive welding mode. In laser welding Mg to Al joint, several intermetallics formed at the bottom of the fusion zone, where some cracks were generated. In laser weld bonding Mg to Al joint, the decomposition of the adhesive caused a baffle effect on the diffusion between the Mg and the Al. The intermetallics formed in the middle of the fusion zone, and the thickness of Mg₁₇Al₁₂ layer was approximately 10 to 20 μm and the Mg₂Al₃ layer was less than 5 μm, which influenced the property of the joint less.     

Studies on Electro Mechanical Aspects in Ultrasonically Welded Al/Cu Joints

Ultrasonic welding has been widely used to bond dissimilar conductive wires, battery cell terminals in relay applications. In this paper, dissimilar metals, Al/Cu were joined using ultrasonic welding for conductive applications. Welding trials were carried out by varying three control parameters: (1) vibrational amplitude (40, 60, and 80 µm), (2) clamping pressure (1, 1.2, and 1.4 bar), (3) weld time (0.1, 0.2, and 0.3 s). Experimental trails were designed based on L₉ Taguchi method. Interpretation of tensile strength and microhardness results revealed that the satisfactory weldments were obtained for higher welding energies when compared to low welding energies. From the microstructural analysis, the bond formation of metals and failure modes were studied. SEM and XRD images revealed the four major intermetallic compounds at the interface of joint; AlCu, Al₂Cu, Al₃Cu₄, and Al₄Cu₉ with resistivity values of 11.415, 8.027, 10.612, and 14.243 Ω-cm respectively. The resistivity values of intermetallic compounds observed in the joint was almost 5–6 times higher than the Al.     

Structural stability of super duplex stainless weld metals and its dependence on tungsten and copper

Three different superduplex stainless weld metals have been produced using manual metal arc welding under identical welding conditions. The concentration of the alloying elements tungsten and copper corresponded to the concentrations in commercial superduplex stainless steels (SDSS). Aging experiments in the temperature range 700 °C to 1110 °C showed that the formation of intermetallic phase was enhanced in tungsten-rich weld metal and also dissolved at higher temperatures compared with tungsten-poor and tungsten-free weld metals. It could be inferred from time-temperature-transformation (TTT) and continuous-cooling-transformation (CCT) diagrams produced in the present investigation that the critical cooling rate to avoid 1 wt pct of intermetallic phase was 2 times faster for tungsten-rich weld metal. Microanalysis in combination with thermodynamic calculations showed that tungsten was accommodated in χ phase, thereby decreasing the free energy. Experimental evidence supports the view that the formation of intermetallic phase is enhanced in tungsten-rich weld metal, owing to easier nucleation of nonequilibrium χ phase compared with σ phase. The formation of secondary austenite (γ₂) during welding was modeled using the thermodynamic computer program Thermo-Calc. Satisfactory agreement between theory and practice was obtained. Thermo-Calc was capable of predicting observed lower concentrations of chromium and nitrogen in γ₂ compared with primary austenite. The volume fraction of γ₂ was found to be significantly higher in tungsten-rich and tungsten + copper containing weld metal. The results could be explained by a higher driving force for precipitation of γ₂ in these.     

The Mechanism of Grain Coarsening in Friction-Stir-Welded AA5083 after Heat Treatment

Friction stir welding (FSW) takes place in the solid state, thus providing potential advantages of welds of high strength and ductility because of fine microstructures. However, post-FSW heat treatment can create very coarse grains, potentially reducing mechanical properties. AA5083-H18 sheets were friction-stir butt welded using three sets of welding parameters representing a wide range of heat input. They were then heat treated for 5 minutes at 738 K (465 °C), producing grain sizes exceeding 100 μm near the top weld surfaces, with the coarse grains extending toward the bottom surface to various degrees depending on the welding parameters. Electron backscatter diffraction (EBSD), transmission electron microscopy (TEM), scanning electron microscopy (SEM), optical metallography, inductively coupled plasma–mass spectrometry, and Vickers hardness testing were used to characterize the regions within welds. Particle pinning was determined quantitatively and used with Humphreys’ model of grain growth to interpret the behavior. The mechanism responsible for forming the large grains was identified as abnormal grain growth (AGG), with AGG occurring only for regions with pre-heat-treatment grain sizes smaller than 3 μm. Second-phase particle volume fractions and sizes, textures, and solute concentrations were not significantly different in AGG and non-AGG regions. Ultrafine grain layers with grain diameters of 0.3 mm were characterized and had high densities of pinning particles of MgSi₂, Al₂O₃, and Mg₅Al₈. Strategies to eliminate AGG by alloy and weld process design were discussed.     

Microstructure and phase constituents in the interface zone of Mg/Al diffusion bonding

The microstructure and phase constituent for the Mg/Al diffusion-bonded joint were studied via scanning electron microscope (SEM), microhardness test, electron probe microanalyzer (EPMA), and X-ray diffraction (XRD). The test results indicated that the new compact phase was formed near the transition region of the Mg/Al diffusion interface. There are three new phase layers in the transition region. The microhardness of the diffusion zone is higher than that of the Mg and Al substrate. The fracture morphology mainly consists of a coarse and gray fracture, and the fracture is mainly the mixed fracture of cleavage and intergranular. X-ray diffraction tests indicate that the diffusion zone of the Mg/Al diffusion-bonded joint consists of intermetallic compounds MgAl, Mg₃Al₂, and Mg₂Al₃. With the increase of temperature, the content of Mg₃Al₂ and Mg₂Al₃ phases with good stability was increased.     

Nanocomposites of Aluminum Alloys by Rapid Solidification Processing

Aluminium alloys reinforced with transition metal aluminide (Al₃Ti, Al₃Fe, Al₃Ni, etc.) particles possess high specific strength both at ambient and elevated temperature. The improved strength of these alloys are the results of slower coarsening rate of the intermetallic particles due to low diffusivity of the transition metals in aluminium. However, the strength can be enhanced further by refining the microstructure of the alloys to nanometer range. The authors have successfully attempted two important non-equilibrium processing techniques i.e. rapid solidification processing (RSP) and mechanical alloying for the refinement of the microstructure in various aluminium alloys. In this report, authors present a short review of their work on RSP of Al?CTi and Al?CFe alloys to produce nanocomposites.     

Influence of Tool Offsetting on the Structure and Morphology of Dissimilar Aluminum to Copper Friction-Stir Welds

In this work, a systematic analysis of the effect of tool offsetting on the morphological, structural, and mechanical properties of 6082-T6 aluminum to copper-DHP friction-stir welds was performed, enabling full understanding of Al-Cu bonding structure and failure mechanisms. Important relations between tool positioning and the thermomechanical phenomena taking place during welding were established. Tool offsetting was revealed to be an effective way of solving one of the most important concerns in Al/Cu friction-stir welding, i.e., the formation of large amounts of intermetallic-rich structures, which deeply influence the final strength and surface morphology of the welds. Actually, for welds produced without tool offsetting, it was found that the formation of fluidized intermetallic-rich structures promote the formation of internal decohesion areas inside the nugget, which have a detrimental effect on weld strength. For welds carried out with tool offsetting, intermetallic formation is almost suppressed, but important metallurgical discontinuities in the vicinity of large copper fragments, dispersed over the nugget, and at the nugget/copper interface were also found to have a detrimental effect on weld strength.     

Microstructural Investigation and Phase Relationships of Fe-Al-Hf Alloys

The effect of Hf addition on microstructures, phase relationships, microhardness, and magnetic properties of Fe₅₀Al_50?n Hf _n alloys for n = 1, 3, 5, 7, and 9 at. pct has been investigated. At all investigated compositions, the ternary intermetallic HfFe₆Al₆ τ ₁ phase forms due to the limited solid solubility of Hf in FeAl phase and tends to develop a eutectic phase mixture with the Fe-Al-based phase. The Hf concentration of the eutectic composition is found to be 7 at. pct from the microstructural examinations and the eutectic phase transition temperature is determined as 1521 K (1248 °C) independent of Hf amount by differential scanning calorimetry measurements. Furthermore, the enthalpies and activation energies (based on Kissinger and Ozawa methods) of eutectic phase transitions are reported. The minimum activation energy is calculated for the fully eutectic composition. Moreover, variation of the microhardness of Fe-Al-based alloys as a function of the Hf content is investigated, and its dependence on the thermal history of the alloys is explained.     

Microstructure and mechanical properties of ternary intermetallic compound dispersed P/M Al-Mn-X-Zr (x=Cu,Ni) alloys

Heat-resistant aluminum alloys are generally developed by dispersing stable intermetallic compounds by adding transition metals (TM) whose diffusion coefficient in aluminum alloys is low even at high temperatures. Commonly used intermetallic compounds include Al-TM binary intermetallic compounds, for example, Al₆Fe, Al₃Ti and Al₃Ni. By contrast, multicomponent intermetallic compounds are hardly used. The present study focuses on Al-Mn-Cu and Al-Mn-Ni ternary intermetallic compounds, and by finely dispersing these intermetallic compounds, attempts to develop heat-resistant alloys. Through the atomization method, Al-(4.96–5.96)Mn-(6.82–7.53)Cu-0.4Zr and Al-(5.48–8.76)Mn-(2.23–4.32)Ni-0.4Zr (in mass%) powders were fabricated, and by degassing these powders at 773 K, intermetallic compounds were precipitated. These powders were then solidified into extrudates by hot extrusion at 773 K. The microstructural characterization of powders and exrudates was carried out by XRD analysis, SEM/EDX and TEM. The mechanical properties of extrudates were determined at room temperature, 523 K and 573 K. In Al-Mn-Cu alloys, while a small amount of Al₂Cu was crystallized, precipitated Al₂₀Mn₃Cu₂ intermetallic compounds were mainly dispersed. In Al-Mn-Ni alloys, while a small amount of Al₆Mn intermetallic compounds was precipitated, the precipitated A₆₀Mn₁₁Ni₄ intermetallic compounds were mainly dispersed. Both ternary intermetallic compounds were about 200 nm in size. The compounds were elliptical, and their longitudinal direction was oriented along the extrusion direction. In the Al-Mn-Cu alloys, since the work hardening at room temperature was high, the tensile strength became 569 MPa. At elevated temperatures, since hardly any work hardening was observed, the tensile strength decreased markedly. However, in Al-Mn-Ni alloys, since the work hardening is low even at room temperature, the roomtemperature strength is not high. Thus, the decrease in tensile strength at elevated temperatures is relatively small and a high strength was obtained at 523 K and 573 K: 276 MPa and 207 MPa, respectively.     

Resistance-Spot-Welded AZ31 Magnesium Alloys: Part I. Dependence of Fusion Zone Microstructures on Second-Phase Particles

A comparison of microstructural features in resistance spot welds of two AZ31 magnesium (Mg) alloys, AZ31-SA (from supplier A) and AZ31-SB (from supplier B), with the same sheet thickness and welding conditions, was performed via optical microscopy, scanning electron microscopy (SEM), X-ray diffraction (XRD), and transmission electron microscopy (TEM). These alloys have similar chemical composition but different sizes of second-phase particles due to manufacturing process differences. Both columnar and equiaxed dendritic structures were observed in the weld fusion zones of these AZ31 SA and SB alloys. However, columnar dendritic grains were well developed and the width of the columnar dendritic zone (CDZ) was much larger in the SB alloy. In contrast, columnar grains were restricted within narrow strip regions, and equiaxed grains were promoted in the SA alloy. Microstructural examination showed that the as-received Mg alloys contained two sizes of Al₈Mn₅ second-phase particles. Submicron Al₈Mn₅ particles of 0.09 to 0.4 μm in length occured in both SA and SB alloys; however, larger Al₈Mn₅ particles of 4 to 10 μm in length were observed only in the SA alloy. The welding process did not have a great effect on the populations of Al₈Mn₅ particles in these AZ31 welds. The earlier columnar-equiaxed transition (CET) is believed to be related to the pre-existence of the coarse Al₈Mn₅ intermetallic phases in the SA alloy as an inoculant of α-Mg heterogeneous nucleation. This was revealed by the presence of Al₈Mn₅ particles at the origin of some equiaxed dendrites. Finally, the columnar grains of the SB alloy, which did not contain coarse second-phase particles, were efficiently restrained and equiaxed grains were found to be promoted by adding 10 μm-long Mn particles into the fusion zone during resistance spot welding (RSW).     

Characterization of dispersed intermetallic Phases in Rapidly Quenched Al-Ti-Ce Alloys

The microstructures of Al-3Ti-lCe (wt pct) and Al-5Ti-5Ce alloys melt-spun under controlled He atmosphere have been characterized using analytical electron microscopy. The rapidly so- lidified microstructures comprise uniform, fine-scale dispersions of intermetallic phase in an aluminum matrix, and particular attention has been given to identification of the dispersed phases. In the Al-3Ti-lCe alloy, the dispersed particles are polycrystalline with a complex twinned substructure and a diamond cubic crystal structure (α _o = 1.44 ± 0.01 nm) and composition consistent with the ternary compound Al₂₀Ti₂Ce (Al₁₈Cr₂Mg₃ structure type, space group Fd3m). In the Al-5Ti-5Ce alloy, there is, in addition to the dispersed ternary phase, a separate uniform array of fine-scale particles of the binary compound Al₁₁Ce₃. The majority of such particles have the body-centered orthorhombic structure of the low-temperature polymorph, α-Al₁₁Ce₃, but there is evidence to suggest that at least some particles developvia initial formation of the high-temperature body-centered tetragonal phase, β-Al₁₁Ce₃. The accumulated evidence sug- gests that both binary and ternary particles formed as primary phases directly from the melt during rapid solidification, leaving only small concentrations of solute in aluminum matrix solid solution. Both phases are observed to be resistant to coarsening for up to 240 hours at 400 °C.