1045 carbon steel was joined to 304 stainless steel by friction welding, and the joint was heat treated at different post-weld heat treatment (PWHT) temperature. The weld interface of as-welded joint was straight and obvious, whereas, it became indistinct after PWHT characterized by severe element diffusion. Chromium carbides such as (Cr, Fe)7C3 and Cr7C3 formed at the weld interface, the amount of which increased with the PWHT temperature increasing. In thermo-mechanically affected zone (TMAZ), the microstructure at carbon steel side of as-welded joint was quite heterogeneous, while it became homogeneous after PWHT at 400 °C. Analyses of fracture surfaces showed the amount of chromium carbide and the heterogeneous microstructure in TMAZ at carbon steel side were crucial factors in influencing fracture mechanism and properties of joints. Tensile strength and elongation of the joint were improved substantially after PWHT at 400 °C, which could reach up to the equivalent strength of stainless steel and elongation of carbon steel, because the microstructure became homogeneous to some extent and the amount of chromium carbides barely increased. The fracture mode also altered from quasi-cleavage fracture in as-welded joint to the combination of dimple fracture and quasi-cleavage fracture after PWHT.

A computational fluid dynamics model is developed to investigate undercut defect formation in high speed gas tungsten arc welding (GTAW) process. Double-ellipse arc shear stress model and modified double-ellipse arc heat source and arc pressure models are used, which are self-adaptive to weld pool surface evolution. The heat and mass transfer in weld pool and solidified weld bead profile are simulated and the undercut mechanism are discussed. The prematurely solidified periphery part at maximum width of weld pool is the initiation of undercut defect and the inward velocity component at trailing periphery due to teardrop-shaped weld pool profile promotes subsequent undercut formation, which provides an explanation to the high tendency of undercut formation during high current and high speed welding. The undercut morphology is unremarkably influenced by Marangoni force and the capillary pressure hinders undercut formation to some extent. The developed model is validated by comparing undercut morphology and gouging region profile from both simulation and experiment.

•Effect of welding conditions during Al–Mg FSW was investigated.•Heat input in Al–Mg FSW was calculated from torque.•Effect factors of welding conditions were revealed.•Sound joints were obtained with 73% tensile strength of Mg BM.Sound friction stir welded joints of 6061-T6 aluminum alloy to AZ31B magnesium alloy are obtained with the combination of intermediate rotation rate of tool (600–800 rpm) and low traverse speed (30–60 mm/min) when Mg was on advancing side, tool offset to Mg 0.3 mm, and the tensile strength of the joints could reach up to 70% of that of Mg base metal. Intermetallics consisted of Al12Mg17 and Al3Mg2 formed and the eutectic structure existed in the nugget zone. Heat input in Al–Mg dissimilar metal FSW could be calculated accurately based on measuring x-axis torque and spindle torque. Because of the differences in friction coefficient, liquation tendency and deformability between Al alloy and Mg alloy, as well as the extent of mixing, the heat input increased by placing Al on advancing side and tool offsetting to Al, the increased rotation rate and traverse speed could decrease heat input. The effect of welding conditions was explained based on the extent of material intermixing and heat input. Welding conditions which resulted in sufficient intermixing between dissimilar materials and the intermediate heat input were helpful to obtain high weld properties.

•FEM was used to study the large spot laser + MIG arc brazing–fusion welding.•The temperature field and thermal cycles were simulated and agreed well with experiments.•A fundamental energy condition was provided for this process.A finite element model was developed to investigate the thermal process of large spot laser + MIG arc brazing–fusion welding. The laser was treated as a Gaussian plane heat source, the MIG arc was performed as a modified double ellipse Gaussian plane heat source, in which the arc distortion was taken into consideration and the overheated droplet was treated as a uniform body heat source. The calculated weld bead geometry and heat-affected width of zinc coating had good agreement with experimental results. The temperature field, especially for the brazed interface, showed non-uniform and asymmetric distribution. The thermal cycles at brazed interface had obvious bimodal characteristic at arc center and laser spot center and the high-temperature zone at the brazed interface was widened duo to the introduction of laser beam compared with conventional MIG brazing–fusion welding. A fundamental processing window was determined based on the founded model to satisfy a certain energy condition, in which the Al alloy was fully penetrated and steel plate was not melted.

Dissimilar welding of aluminum bars and magnesium bars was produced by the friction welding technique. The interfacial microstructure characteristics was evaluated after friction welding of Al–Mg alloy using optical microscopy, scanning electron microscopy, as well as X-ray diffraction analysis. Friction and forge pressure were selected as variable parameters. The friction time was maintained at 10 s for a rotational speed of 2800 rpm. The chemical compositions of the interfaces of the welded joints were determined by using energy dispersive spectroscopy. Experimental results showed that intermetallic compounds (IMCs), consisting of phase β-Al3Mg2 and γ-Al12Mg17, were generated in the interfaces of the Al and Mg alloys. When the friction and forge pressure increased the thickness of IMCs layer at the interfaces decreased as a result of more mass discarded from the welding interfaces. Heavy thickness of IMCs layer seriously deteriorated the mechanical properties of the joints. Microcracks were generated along the welded interfaces of all the welded samples. Formation of microcracks could be controlled effectively under the higher friction and forge pressure. Mechanical evaluations were conducted by determining microhardness and the tensile tests. It was observed that the tensile strength of the joints depended on the friction and forge pressure and the maximum tensile strength was 138 MPa.

•TIG–MAG hybrid arc welding can increase welding speed significantly.•High mechanical properties of hybrid arc weld were obtained.•The assistant TIG arc can stabilize the MAG welding current and arc voltage.•Stable hybridization is a key factor to stabilize the welding process.A TIG–MAG hybrid arc welding process was proposed to achieve high speed welding. The influences of hybrid arc welding parameters on welding speed and weld appearance were studied through orthogonal experiment and the microstructures and mechanical properties of weld were tested and compared with that of the conventional MAG weld. The TIG–MAG hybrid arc welding speed could reach up to 3.5 m/min for bead-on-plate welding of 2.5 mm thick mild steel plate under the condition of high quality of weld appearance and 4.5 m/min for butt welding of 2 mm thick mild steel plate, respectively. The mechanical properties of hybrid arc weld were not lower than that of the conventional MAG weld. The assistant TIG arc could effectively stabilize the MAG welding current and MAG arc voltage in high speed TIG–MAG hybrid arc welding process. The stable hybridization obtained by balance between TIG and MAG welding current and proper wire-electrode distance was a key factor to stabilize the welding process.

•Large spot laser assisted GMA brazing–fusion welding technology was developed.•Large spot laser assisted GMA welding could achieve high speed and quality joining.•Effect of parameters on weld shape, mechanical properties was observed.•The function of laser and metallurgical effect of zinc were joining mechanism.Through positioning the leading laser as an auxiliary role, and the trailing arc as the main heat source, aluminum alloy (Al) was joined to galvanized steel plate with lap joint. The brazed seam width w increased with the increasing of heat input. The appropriate laser-wire distance Dlw and defocusing distance df to obtain the good fusion weld appearance were 5 mm and +20 mm, respectively. The fracture position of tensile test sample was divided into brazed interface fracture (P < 0.6 kW) and HAZAl fracture (P > 0.6 kW). The maximum tensile strength of dissimilar joint reached 75% of that of Al. The shear strength was mainly decided by heat input, and the brazed seam had the highest microhardness. Joining mechanism of this process was summarized into two factors: the effective function of laser and the metallurgical function of zinc. Compared to gas metal arc welding (GMAW), large spot laser assisted GMA process improved weld appearance and enhanced the process stability and its time-efficiency.

Aluminum alloy plates were joined to galvanized steel sheets with lap joint by laser-MIG arc hybrid brazingfusion welding with AlSi5, AlSi12, AlMg5 filler wires, respectively. The influences of Si and Mg on the microstructure and mechanical properties of the brazed-fusion welded joint were studied. The increase of Si element in the fusion weld can make the grain refined, and increase the microhardness of the fusion weld. Therefore, the microhardness in fusion weld made from AlSi12 and AlSi5 filler wires can be up to 98.4 HV0.01 and 96.8 HV0.01, which is higher than that from AlMg5 filler wire of 70.4 HV0.01. The highest tensile strength can reach 178.9 MPa made with AlMg5 filler wire. The tensile strength is 172.43 MPa made with AlSi5 filler wire. However, the lowest tensile strength is 144 MPa made with AlSi12 filler wire. The average thicknesses of the intermetallic compounds (IMCs) layer with AlSi5, AlSi12, AlMg5 filler wires are 1.49–2.64 µm. The IMCs layer made from AlSi5, AlSi12 filler wires are identified as FeAl2, Fe2Al5, Fe4Al13 and Al0.5Fe3Si0.5, that from AlMg5 filler wire are identified as FeAl2, Fe2Al5 and Fe4Al13.

Pulsed MIG arc brazing-fusion welding process was applied to realize the butt joining of galvanized steel and 5052 aluminum alloy with no enclosed slot or groove adopted. A modified flux mixture was developed to improve the butt joint performance. After applying the modified flux, weld appearance became better and the spreadability of filler metal was also greatly improved. During brazing-fusion welding process, flux floating on the surface of welding pool weakened the surface tension between filler metal and Fe and reduced the evaporation of Zn, which both led to the greatly enhanced spreadability. The analysis of intermetallic compounds (IMCs) at brazed interface showed that Fe3Al was formed at upper side of steel plate, while Fe2Al5 was formed at other areas with the consideration of its lowest Gibbs free energy under certain temperature range. With modified flux, the tensile strength of the joint could reach up to 120 MPa, which was about 60% of that of 5052 aluminum alloy base metal. From the fractured surface, two different fracture modes were identified, tear fracture and intergranular fracture, which confirmed that joint fractured in a brittle mode. The crack initiated at the front side of brazed interface due to the thicker IMCs layer, then propagated to the surface before the whole joint failed. The improved spreadability of filler metal on the top and back sides of steel plate due to the application of flux, contributed to the improvement of joint strength.Download high-res image (149KB)Download full-size image

•The continuous drive friction welding of Al alloy to Mg alloy was realized.•A continuous intermetallics layer formed along the friction interface.•The IMCs formation resulted from diffusion transformation.•The highest strength of Al/Mg joint can reach up to 101 MPa.5A33 aluminum alloy bar was joined with AZ31B magnesium alloy bar by continuous drive friction welding. The friction weldability of Al alloy to Mg alloy was investigated. The microstructure of the friction interface in joints was analyzed by optical microscopy, scanning electron microscopy, and X-ray diffraction analysis. The chemical compositions of newly formed phase on the interface were tested by energy dispersive spectroscopy. The results show that the sound joints of Al alloy to Mg alloy can be obtained by continuous drive friction welding process. The tensile strength of the joints increased with increasing friction time, and on average the highest strength could reach up to 101 MPa when friction time was 5 s. All the friction welded samples failed at the friction interface during tensile test. The fracture appearances showed almost flat surface, so the fracture of the as-welded Al/Mg joints in this experiment was brittle mode. A new reaction layer formed on the friction interface consisted of intermetallic compounds (IMCs) layer and Mg solid solution layer, and the IMCs were mainly Mg17Al12 and Al3Mg2. The type of IMCs was variable with increasing friction time. Due to high microhardness of reaction layer, the microhardness value on the interface was dramatically larger than that of the Mg base material. The thickness of hardened layer in the Mg side and softened layer in the Al side increased with increasing friction time.Download high-res image (291KB)Download full-size image