The microstructure evolution and nucleation mechanisms of dynamic recrystallization in hot-deformed Inconel 625 superalloy

Hot compressions tests of Inconel 625 superalloy were conducted using a Gleeble-1500 simulator at different strains between 900 °C and 1200 °C with a strain rate of 0.1 s⁻¹. Optical microscope, transmission electron microscope and electron backscatter diffraction technique were employed to investigate the microstructure evolution and nucleation mechanisms of dynamic recrystallization. It was found that both the size and fraction of dynamically recrystallized grains increase with increasing deformation temperature. However, the size of dynamically recrystallized grains almost remains constant with increasing deformation strain. The dominant nucleation mechanism of dynamic recrystallization in Inconel 625 superalloy deformed at 1150 °C is the discontinuous dynamic recrystallization, which is characterized by the bulging of the original grain boundaries accompanied with twining. The continuous dynamic recrystallization characterized by progressive subgrain rotation occurs simultaneously in dynamic recrystallization process, although it can only be considered as an assistant nucleation mechanism at the early stage of hot deformation.     

Material flow and mechanical behaviour of dissimilar AA2024-T3 and AA7075-T6 aluminium alloys friction stir welds

The scope of this investigation is to evaluate the effect of joining parameters on the mechanical properties, microstructural features and material flow of dissimilar aluminium alloys (3 mm-thick AA2024-T3 and AA7075-T6 sheets) joints produced by friction stir welding. Mechanical performance has been investigated in terms of hardness and tensile testing. Material flow using the stop action technique has also been investigated in order to understand the main features of the mixing process. No onion ring formation has been observed; the boundary between both base materials at the stir zone is clearly delineated, i.e., no material mixing is observed. A non-stable rotational flow inside the threads has been identified due to the formation of a cavity on the rear of the pin. Microstructural observation has revealed the development of a recrystallised fine-grained stir zone, with two different grain sizes resulting from the two different base materials.     

Effect of governing metal thickness and stack orientation on weld quality and mechanical behaviour of resistance spot welding of AA5754 aluminium

A study was carried out to investigate the effect of governing metal thickness (GMT) and stack orientation on weld quality and mechanical behaviour of resistance spot welded (RSW) AA5754 aluminium. Individual samples from 27 different joint stacks in three test geometries; lap-shear, coach-peel and cross-tension were evaluated for quasi-static and fatigue performance; micro examination was also conducted on some of the samples to assess weld quality. The results derived from over 1000 samples show that: the GMT has a significant effect on welding quality by controlling progression of weld nugget from under-developed to over penetrated. The GMT also determines the feasible quasi-static joint strength regardless of stacks in the three joint geometries tested, though the effect differs with respect to test geometry. The fatigue behaviour is dominated by the effect of GMT on attainable weld size, overall joint stiffness and stress concentration, providing good quality of weld nuggets is achieved. No notable effect of stack orientation on weld quality and joint strength was found with respect to the joint stack asymmetry and welding orientation to the electrodes. These fundamental relationships between weld qualities, joint strength, GMT and stack orientation for RSW of aluminium will have significant relevance to design and manufacturing communities.     

Improvement of weld temperature distribution and mechanical properties of 7050 aluminum alloy butt joints by submerged friction stir welding

Submerged friction stir welding (FSW) in cold and hot water, as well as in air, was carried out for 7050 aluminum alloys. The weld thermal cycles and transverse distributions of the microhardness of the weld joints were measured, and their tensile properties were tested. The fracture surfaces of the tensile specimens were observed, and the microstructures at the fracture region were investigated. The results show that the peak temperature during welding in air was up to 380 °C, while the peak temperatures during welding in cold and hot water were about 220 and 300 °C, respectively. The temperature at the retreated side of the joint was higher than that at the advanced side for all weld joints. The distributions of microhardness exhibited a typical “W” shape. The width of the low hardness zone varied with the weld ambient conditions. The minimum hardness zone was located at the heat affected zone (HAZ) of the weld joints. Better tensile properties were achieved for joint welded in hot water, and the strength ratio of the weld joint to the base metal was up to 92%. The tensile fracture position was located at the low hardness zone of the weld joints. The fracture surfaces exhibited a mixture of dimples and quasi-cleavage planes for the joints welded in cold and hot water, and only dimples for the joint welded in air.     

Characterization of microstructure,mechanical properties and corrosion resistance of dissimilar welded joint between 2205 duplex stainless steel and 16MnR

The joint of dissimilar metals between 2205 duplex stainless steel and 16MnR low alloy high strength steel are welded by tungsten inert gas arc welding (GTAW) and shielded metal arc welding (SMAW) respectively. The microstructures of welded joints are investigated using scanning electron microscope, optical microscope and transmission electron microscopy respectively. The relationship between mechanical properties, corrosion resistance and microstructure of welded joints is evaluated. Results indicate that there are a decarburized layer and an unmixed zone close to the fusion line. It is also indicated that, austenite and acicular ferrite structures distribute uniformly in the weld metal, which is advantageous for better toughness and ductility of joints. Mechanical properties of joints welded by the two kinds of welding technology are satisfied. However, the corrosion resistance of the weldment produced by GTAW is superior to that by SMAW in chloride solution. Based on the present work, it is concluded that GTAW is the suitable welding procedure for joining dissimilar metals between 2205 duplex stainless steel and 16MnR.     

Characterization of friction stir welded boron carbide particulate reinforced AA6061 aluminum alloy stir cast composite

Development of welding procedures to join aluminum matrix composite (AMCs) holds the key to replace conventional aluminum alloys in many applications. In this research work, AA6061/B₄C AMC was produced using stir casting route with the aid of K₂TiF₆ flux. Plates of 6 mm thickness were prepared from the castings and successfully butt joined using friction stir welding (FSW). The FSW was carried out using a tool rotational speed of 1000 rpm, welding speed of 80 mm/min and axial force of 10 kN. A tool made of high carbon high chromium steel with square pin profile was used. The microstructure of the welded joint was characterized using optical and scanning electron microscopy. The welded joint showed the presence of four zones typically observed in FSW of aluminum alloys. The weld zone showed fine grains and homogeneous distribution of B₄C particles. A joint efficiency of 93.4% was realized under the experimental conditions. But, FSW reduced the ductility of the composite.     

Microstructure and mechanical properties of resistance spot welded dissimilar thickness DP780/DP600 dual-phase steel joints

In this study, resistance spot welding (RSW) experiments were performed in order to evaluate the microstructure and mechanical properties of single-lap joints between DP780 and DP600. The results show that the weld joints consist of three regions including base metal (BM), heat affected zone (HAZ) and fusion zone (FZ). The grain size and martensite volume fractions increase in the order of BM, HAZ and FZ. The hardness in the FZ is significantly higher than hardness of base metals. Tensile properties of the joints were described in terms of the failure modes and static load-carrying capabilities. Two distinct failure modes were observed during the tensile shear test of the joints: interfacial failure (IF) and pullout failure (PF). The FZ size plays a dominate role in failure modes of the joints.     

Effect of welding speed on microstructural and mechanical properties of friction stir welded Inconel 600

In order to evaluate the properties of a friction stir welded Ni base alloy, Inconel 600 (single phase type) was selected. Sound friction stir welds without weld defect were obtained at 150 and 200 mm/min in welding speed, however, a groove like defect occurred at 250 mm/min. The electron back scattered diffraction (EBSD) method was used to analyze the grain boundary character distribution. As a result, dynamic recrystallization was observed at all conditions, and the grain refinement was achieved in the stir zone, and it was gradually accelerated from 19 μm in average grain size of the base material to 3.4 μm in the stir zone with increasing the welding speed. It also has an effect on the mechanical properties so that friction stir welded zone showed 20% higher microhardness and 10% higher tensile strength than those of base material.     

Microstructure and mechanical properties of friction spot welded 6061-T4 aluminum alloy

Friction spot welding (FSpW) is a relatively new solid state joining technology developed by GKSS. In the present study, FSpW was applied to join the 6061-T4 aluminum alloy sheet with 2 mm thickness. The microstructure of the weld can be classified into four regions, which are stir zone (SZ), thermo-mechanically affected zone (TMAZ), heat affected zone (HAZ) and the base material (BM), respectively. Meanwhile, defects such as bonding ligament, hook and voids are found in the weld, which are associated to the material flow. The hardness profile of the weld exhibits a W-shaped appearance and the minimum hardness is measured at the boundary of TMAZ and SZ. Both the tensile/shear strength and cross-tension strength reach the maximum of 7117.0 N and 4555.4 N at the welding condition of the rotational speed of 1500 rpm and duration time of 4 s. Compared to cross-tension strength, the tensile/shear strength were stable with the variation of processing parameters. Three different fracture modes are observed under tensile/shear loading, which are plug type fracture, shear fracture and plug-shear fracture. There are also there different fracture modes under cross-tension loading, which are plug type fracture (on the upper sheet), nugget debonding and plug type fracture (on the lower sheet).     

Mechanical and microstructural investigation of friction spot welded AA6181-T4 aluminium alloy

Friction spot welding (FSpW) is a solid state welding process suitable for spot joining lightweight low melting point materials like aluminium and magnesium alloys. The process is performed by plunging a rotating three-piece tool (clamping ring, sleeve and pin) that creates a connection between sheets in overlap configuration by means of frictional heat and mechanical work. The result is a spot welded lap connection with minimal material loss and a flat surface with no keyhole. FSpW has been performed in a 1.7 mm-thick AA6181-T4 aluminium alloy using different welding parameters (rotation speed and joining time) aiming to produce high quality connections in terms of microstructure and mechanical performance. Microstructural features of the FSpW connections were analysed by optical microscopy; while mechanical performance was investigated in terms of hardness and tensile testing. Connections with shear strength close to 7 kN were obtained with high reproducibility. The results also showed that geometric features of the connection play an important role on the fracture mechanism and hence on the mechanical performance of the connections.     

Microstructural aspects and mechanical properties of friction stir welded AA2024-T3 aluminium alloy sheet

In the present work, aluminium alloy AA2024-T3 thin sheets were joined by the Friction Stir Welding – FSW – process. Butt joints were obtained in 1.6 mm sheets, using an advancing speed of 700 mm/min. These joints were characterised by optical, scanning electron microscopy, tensile and fatigue mechanical tests. The results showed that the resulting microstructure is free of defects and the tensile strength of the welded joints is up to 98% of the base-metal strength. Fatigue tests result indicates an equivalent stress intensity factor (kt) of approximately 2.0 for the welded samples. Consequently, the FSW process can be advantageous compared to conventional riveting for airframe applications.     

Effect of precipitation on post-heat-treated Inconel 625 alloy after friction stir welding

This study evaluated the mechanical properties of friction stir welded and post-heat-treated Inconel 625 alloy. Friction stir welding (FSW) was performed at rotation and traveling speeds of 200 rpm and 100 mm/min, respectively; heat treatment was carried out after welding at 700 °C for 100 h in vacuum. As a result, the application of FSW on Inconel 625 alloy led to the grain refinement in the stir zone, which resulted in increase in mechanical properties than those of the base material. Especially, applying heat treatment after FSW led to the improvement of mechanical properties of the welds; microhardness and tensile strength increased by more than 30% and 50%, respectively, as compared to FSW alone.     

Microstructural stability and high-temperature mechanical properties of AZ91 and AZ91 + 2RE magnesium alloys

The effects of 2 wt.% rare earth element addition on the microstructure evolution, thermal stability and shear strength of AZ91 alloy were investigated in the as-cast and annealed conditions. The as-cast structure of AZ91 consists of α-Mg matrix and the β-Mg₁₇Al₁₂ intermetallic phase. Due to the low thermal stability of this phase, the strength of AZ91 significantly decreased as the temperature increased. The addition of rare earth elements refined the microstructure and improved both thermal stability and high-temperature mechanical properties of AZ91. This was documented by the retention of the initial fine microstructure and ultimate shear strength (USS) of the rare earth elements-containing material after long-term annealing at 420 °C. The improved stability and strength are attributed to the reduction in the volume fraction of β-Mg₁₇Al₁₂ and retention of the thermally stable Al₁₁RE₃ intermetallic particles which can hinder grain growth during the annealing process. This behavior is in contrast to that of the base material which developed a coarse grain structure with decreased strength caused by the dissolution of β-Mg₁₇Al₁₂ after exposure to high temperature.     

Predicting residual distortion of aluminum alloy stiffened sheet after friction stir welding by numerical simulation

Friction stir welding (FSW) has been widely applied in aluminum alloy manufacturing with its advantages of low residual stress and small deformation. But some FSW cases indicated that the residual distortion was still significant if a large size sheet be welded. In order to reduce the residual distortion of large aluminum alloy sheet after FSW, a general method adopted in this paper was that some stiffeners were designed on the sheet before it be welded. The process of FS-welded structure (sheet with stiffeners) was numerically simulated by finite element analysis (FEA) method. Based on the numerical simulation, the residual distortion of the structure was predicted and the effect of stiffeners was investigated. First, a three-dimensional FEA model was developed to analyze the welding process on a sheet, and the simulation results were verified by FSW experiments. Then, the verified model was applied on structure to compute residual distortion. The prediction displayed that the distortion pattern of structure was convex in longitudinal direction and concave in transversal direction after FSW. In simulation results, the distortion shape of structure was similar to that of sheet, but the distortion value of structure was much smaller. The comparison of simulation results showed that the residual plastic strain generated by FSW on the sheet and the structure was nearly the same, so the part of distortion which was reduced by stiffeners was the distortion induced by buckling.     

Tensile behavior of dissimilar friction stir welded joints of aluminium alloys

The heat treatable aluminium alloy AA2024 is used extensively in the aircraft industry because of its high strength to weight ratio and good ductility. The non-heat treatable aluminium alloy AA5083 possesses medium strength and high ductility and used typically in structural applications, marine, and automotive industries. When compared to fusion welding processes, friction stir welding (FSW) process is an emerging solid state joining process which is best suitable for joining these alloys. The friction stir welding parameters such as tool pin profile, tool rotational speed, welding speed, and tool axial force influence the mechanical properties of the FS welded joints significantly. Dissimilar FS welded joints are fabricated using five different tool pin profiles. Central composite design with four parameters, five levels, and 31 runs is used to conduct the experiments and response surface method (RSM) is employed to develop the model. Mathematical regression models are developed to predict the ultimate tensile strength (UTS) and tensile elongation (TE) of the dissimilar friction stir welded joints of aluminium alloys 2024-T6 and 5083-H321, and they are validated. The effects of the above process parameters and tool pin profile on tensile strength and tensile elongation of dissimilar friction stir welded joints are analysed in detail. Joints fabricated using Tapered Hexagon tool pin profile have the highest tensile strength and tensile elongation, whereas the Straight Cylinder tool pin profile have the lowest tensile strength and tensile elongation. The results are useful to have a better understanding of the effects of process parameters, to fabricate the joints with desired tensile properties, and to automate the FS welding process.     

Effects of trace Er addition on the microstructure and mechanical properties of Mg–Zn–Zr alloy

The effects of trace Er addition on the microstructure in Mg–9Zn–0.6Zr alloy during casting, homogenization, pre-heating, and hot extrusion processes were examined. The mechanical properties of alloys with and without Er were compared. The results showed that Er exhibited a lower solubility in solid magnesium and formed thermally stable Er- and Zn-bearing compounds. The Er-bearing alloy exhibited a considerably improved deformability, as well as a fine and uniform microstructure. Moreover, dynamic precipitation of fine MgZn₂ particles with a modified spherical morphology occurred during hot extrusion, resulting in a tensile yield strength of 313 MPa and a high elongation to failure value of 22%. Further aging of the Er-bearing alloy led to an increment of another 30 MPa in yield strength. In addition, Er markedly increased the thermal stability of the alloy structure.     

Influence of friction stir welding process and tool parameters on strength properties of AA7075-T6 aluminium alloy joints

The aircraft aluminium alloys generally present low weldability by traditional fusion welding process. The development of the friction stir welding has provided an alternative improved way of satisfactorily producing aluminium joints, in a faster and reliable manner. In this present work, the influence of process and tool parameters on tensile strength properties of AA7075-T₆ joints produced by friction stir welding was analysed. Square butt joints were fabricated by varying process parameters and tool parameters. Strength properties of the joints were evaluated and correlated with the microstructure, microhardness of weld nugget. From this investigation it is found that the joint fabricated at a tool rotational speed of 1400 rpm, welding speed of 60 mm/min, axial force of 8 kN, using the tool with 15 mm shoulder diameter, 5 mm pin diameter, 45 HRc tool hardness yielded higher strength properties compared to other joints.     

Effect of tool rotational speed and probe length on lap joint quality of a friction stir welded magnesium alloy

The effects of tool rotational speed and probe length on lap joint quality of friction stir welded 2-mm AZ31B-H24 magnesium alloy were investigated in terms of welding defects, microstructure and mechanical properties. Tensile shear load initially increases with increasing tool rotational speed but decreases with further increase. However, the tensile shear load always increases with increasing probe length. The highest shear strength is obtained using a set of welding parameters resulting in a downward hooking defect at the maximum stress location of the top sheet. Sound lap joints with low distortion, lack of cavity, minor kissing bond and preferable hooking defects, and high tensile shear strength were successfully obtained, indicating the great potential of friction stir welding technique for magnesium alloys.     

Effects of Sn addition on as-cast microstructure, mechanical properties and casting fluidity of ZA84 magnesium alloy

The effects of Sn addition on the as-cast microstructure, mechanical properties and casting fluidity of the ZA84 magnesium alloy are investigated. The results indicate that adding 0.5–2.0 wt.%Sn to the ZA84 alloy not only can result in the formation of Mg₂Sn phase but also can refine the Mg₃₂(Al, Zn)₄₉ phase and suppress the formation of Mg₃₂(Al, Zn)₄₉ phase, and with the increase of Sn amount from 0.5 wt.% to 2.0 wt.%, the morphology of Mg₃₂(Al, Zn)₄₉ phase gradually changes from coarse continuous and/or quasi-continuous net to relatively fine quasi-continuous and/or disconnected shapes. In addition, adding 0.5–2.0 wt.%Sn to the ZA84 alloy can improve the tensile and creep properties, and casting fluidity of the alloy. Among the Sn-containing ZA84 alloys, the ZA84 alloy added 1.0 wt.%Sn exhibits the best ultimate tensile strength, elongation and casting fluidity while the ZA84 alloy added 2.0 wt.%Sn has the best yield strength and creep properties.     

Interface microstructure and mechanical properties of arc spot welding Mg–steel dissimilar joint with Cu interlayer

A novel technology was developed for the arc spot welding of AZ31 Mg alloy to Q235 steel with Cu as interlayer. The mechanisms of bonding dissimilar materials were investigated using mechanical and metallurgical examinations. Results show that the joining of Mg alloy to steel with Cu involved two bonding mechanisms: weld-brazing by the Cu transition layer at the interface edge and bonding by a micron-scale composite transition layer of Al₃Cu₄Fe₃ and Fe₄Cu₃ intermetallic phases at the interface center. The additional reaction of Cu increased the reaction temperature and composition ranges at the interface. It also elicited a bridge effect that improved the weldability of Mg alloy and steel by new formed phases.