Increasing Welding Energy Research Articles

Dissimilar ultrasonic spot welding of rare-earth containing ZEK100 magnesium-to-aluminum alloy with a zinc interlayer: Microstructural evolution and mechanical properties

The purpose of the present study was to identify microstructural changes at both Mg/Zn and Al/Zn interfaces of ultrasonic spot welded joints between a rare-earth containing ZEK100 magnesium alloy and an AA6022 aluminum alloy with a zinc interlayer, focusing on their effect on the tensile lap shear strength. During ultrasonic spot welding (USW), the diffusion or reaction was observed to occur at both interfaces, with the reaction being much faster at the Mg/Zn interface than at the Al/Zn interface. This was attributed to the higher diffusion rate of Zn in Mg and lower eutectic temperature in the Mg-Zn system along with the higher specific heat capacity and lower thermal conductivity of magnesium. With increasing welding energy, the diffusion layer at both interfaces increased. However, the diffusion layer at the Al/Zn interface was appreciably thinner than that at the Mg/Zn interface. While the eutectic (or eutectoid) structure containing Mg7Zn3, MgZn2 and MgZn intermetallic phases appeared at the Mg/Zn interface, the diffusion layer at the Al/Zn interface consisted of only Al and Zn based solid solution. The joining mechanisms were mainly mechanical interlocking and metallurgical bonding which were enhanced by grain refinement at both interfaces stemming from dynamic crystallization during USW. The peak tensile lap shear strength of the present Mg-Al dissimilar joints was attained at a welding energy of 1000 J, and the failure occurred in the diffusion layer at the Mg/Zn interface.

Ultrasonic spot welding of a clad 7075 aluminum alloy: Strength and fatigue life

The purpose of this study was to examine weldability of an AA7072-cladded high-strength AA7075-T6 via ultrasonic spot welding, focusing on the influence of welding energy on mechanical properties and failure mechanisms. Defect-free sound joints were achieved with robust bonding and grain refinement via dynamic recrystallization at the weld interface, leading to superior static and fatigue properties. The nominal tensile lap shear strength increased with increasing welding energy, and exceeded AWS D17.2 standards at higher energy levels. Interfacial failure at higher cyclic loads with distinctive elongated shear dimples and fatigue striations and transverse through-thickness failure at lower cyclic loads were observed.

Microstructure and mechanical properties of Mg-to-Al dissimilar welded joints with an Ag interlayer using ultrasonic spot welding

Lightweight ZEK100-O Mg alloy and Al6022-T43 Al alloy with an Ag interlayer were joined via ultrasonic spot welding (USW), focusing on the microstructural change and tensile lap shear strength of the welded joints in relation to welding energy. Mg/Al interface was superseded by Mg/Ag and Al/Ag interfaces, and unfavorable Mg17Al12 intermetallic compound was eliminated. Ag foil was observed to be intact in the nugget center, while it was broken or dissolved at the nugget edge at high welding energy levels. The diffusion layer at the Mg/Ag interface consisted of two distinctive sub-layers: Mg3Ag intermetallic compound adjoining Ag foil, and Mg3Ag + Mg eutectic structure adjacent to Mg. Only a thin diffusion layer consisting mainly of Ag3Al occurred at the Al/Ag interface. The tensile lap shear strength first increased, reached its peak value, and then decreased with increasing welding energy. The shear strength achieved in the present study was ∼31% higher than that of the joint without interlayer. Interfacial failure occurred at all energy levels, with Ag foil particles or fragments being stuck on both Mg and Al sides due to its intense interaction with Mg and Al via accelerated diffusion during USW. The results obtained pave the way for the challenging dissimilar welding between Mg and Al alloys.

Mechanical Behavior and Microstructure of Ultrasonic-Spot-Welded Al/Cu Dissimilar Joints with Zn Interlayer

The effect of Zn interlayer on the microstructural evolution and mechanical behavior of dissimilar ultrasonic-spot-welded Al/Cu joints was investigated. The tensile lap shear strength in relation to welding energy was analyzed. The experimental results show that two intermetallic compounds, Cu5Zn8 and Al2Cu, were generated at the interface of the ultrasonic-spot-welded Al/Cu joint with a Zn interlayer. The primary joining mechanisms of the joint included the intermetallic compound bonding and metallic bonding caused by solid shear plastic deformation. Meanwhile, with increasing welding energy, the plastic deformation of the material became more substantial. With increasing welding energy, the tensile lap shear strength of the joints first increased and then decreased for the ultrasonic-spot-welded Al/Cu joints with and without Zn interlayers. Under the energy input of 700 J, the bearing load capacity of the ultrasonic-spot-welded Al/Cu joints with a Zn interlayer improved significantly due to the observed intermetallic compound (Cu5Zn8).

Ultrasonic spot welding of 5182 aluminum alloy: Evolution of microstructure and mechanical properties

The present study is focused on the evaluation of microstructures, tensile lap shear strength and fatigue resistance of ultrasonic spot welded joints of lightweight AA 5182 aluminum alloy. The temperature quickly increased during ultrasonic spot welding (USW), with the peak temperature increasing with increasing welding energy. A “necklace”-like ultrafine structure occurred at the weld interface due to the presence of dynamic recrystallization. The tensile lap shear strength increased with decreasing number of impedance setting. When the impedance was set below 5, the tensile lap shear failure load of nearly 6 kN was attained, surpassing the requirement for the average peak load of AWS D17.2 standards. The tensile lap shear strength, failure energy, and critical stress intensity factor first increased, reached their maximum and then decreased with increasing welding energy. The maximum tensile lap shear strength of ∼150 MPa was achieved at a welding energy of 4000 J and an impedance setting of 2. The load-controlled fatigue behavior displayed a bi-linear characteristic due to the change in the failure mode from interfacial failure at the higher cyclic loads to transverse-through thickness (TTT) crack growth mechanism at the lower cyclic loads.

Ultrasonic Spot Welding of an Aluminum Alloy for Automotive Applications

To reduce fuel consumption and the resulting environment-damaging, climate-changing, costly, human death-causing emissions, lightweight aluminum alloys have been increasingly used in the transportation industry due to their low density, high specific strength, superior ductility, machinability, recyclability, and environmental friendliness. The structural applications of such aluminum alloys in the automotive industry unavoidably entail welding and joining process. While it is challenging to weld aluminum sheets via the conventional resistance spot welding developed mainly for joining steel sheets, an emerging solid-state joining technology known as ultrasonic spot welding (USW) is promising for joining aluminum alloys. This study was aimed to examine the feasibility of welding a 6022 Al alloy similarly (AA6022-to-AA6022) and dissimilarly (AA6022-to-ZEK100) in relation to welding energy. It was observed that there was a significant change in the interface grain size in the similar welding, exhibiting a characteristic fine-grained “necklace”-like structure along the welding line, while an intermetallic diffusion layer was present in the dissimilar welding of aluminum-to-magnesium alloys, and its thickness increased as the welding energy increased. The tensile lap shear strength first increased, reached its peak value at a certain energy, and then decreased with increasing welding energy. The strength of the dissimilar welded joints was about 55% of that of the similar welded joints. Three different modes of interfacial failure, nugget pull-out, and transverse through-thickness (TTT) crack growth were observed in the similar welded joints, while only interfacial failure was present in the dissimilar welded joints.

Ultrasonic spot welding of magnesium-to-aluminum alloys with a copper interlayer: Microstructural evolution and tensile properties

Ultrasonic spot welding of ZEK100-O Mg alloy to Al6022-T43 Al alloy with a Cu interlayer was successfully achieved, and relevant technical issues were investigated with focus on the microstructural evolution at Mg/Cu and Al/Cu interfaces and the tensile lap shear strength in relation to welding energy. While an interface diffusion layer occurred at both interfaces, the Mg/Cu interface diffusion layer mainly consisting of α-Mg and Mg2Cu eutectic structure grew much faster and became significantly thicker with increasing welding energy. A thin interface diffusion layer composed of primarily Al2Cu was present at the Al/Cu interface, alongside the formation of characteristic “barb”-like interlocks at lower energy levels and core-shell microstructures at higher energy levels. With increasing welding energy the tensile lap shear strength of the tri-dissimilar joints first increased, reached a peak value at an energy of 1500 J, and then decreased. The strength of ZEK100-Al6022 dissimilar joints was attained to be ∼35% higher with Cu interlayer than without Cu interlayer, reached ∼80% that of Al6022-Al6022 similar joints and almost the same strength level of ZEK100-ZEK100 similar joints. Interfacial failure was observed to occur at the Al/Cu interface at the lower energy and at the Mg/Cu interface at the higher energy.

Ultrasonic spot welding of dissimilar 2024Al alloy and SiCp/2009Al composite

Dissimilar welding of 2024Al alloy to SiC/2009Al composite was conducted via solid-state ultrasonic spot welding technique. Defect-free sound spot welds were successfully achieved. SiCp particles were observed to migrate from the composite side to 2024Al side along with the interfacial interlocking during the intense high-frequency rubbing, which increased with increasing welding energy. The microhardness across the ultrasonic spot welded similar and dissimilar joints remained nearly constant. The tensile lap shear failure load increased with increasing welding energy, and satisfied the requirements of AWS standard D17.2 for spot welding. In addition to room temperature tensile lap shear tests, all the similar and dissimilar joints were subjected to tensile tests at varying temperatures and their respective results were discussed.

Microstructure and Mechanical Properties of Ultrasonic Spot Welded Mg/Al Alloy Dissimilar Joints

Lightweight structural applications of magnesium and aluminum alloys inevitably necessitate welding and joining, especially dissimilar welding between these alloys. The objective of this study was to examine the feasibility of joining ZEK100 Mg alloy to Al6022 alloy via ultrasonic spot welding, focusing on effects of welding energy. An interface diffusion layer consisting of α-Mg and Al12Mg17 eutectic structure was observed to form, with its thickness increased from ~0.5 µm to ~30 µm with increasing welding energy from 500 J to 2000 J. The tensile lap shear peak load or strength and critical stress intensity of the welded joints first increased and then decreased with increasing welding energy, with their peak values achieved at 750 J. Fatigue life of the joints made at 750 J and 2000 J was equivalent at the lower cyclic loading levels, while it was longer for the joints made at 750 J at the higher cyclic loading levels. Fatigue fracture mode changed from interfacial failure to mainly transverse-through-thickness crack growth with decreasing cyclic loading level, which corresponded well to the bi-linear characteristic of S-N curves. Crack initiation basically occurred at the weld nugget border and at the interface between the two sheets, which can be understood via a theoretical stress analysis.

Microstructure and Mechanical Properties of an Ultrasonic Spot Welded Aluminum Alloy: The Effect of Welding Energy.

The aim of this study is to evaluate the microstructures, tensile lap shear strength, and fatigue resistance of 6022-T43 aluminum alloy joints welded via a solid-state welding technique–ultrasonic spot welding (USW)–at different energy levels. An ultra-fine necklace-like equiaxed grain structure is observed along the weld line due to the occurrence of dynamic crystallization, with smaller grain sizes at lower levels of welding energy. The tensile lap shear strength, failure energy, and critical stress intensity of the welded joints first increase, reach their maximum values, and then decrease with increasing welding energy. The tensile lap shear failure mode changes from interfacial fracture at lower energy levels, to nugget pull-out at intermediate optimal energy levels, and to transverse through-thickness (TTT) crack growth at higher energy levels. The fatigue life is longer for the joints welded at an energy of 1400 J than 2000 J at higher cyclic loading levels. The fatigue failure mode changes from nugget pull-out to TTT crack growth with decreasing cyclic loading for the joints welded at 1400 J, while TTT crack growth mode remains at all cyclic loading levels for the joints welded at 2000 J. Fatigue crack basically initiates from the nugget edge, and propagates with “river-flow” patterns and characteristic fatigue striations.

Microstructure, tensile and fatigue properties of ultrasonic spot welded aluminum to galvanized high-strength-low-alloy and low-carbon steel sheets

The microstructure evolution, tensile lap shear strength and fatigue properties of dissimilar ultrasonic spot welded (USWed) joints of aluminum to two commercial steel sheets at different welding energies were investigated. The main intermetallics at the weld interface were θ (FeAl3) in both joints along with eutectic Al-Zn in Al-to-galvanized high-strength-low-alloy (HSLA) steel joints and Fe3Al in Al-to-ASTM A36 steel joints. The welding strengths of both joints were higher than those of other dissimilar joints reported in the literature. With increasing welding energy, the maximum tensile lap shear strength increased in the Al-to-galvanized HSLA steel joints, while the lap shear strength increased up to a peak value and then decreased in the Al-to-ASTM A36 steel joints. Both the average peak welding strength and fracture energy of the Al-to-galvanized HSLA steel joints were higher than those of the Al-to-ASTM A36 steel joints. The fatigue lives of both welded joints were in agreement with or somewhat longer than other Al-to-steel USWed joints in the literature. The fatigue fracture mode changed with increasing cyclic loads in both welded joints. Fatigue crack growth was mainly characterized by the formation of fatigue striations perpendicular to the fatigue crack propagation direction.

Effect of interlayer on microstructure and mechanical properties of Al–Ti ultrasonic welds

Joints of Al6061 and Ti6Al4 V alloys with pure Al-particle interlayers were conducted using ultrasonic spot welding. The microstructure, hardness, lap shear strength and fracture energy were measured for different welding energies. With increasing welding energy delivered through the sonotrode, the lap shear strength of the joints increased, reaching about 106 MPa at a welding energy of 1100 J, at which failure occurred in the pull-out mode. In the weld region, the hardness of Al6061 alloy increased with increasing weld energy, whereas the hardness of Ti6Al4 V did not change discernibly. No brittle intermetallic compounds were observed in the joints. Moreover, two simple mechanisms were described for the formation of ultrasonic spot-welded Al–Ti joints with and without the pure Al interlayer.

Similar and Dissimilar Ultrasonic Spot Welding of 5754 Aluminum Alloy for Automotive Applications

Aluminum (Al) alloys are increasingly used in the transportation industry to reduce the weight of vehicles due to their high strength-to-weight ratio. These applications unavoidably involve similar and dissimilar joining of an automotive grade 5754 Al alloy to manufacture multi-material vehicle body structures and parts. Ultrasonic spot welding (USW), an emerging and promising solid-state joining technology, can be suitably applied to join Al alloys. In this study, 5754 Al alloy was welded in similar (Al5754-Al5754) and dissimilar (Al5754-ZEK100 Mg alloy, Al5754-HSLA steel) configurations at varying levels of welding energy. It was observed that USW had a strong effect on the interface microstructure, with fine grains present at the weld interface via dynamic recrystallization in the similar welding, while an interface diffusion layer formed in the dissimilar welding. The tensile lap shear strength increased with increasing welding energy, reached its optimum value, and then decreased with further increasing welding energy. The strength of dissimilar Al5754-ZEK100 and Al5754-HSLA steel joints was about 55% and 88% of that of the similar Al-Al joints, respectively. The dissimilar Al5754-HSLA steel joints exhibited the longest fatigue life due to the reduced stress concentration and additional strengthening arising from the brazing effect of the squeezed-out Al-Zn eutectic structure at the nugget edge.

Ultrasonic spot welded 6111-T4 aluminum alloy to galvanized high-strength low-alloy steel: Microstructure and mechanical properties

Multi-material design of automotive structures for lightweighting solutions inevitably involves dissimilar joining of lightweight aluminum alloys with steels. Solid-state ultrasonic spot welding (USW) has recently received significant attention due to its high energy efficiency in comparison with conventional fusion welding techniques. In this study USW technique was used to join Al alloy 6111-T4 to galvanized high-strength low-alloy (HSLA) steel at different welding energy levels. During USW rapid interdiffusion of Zn and Al resulted in the formation of Al-Zn eutectoid/eutectic interface layer due to fast temperature rise and high strain rate. Part of this diffusion layer was squeezed out around the nugget edge in higher welding energy conditions, creating an additional brazing effect. The tensile lap shear load increased with increasing welding energy, reached a peak value, then decreased with a further increase in energy. Two major types of failure mode, namely interfacial failure and transverse-through-thickness (TTT) failure, were observed during tensile lap shear tests. The samples welded at 2000J exhibited a longer fatigue life than those welded at a welding energy of 1000J. During fatigue interfacial failure occurred at higher cyclic load levels, while TTT failure happened at lower cyclic load levels.

Effect of welding energy on microstructure and strength of ultrasonic spot welded dissimilar joints of aluminum to steel sheets

Two dissimilar ultrasonic spot welded joints of aluminum to commercial steel sheets at different levels of welding energy were investigated. The tensile lap shear tests were conducted to evaluate the failure strength in relation to microstructural changes. The main intermetallics at the weld interface in both joints was θ (FeAl3), along with ɳ (Fe2Al5) phase in Al-to-AISI 304 stainless steel joint and Fe3Al phase in Al-to-ASTM A36 steel joint, respectively. The welding strength of Al-to-AISI 304 stainless steel weld samples was slightly higher than Al-to-ASTM A36 steel weld samples, whereas the fracture energies of Al-to-AISI 304 stainless steel weld samples were significantly higher as compared with Al-to-ASTM A36 steel weld samples. The welding strength of both Al-to-Steel welds were higher than other reported dissimilar USW joints in literature. The fracture surfaces of both weld joints exhibits the growth of IMC layer with increasing welding energy or time, whose inherent brittleness compromises the integrity of joints. In both cases, the lap shear tensile fracture occurred from the Al/Fe interface at lower energy inputs and the failure mode at higher welding energy inputs became the “transverse through-thickness crack growth” at the edge of the nugget zone on the softer Al side.

Ultrasonic spot welding of rare-earth containing ZEK100 magnesium alloy to 5754 aluminum alloy

The aim of this study was to examine the feasibility of joining a low rare-earth containing ZEK100 Mg alloy to 5754 Al alloy via solid-state ultrasonic spot welding (USW), and evaluate the interface microstructure, tensile lap shear strength, and fatigue life. A diffusion layer consisting of eutectic structure of α-Mg and Al12Mg17 intermetallic compound was observed to form at the interface during USW, and its thickness increased with increasing welding energy. The tensile lap shear strength first increased, reached a peak value and then decreased with increasing welding energy. The optimal average strength of ZEK100-Al5754 dissimilar joints arrived at ~78% of ZEK100-ZEK100 similar joints and ~55% of Al5754-Al5754 similar joints. The tensile lap shear test samples failed in an interfacial mode with partial cohesive failure through the interface diffusion layer and partial failure through Mg alloy, with Mg alloy sticking on the Al side. Fatigue life of samples welded at a welding energy of 500J was longer than that at 1000J at higher cyclic loading levels, and it became equivalent at lower cyclic loading levels. The bi-linear behavior of S-N curves corresponded well to the change in the failure characteristics. At higher cyclic loading levels interfacial failure prevailed, while the formation of transverse-through-thickness (TTT) crack at the nugget edge occurred at lower cyclic loading levels.

Ultrasonic Spot Welding of a Rare-Earth Containing ZEK100 Magnesium Alloy: Effect of Welding Energy

Ultrasonic spot welding was used to join a low rare-earth containing ZEK100 Mg alloy at different levels of welding energy, and tensile lap shear tests were conducted to evaluate the failure strength in relation to the microstructural changes. It was observed that dynamic recrystallization occurred in the nugget zone; the grain size increased and microhardness decreased with increasing welding energy arising from the increasing interface temperature and strain rate. The weld interface experienced severe plastic deformation at a high strain rate from ~500 to ~2100 s−1 with increasing welding energy from 500 to 2000 J. A relationship between grain size and Zener–Hollomon parameter, and a Hall–Petch-type relationship between microhardness and grain size were established. The tensile lap shear strength and failure energy were observed to first increase with increasing welding energy, reach the maximum values at 1500 J, and then decrease with a further increase in the welding energy. The samples welded at a welding energy ≤1500 J exhibited an interfacial failure mode, while nugget pull-out occurred in the samples welded at a welding energy above 1500 J. The fracture surfaces showed typical shear failure. Low-temperature tests at 233 K (−40 °C) showed no significant effect on the strength and failure mode of joints welded at the optimal welding energy of 1500 J. Elevated temperature tests at 453 K (180 °C) revealed a lower failure load but a higher failure energy due to the increased deformability, and showed a mixed mode of partial interfacial failure and partial nugget pull-out.

Microstructure and mechanical properties of ultrasonic spot welded copper-to-magnesium alloy joints

Abstract High-power ultrasonic spot welding (USW) was used to join copper-to-AZ31B magnesium alloy at different welding energy levels, focusing on the interfacial microstructure and strength of the dissimilar joints. The enhanced diffusion during USW led to the presence of an interface diffusion layer mainly consisting of a eutectic structure of Mg and Mg 2 Cu. The thickness of the interface diffusion layer increased with increasing welding energy or temperature at the joint interface. A unique diffusion pattern formed at high levels of welding energy of 2000 and 2500 J was attributed to the outburst of near-eutectic liquid at localized hot spots under internal pressure, which was explained in four stages. The tensile lap shear strength of the joints was observed to increase initially, reach a peak value, and then decrease with increasing welding energy. The failure of the joints made with the optimum welding parameters of 1500 J and 0.75 s occurred in the mode of cohesive failure in the eutectic structure of the interface diffusion interlayer.

Solid-state ultrasonic spot welding of SiCp/2009Al composite sheets

It has been challenging to join aluminum matrix composites (AMCs) using conventional fusion welding processes due to the occurrence of segregation and deleterious reactions between the reinforcement particles and liquid aluminum in the fusion zone. Development of robust welding processes to join AMCs thus holds the key in advanced lightweighting structural applications in the transportation sectors. The purpose of this study was to explore the weldability of AMCs via a solid-state welding technique – ultrasonic spot welding (USW). In this study 1.5mm thick 17vol.% SiCp/2009Al composite sheets in the annealing (O) and T6 conditions were subjected to USW, respectively, with the aim to demonstrate the welding feasibility of the composites. Microstructure, X-ray diffraction, microhardness and lap shear tensile tests were performed to evaluate the weld zone (WZ) characteristics in the USW joints. A characteristic band consisting of finer and denser crushed SiC particles that were uniformly embedded in the aluminum matrix was observed to occur in the WZ. The WZ of both types of joints had a much higher hardness than that of their respective base metal. The lap shear tensile fracture load increased with increasing welding energy and satisfied the requirement of AWS standard D17.2 for spot welding.

Ultrasonic spot welding of Al/Mg/Al tri-layered clad sheets

Solid-state ultrasonic spot welding (USW) was used to join Al/Mg/Al tri-layered clad sheets, aiming at exploring weldability and identifying failure mode in relation to the welding energy. It was observed that the application of a low welding energy of 100J was able to achieve the optimal welding condition during USW at a very short welding time of 0.1s for the tri-layered clad sheets. The optimal lap shear failure load obtained was equivalent to that of the as-received Al/Mg/Al tri-layered clad sheets. With increasing welding energy, the lap shear failure load initially increased and then decreased after reaching a maximum value. At a welding energy of 25J, failure occurred in the mode of interfacial failure along the center Al/Al weld interface due to insufficient bonding. At a welding energy of 50J, 75J and 100J, failure was also characterized by the interfacial failure mode, but it occurred along the Al/Mg clad interface rather than the center Al/Al weld interface, suggesting stronger bonding of the Al/Al weld interface than that of the Al/Mg clad interface. The overall weld strength of the Al/Mg/Al tri-layered clad sheets was thus governed by the Al/Mg clad interface strength. At a welding energy of 125J and 150J, thinning of weld nugget and extensive deformation at the edge of welding tip caused failure at the edge of nugget region, leading to a lower lap shear failure load.