J.C. Feng

Microstructure of Al–Mg dissimilar weld made by cold metal transfer MIG welding

Abstract A modified metal inert gas welding process called cold metal transfer was successfully applied to dissimilar Mg and Al welding with AlSi5 filler metal. Optical microscopy, scanning electron microscopy, energy dispersive X-ray spectroscopy and X-ray diffraction were used to characterise the microstructure, alloy element distribution and phase constituents of the welded joint. The microhardness distribution was determined by microhardness tester on the fusion zone. The super low heat input and the addition of Si inhibited the creation and growth of brittle intermetallic compound in weld metal, which improved the strength of the joint. An obvious multilayer microstructure, which comprised of solid solution, eutectic structure, Mg17Al12, and Mg2Al3 layer, was observed in the fusion zone adjacent to the Mg substrate. It degraded the strength, leading to the fracture of the joint in tensile test. The fracture morphology presented the typical brittleness fracture mode.

Effect of heat treatment on tensile properties of friction stir welded joints of 2219-T6 aluminium alloy

Abstract The effect of post-weld heat treatment (PWHT) on the tensile properties of friction stir welded (FSW) joints of 2219-T6 aluminium alloy was investigated. The PWHT was carried out at aging temperature of 165°C for 18 h. The mechanical properties of the joints were evaluated using tensile tests. The experimental results indicate that the PWHT significantly influences the tensile properties of the FSW joints. After the heat treatment, the tensile strength of the joints increases and the elongation at fracture of the joints decreases. The maximum tensile strength of the joints is equivalent to 89% of that of the base material. The fracture location characteristics of the heat treated joints are similar to those of the as welded joints. The defect free joints fracture in the heat affected zone on the retreating side and the joints with a void defect fracture in the weld zone on the advancing side. All of the experimental results can be explained by the hardness profiles and welding defects in the joints.

Effects of post-weld heat treatment on microstructure and mechanical properties of friction stir welded joints of 2219-O aluminium alloy

Abstract Post-weld heat treatment (PWHT) of 2219-O aluminium alloy friction stir welding joints was carried out at solution temperatures of 480, 500 and 540°C for 32 min followed by aging at 130°C for 9 h. The effects of PWHT on the microstructure and mechanical properties of the joints were investigated. Experimental results show that PWHT causes coarsening of the grains in the weld, and the coarsening degree increases with increasing solution temperature. The tensile strength of the heat treated joints increases with increasing solution temperature. The maximum tensile strength can reach 260% that of the base material at the solution temperature of 540°C. PWHT has a significant effect on the fracture locations of the joints. When the solution temperature is lower than 500°C, the joints fracture in the base material; when the temperature is higher than 500°C, the joints fracture in the weld. The change of the fracture locations of joints is attributed to the presence of precipitate free zones beside the grain boundaries and coarsening equiaxed grain structures in the weld.

Nugget growth characteristic for AZ31B magnesium alloy during resistance spot welding

Abstract An axisymmetrical finite element model for studying the distribution of temperature for resistance spot welding (RSW) to predict weld nugget growth of AZ31B Mg alloy was developed by employing a contact resistance model based on the microcontact theory. The RSW of a Mg alloy, with regard to nugget formation, consists of the initiation of a nugget in the first cycle, a rapid growth of the nugget in the following 2–3 cycles and a plateau of nugget growth after ∼4 cycles. Because of its high thermal conductivity, low melting point and low volumetric heat capacity, Mg alloy has many characteristics during nugget formation, compared with Al alloy and mild steel. In the RSW of a Mg alloy, the contact resistance in the interface has an important effect on the nugget formation; the welding time is similar to that in Al alloy but smaller than that in low carbon steel; and the welding current lever is required slightly lower than that in Al alloy but higher than that in low carbon steel. The computational simulations based on this model agree well with the experimental data.

New technique of filling friction stir welding

Abstract A new technique of filling friction stir welding (FFSW) relying on a semiconsumable joining tool has been developed to repair the keyhole left at the end of friction stir welding (FSW) seam. The conventional non-consumable tool of FSW was transformed, and a semiconsumable joining tool consisting of alloy steel shoulder and aluminium alloy joining bit was designed to create a solid state joint. Using the combined plastic deformation and flow of the consumable joining bit and the wall of the keyhole, the FFSW process is able to repair the keyhole with both metallurgical and mechanical bonding characteristics, and the FSW seam can be achieved without keyhole or other defects. The relative tensile strength and elongation of the FFSW joint are 84·3 and 98·9% of the base weld without defects respectively.

Interface behaviours and mechanical properties of filling friction stir weld joining AA 2219

Abstract In the present work, 7·8 mm thick AA 2219 rolled plates were successfully joined without keyholes using semiconsumable tools by filling friction stir welding (FFSW). The shoulder further effect was performed to enhance mechanical stir, and mechanical properties were enhanced effectively. The results showed that the bonding interface of AA 2219 bit and keyhole is defect free. The softened region on the advanced side is the weakest part of FFSW joint rather than the bonding interface of the keyhole. The average ultimate tensile strength and elongation are 172 MPa and 11·2%, equal to 90 and 82% of the base weld without defects respectively. Excellent bonding interface and mechanical properties of FFSW joints have been exhibited.

Interfacial phenomena of cold metal transfer (CMT) welding of zinc coated steel and wrought aluminium

Abstract The interfacial microstructures and intermetallic compounds produced by cold metal transfer (CMT) welding of wrought aluminium to zinc coated steel have been investigated. The results showed that the reaction layer mainly consists of FeAl3 phase and the thickness of the layer was decreased to 7–8 μm owing to the low heat input of the CMT method. And the thin reaction layer guarantees the tensile property of the joint. At last, according to the interface characteristic of the dissimilar metal joint, the interfacial reaction process was described in detail.

Effect of Silver Content on Microstructure and Properties of Brass/steel Induction Brazing Joint Using Ag-Cu-Zn-Sn Filler Metal

The induction brazing of brass to steel using Ag-Cu-Zn-Sn filler metal was investigated in this study. The influence of Ag content on the microstructure and properties were analyzed by means of optical microscopy, scanning electron microscopy and electron probe microanalysis. Defect free joint was achieved using Ag-Cu-Zn-Sn filler metal. The microstructure of the joint was mainly composed of Ag-based solid solution and Cu-based solid solution. The increase of Ag content and the cooling rate both led to the increase of the needle like eutectic structure. The tensile strength decreased with the increase of Ag content. The tensile strength at room temperature using Ag25CuZnSn filler metal reached 445 MPa. All fractures using Ag-Cu-Zn-Sn filler metal presented ductile characteristic.

Microstructure characteristics of resistance spot welds of AZ31 Mg alloy

Abstract The experimental investigation was carried out to study the weld microstructure of resistance spot welding of AZ31 Mg alloy 1 mm thick. A fine and homogeneous non-equilibrium microstructure of globular α grains, surrounded by eutectic mixtures of α and β (Mg17Al12), was achieved. The thermal–electrical–mechanical analysis model was employed to simulate the thermal history and the temperature gradient. It was found that a combination of the welding conditions and the particular thermophysical properties of the AZ31Mg alloy established a uniform temperature distribution throughout the weld pool and this thermal condition is ideal for nucleation throughout the melt metal and equiaxed grain structure forming.

Effect of welding time on microstructure and tensile shear load in resistance spot welded joints of AZ31 Mg alloy

Abstract Magnesium alloys are becoming widely recognised as the candidates to replace steel and aluminium alloys, especially in automotive industries and thereby they have potential to be widely used. In the present paper, resistance spot welding (RSW) was performed on 1 mm thick AZ31B magnesium alloy sheets. The effect of welding time on the microstructure and tensile shear load was investigated. Some interesting characteristics for nugget growth were observed during the RSW of Mg alloy. The nuggets were initiated in the first cycle, grown rapidly in the following 2–4 cycles and reached a plateau after around 6 cycles. The microstructure of the nugget consisted of non-equilibrium a -phase dendrites surrounded by a certain amount of β -Mg17 Al12 precipitated in the grain boundaries. With increased welding time, the size and shape of grains in the nugget would be more uniform and the width of corona bonds would be larger. Tensile shear tests showed that the tensile shear load of the joints increased with increasing welding time when the welding time was , <10 cycles. The maximum tensile shear load is up to 1980 N.