Microstructural and mechanical properties of friction stir welded aluminum/copper lap joints

This paper focuses on the microstructural and mechanical properties of the friction stir welding (FSW) of 1060 aluminum alloy to a commercially pure copper. A number of FSW experiments were carried out to obtain the optimum mechanical properties by adjusting the rotational speed and welding speed in the range of 750–1500 rpm and 30–375 mm/min, respectively. Various microstructures with different morphologies and properties were observed in the stir zone. The results indicated that Al₄Cu₉, AlCu and Al₂Cu are the main intermetallic compounds formed in the interfacial region. The effect of formation of hard and brittle intermetallic phase at the interface of the joints on the shear strength of the joint is discussed.     

Formation of interfacial microstructure in a friction stir welded lap joint between aluminium alloy and stainless steel

The interfacial microstructure produced through tool transit of a friction stir welded lap joint between an aluminium alloy and stainless steel was studied by transmission electron microscopy in order to clarify its early stages of formation. Transmission electron microscopy studies of the bottom surface of the exit hole revealed the presence of several mixed layers of an ultrafine intermetallic compound (IMC) and stainless steel. The joining between dissimilar materials was achieved through a continuous flow of the stirred aluminium alloy into the mixed layers and the resultant growth of the ultrafine IMCs due to the heat induced by the friction between the tool and the specimen. The continuous thin reaction layer finally produced at the interface was found to be stronger than the base aluminium alloy.     

Microscale evaluation of mechanical properties of friction stir welded A6061 aluminium alloy/304 stainless steel dissimilar lap joint

Abstract

Microscale evaluation of the mechanical properties of a friction stir welded A6061/SUS 304 grooved lap joint was performed using a microtensile test and transmission electron microscopy. The microtensile test revealed that ～62% of the area along which the rotating tool passed the specimen was regarded as the bonded region and that the joint was fractured at the A6061 matrix owing to the formation of very thin interfacial reaction layers. Equiaxed aluminium grains were observed at the interface of the specimen after it was fractured, indicating that the interface deformed only slightly during the microtensile test. It should be noted that although the maximum tensile strength of the joint was approximately the same as that of the base alloy, the proof stress of the joint decreased with the dissolution of the β″ phase in the A6061 aluminium alloy.     

Numerical and experimental investigation on friction stir lap welding of aluminium to steel

Use of multimaterial fabrication such as aluminium to steel to reduce overall vehicular body weight has gained significant attention in the automotive industries. Since fusion welding of aluminium to steel is difficult, friction stir welding of the same is considered as an effective recourse. Quantitative studies on friction stir welding of aluminium to steel are thus important but scarce in the literature. We present here a numerical and experimental study on friction stir lap welding of AA6061 to high strength interstitial free coated steel sheets under different combinations of tool rotational speed and welding speed. The computed values of thermal cycle, torque and traverse force are found to be in good agreement with the corresponding experimentally measured values. The computed thermal cycles along the AA6061 to steel interfaces are related qualitatively with the experimentally measured trend and distribution in Fe–Al intermetallics along the weld joint interface.     

High strength lap joint of aluminium and stainless steels fabricated by friction stir welding with cutting pin

Abstract

Dissimilar lap joints of aluminium and stainless steel were first friction stir welded by the tool with a cutting pin. The results showed that sound joints could be obtained by this method. When the pin was inserted into the lower steel sheet, macrointerlocks were formed by the steel flashes plugging into the upper aluminium at both sides of the nugget bottom. At the aluminium/steel interface, a thin intermetallic compound (IMC) layer and the mechanical bonding of microinterlocks were formed. In addition, the aluminium near the interface was also strengthened by grain refinement and IMC particles. Therefore, the beneficial effect of the macrointerlocks provided by the steel flashes was removed, the shear strength of the joint reached 89·7 MPa, which was even higher than that of the base metal of aluminium.     

Evaluation of heat input during friction stir welding of aluminium alloys

Heat input is one of the key parameters governing the quality and service properties of friction stir welds. By using a calorimetric technique, the heat inputs generated during the friction stir welding of the aluminium alloys, 1100 and 5083, were measured over a wide range of welding parameters. An empirical equation to estimate the heat input using the welding parameters was established based on a multiple regression analysis of the results. The effect of the heat input on the final grain size of the stir zone was also investigated by the electron backscatter diffraction method. The quantitative relationships between the input variables, heat inputs, and final grain sizes in the stir zone were derived.     

Interface shape and microstructure controlled dissimilar friction stir lap welded steels

Abstract

When fusion welding is conducted on the dissimilar materials between a reduced activation ferritic/martensitic steel F82H steel and an austenite stainless steel SUS 316 steel, δ ferrite is generally formed and inevitably deteriorates the weld properties. In this study, dissimilar welding of F82H to SUS 316 steel was successfully achieved by friction stir lap welding technique. It revealed that the shape and microstructure of the joint interface can be controlled by controlling the welding temperature, in another word, by changing the applied load. By controlling the welding temperature at ～710°C, a sound dissimilar joint can be obtained with a smooth joint interface and no mixed microstructure, despite the relative overlapping position of the steel plates. All the dissimilar joints showed high shear tensile strength and fracture in the base metal of F82H steel plate, which has lower strength than the SUS 316 steel plate at room temperature.     

Fatigue properties of friction stir welded joint of ultrafine-grained 2024 aluminium alloy

Ultrafine-grained (UFG) 2024 aluminium alloy prepared by the equal channel angular pressing was friction stir welded (FSW). The high cycle fatigue and crack growth behaviour of the FSW joint were investigated in air and NaCl solution, respectively. This study demonstrated that FSW was a viable technique for joining UFG materials. The UFG microstructure was retained in the nugget zone (NZ). Compared with the UFG base metal (BM), FSW joint exhibited lower ultimate tensile strength and hardness, and the minimum hardness value was located in the heat affected zone (HAZ). NaCl solution significantly reduced the fatigue strength of FSW joint. Fatigue crack propagation rates in the NZ and HAZ were slower than that in the BM in the whole fatigue life.     

Influence of aluminium alloy type on dissimilar friction stir lap welding of aluminium to copper

A heat-treatable (AA 6082) and a non-heat treatable (AA 5083) aluminium alloys were friction stir lap welded to copper using the same welding parameters. Macro and microscopic analysis of the welds enabled to detect important differences in welding results, according to the aluminium alloy type. Whereas important internal defects, resulting from ineffective materials mixing, were detected for the AA 5083/copper welds, a relatively uniform material mixing was detected in the AA 6082/copper welds. Micro-hardness testing and XRD analysis also showed important differences in microstructural evolution for both types of welds. TEM and EBSD-based study of the AA 5083/copper welds revealed the formation of submicron-sized microstructures in the stirred aluminium region, for which untypically high hardness values were registered.     

Microcavities accompanying a zigzag line in friction stir welded A6082-T6 alloy joint

Abstract

An Al–Mg–Si alloy was friction stir welded (FSW), and the microstructure of the zigzag line in the welds was investigated using optical microscopy, energy dispersive X-ray spectroscopy and scanning electron microscopy. The effect of the zigzag line on the mechanical properties of the as welded joints was also examined. It was found that in the welds with high heat input, small discontinuous cavities or microcracks of several micrometres in size exist along the zigzag line, and the microcracked zigzag line was found to significantly affect the face bend properties of the FSW joint, but had limited influence on the tensile properties of the butt welds. In joints with low heat input, the zigzag line was only composed of oxide particles, no cavities or microcracks were detected at the zigzag line.     

Filling exit holes of friction stir welding lap joints using consumable pin tools

Abstract

In this study, filling friction stir welding was used to remove the exit hole of friction stir welding lap joints made from AA5456 sheets. For this purpose, the exit holes were filled by consumable pins with various geometries and different pin applying methods. Then, the structures and mechanical properties of the resulting joints were investigated. Results showed that the strength of 7% higher than the strength of the joint with the non-filled exit hole, ～91% of the corresponding defect free joints, is obtainable with this technique. The best results were found by a pin with 11° cone angle, 8?mm diameter and 7?mm length, and with a 6?mm plunge without rotation.     

Analysis of process parameters effects on dissimilar friction stir welding of AA1100 and A441 AISI steel

Abstract

A prominent benefit of friction stir welding process is to join plates with dissimilar material. In this study, an attempt is made to find effects of tool offset, plunge depth, welding traverse speed and tool rotational speed on tensile strength, microhardness and material flow in dissimilar friction stir welding of AA1100 aluminium alloy and A441 AISI steel plates. Here, one factor at a time experimental design was utilised for conducting the experiments. Results indicated the strongest joint obtained at 1·3?mm tool offset and 0·2?mm plunge depth when the tool rotational speed and linear speed were 800?rev min^??1 and 63?mm min^??1 respectively. The maximum tensile strength of welded joints with mentioned optimal parameters was 90% aluminium base metal. Fracture locations in tensile test at all samples were in aluminium sides. Owing to the formation of intermetallic compounds at high tool rotational speed, the microhardness of joint interface goes beyond that of A441 AISI steel.     

Interface structure in a Fe–Ni friction stir welded joint

Friction stir welding of commercially pure iron and nickel was carried out using a polycrystalline cubic boron nitride tool. The macroscopic flow of the two phases generally corresponded to that predicted by the mixing models proposed in the literature. The interface region, however, revealed striations of the phases suggesting a complex flow pattern. Transmission electron microscopy and high resolution Auger analyses revealed that the width of the inter-diffusion zone was about 1.5 μm in single pass and 1.8 μm in double pass joints. The measured concentration profiles could be fitted using calculated profiles generated with static diffusion rates reported for iron and nickel. The diffusion profiles also indicated stabilization of the austenite phase at the interface, which it is proposed are the result of high levels of plastic strain.     

Influence of Zn coating on friction stir spot welded magnesium-aluminium joint

For friction stir spot welded (FSSW) magnesium–aluminium joints, the formation of Mg–Al intermetallics in the hook region and cracking at the interface damaged seriously the strength of Mg–Al joints, resulting in lower joint load of only 0.8?±?0.2?kN. When adding the hot-dipped Zn coating on the Al substrate surface prior to FSSW, a brazed layer, composed of Mg–Zn and Al–Zn diffusion zones at the edge of the shoulder, and a transition layer, composed of MgZn₂, Zn-rich zone and residual Zn in the hook region, were formed in the FSSW Mg–Al joint, eliminating the cracking and Mg–Al intermetallics in the FSSW joint without the Zn coating. The load of the joint with the Zn coating increased to 3.7?±?0.3?kN.     

Characterization of aluminum/steel lap joint by friction stir welding

The welding of a lap joint of a commercially pure aluminum plate to a low carbon steel plate (i.e., Al plate top, and steel plate bottom) was produced by friction stir welding using various rotations and traveling speeds of the tool to investigate the effects of the welding parameters on the joint strength. The joint strength depended strongly on the depth of the pin tip relative to the steel surface; when the pin depth did not reach the steel surface, the joint failed under low applied loads. Meanwhile, slight penetration of the pin tip to the steel surface significantly increased the joint strength. The joint strength tended to increase with rotationspeed and slightly decrease with the increase in the traveling speed, although the results were quite scattered. The effects of the welding parameters were discussed metallographically based on observations with optical and scanning electron microscopes.     

Effects of microstructural asymmetries across friction stir welded AA2024 joints on mechanical properties

The effect of offset of the centre of a tensile specimen to the weldline on global tensile properties of friction stir welded AA2024 joints was investigated using experiments and numerical analysis. The size and geometry of these discrete zones, such as the central low hardness zone, the low hardness zone-I near the thermo-mechanically affected zone, the low hardness zone-II near the base metal and the base metal, were determined from a cross-sectional hardness map. Results show that tensile specimens with different area of the joint placed in the centre of the specimen do not affect the tensile strength and fracture path of a joint, strain is affected. Predictions based on local tensile properties follow well the measured global tensile behaviour.     

Microstructural investigation and mechanical properties of dissimilar friction stir welded magnesium alloys

Abstract

Dissimilar joints of magnesium alloys were obtained by friction stir welding. Detailed microstructure and texture examinations were performed on the joints. Significant difference in microtexture distribution and microstructural features is observed between crown and stir zones in the dissimilar joints. However, an overall effect of these factors on mechanical properties of layered joints is not obvious as both the up and middle samples present quite similar yield strength and strain hardening behaviour. Moreover, both samples fracture in the Mg–3Al–1Zn alloy side. The fracture of the middle sample starts at the boundary of transition and stir zones, while the up sample starts in the crown zone side.     

Material flow and core/multi-shell structures in a friction stir welded aluminum alloy with embedded copper markers

The objective of this study was to reveal the material flow and temperature distribution in a thick aluminum plate during friction stir welding and examine the subsequent microstructural change with particular attention to the reaction between copper marker and aluminum matrix. It is shown that the material adjacent to the threaded pin was transported from the top to bottom non-symmetrically, and then was forced to move upwards at a small distance from the pin due to the constraint of an extrusion-die-like configuration. The interaction between the copper marker and aluminum matrix led to the formation of (i) a unique core/multi-shell microstructure consisting of copper core, inner shell of AlCu and outer shell of Al₂Cu, and (ii) a composite band containing uniformly-distributed Al₂Cu particles with refined grains due to the lower stacking fault energy of copper and the pinning role of Al₂Cu particles.