AZ31 Magnesium Alloy Joints Research Articles

Effect of the gap width in AZ31 magnesium alloy joints obtained by friction stir welding

Thin AZ31 magnesium alloy sheets, i.e., 2 mm thick, are welded by Friction Stir Welding (FSW) in butt joint configuration using gap width up to 1.15 mm. All welds present good surface finishing and no internal defects, except for the weld produced using the maximum gap width. A reduction of the weld thickness within the Stir Zone is seen with the increase in gap width, leading to a maximum thickness reduction of 8.5%. Microstructure and Vickers hardness investigations reveal no influence of the gap width on these properties. Up to a gap width of 0.51 mm, a slight decrease in the Ultimate Tensile Strength (UTS) is observed with increasing gap width. For larger gap widths, the UTS, as well as the fracture strain, are constant. To keep the metallurgical integrity, a maximum gap width of 1 mm seems acceptable for joints of thin AZ31 magnesium alloy sheets produced by FSW.

The Activation and Evolution of Twinning during Bending of Friction Stir Welded AZ31 Magnesium Alloys

AZ31 magnesium alloy joints obtained by friction stir welding with rotation speed of 1400 rpm and welding speed of 200 mm/min were subsequently subjected to the three-point bending process. The bending behavior, microstructure evolution and twinning mechanism were investigated. The results indicate that the stress-strain curve appeared as power-law shape during tension and the stress-strain curve appeared as work hardening shape during compression. However, the stress-strain curve during bending is different and macrographs of face and base bending indicated that the severe strain localization was present during bending of FSWed AZ31 magnesium alloy joint. Three concave regions formed due to texture distribution and stress state in the weld zone. In those regions, the grains had favorable orientation with c-axis parallel to the direction of tensile stress and abundant twins were activated. It can be proved by electron backscatter diffraction (EBSD) analysis—two twinning mechanisms were activated during bending—that is, ~56° { 10 1 ¯ 1 } contraction twin and ~86° { 10 1 ¯ 2 } expansion twin, in which { 10 1 ¯ 2 } twinning was main plastic deformation mechanism of joint and the number of twins was proportional to the compressive stress in corresponding areas. The twinning resulted in lattice rotation about 86° around < 1 2 ¯ 10 > direction and changed the orientation distribution of original crystal.

Microstructure and properties of probeless friction stir spot welding of AZ31 magnesium alloy joints

Abstract The AZ31 magnesium alloy with a thickness of 1.8 mm was welded by the probeless friction stir spot welding process without Zn interlayer. The influence of process parameters on joint microstructure and mechanical properties was investigated by using different rotating speeds and dwell time. Microstructure of joints is divided into three regions: stir zone, thermomechanically-affected zone and heat-affected zone. With the increase of rotation speed and dwell time, the depth of stir zone gradually increases, and hook defects extend from the interface of two plates to the surface of the upper plate. The tensile shear strength of joints and two fracture modes (shear fracture and plug fracture) are closely related to hook defects. The maximum tensile shear strength of the joint is 4.22 kN when rotation speed and dwell time are 1180 r/min and 9 s, respectively. Microhardness value and its fluctuation in upper sheet are evidently higher than those of the lower sheet.

Effects and distribution of TiC on the nanoparticle strengthening A-TIG welded AZ31 magnesium alloy joints

AZ31 magnesium alloy sheets were A-TIG welded using mixed activating flux, which contained different ratio of TiC nanoparticles (TiCp) and TiO2 powder, as surface coating. From mechanical test, the values of ultimate tensile strength and microhardness increased from 120.2 MPa to 181.7 MPa, 46.4 HV to 84.2 HV, respectively, which mainly resulted from the grain boundary strengthening of α-Mg and the dispersion strengthening of TiCp. The theoretical calculation suggested that the improvement of the yield strength was mainly the combination of the Hall-Petch and Orowan strengthening mechanisms, and the mismatch strain caused by different coefficient of thermal expansion between the matrix and the particles. Numerical simulation showed that the TiCp had a centralized tendency toward middle and bottom part of welding pool and was confirmed by the micro-zone microhardness test results in the fusion zoon.

The effect of axial external magnetic field on tungsten inert gas welding of magnesium alloy

The influences of axial external magnetic field on the microstructure and mechanical property of the AZ31 magnesium (Mg) alloy joints were studied. The microstructure of Mg alloy joint consisted of the weld seam, heat affected zone and base metal zone. The average grain size of weld seam welded with magnetic field is 39 μm, which is 38% smaller than that of the joint welded with absence of magnetic field. And the microhardness of weld seam increases with the help of magnetic field treatment, owing to the coarse grain refinement. With coil current of 2.0A, the maximum mechanical property of joint increases 6.7% to 255 MPa over the specimen without magnetic field treatment. Furthermore, fracture location is near heat affected area and the fracture surface is characterized with ductile fracture.

In-process control strategies for friction stir welding of AZ31 sheets with non-uniform thickness

Two different in-process control strategies were developed and compared with the aim to produce AZ31 magnesium alloy joints by friction stir welding on sheet blanks with a non-uniform thickness. To this purpose, sheets with dip or hump zones were welded by either changing the rotational speed or the tool plunging in order to keep constant the value of the vertical force occurring during the welding stage of the process. The influence of the main process parameters on the tool force, the micro- and macromechanical properties, and the joints microstructures in the dip and hump zones were analyzed. The results showed that using the rotational speed change-based approach, the hump zones are subjected to increased heat input with consequent increase of the heat-affected zone extension and average grain size (up to 11 μm). On the contrary, using the tool plunge change-based approach, the weld seam thickness in the dip zones is reduced, resulting in a decreased joint strength, with respect to tool rotation approach, equal to about 18%.

Development of liquid-nitrogen-cooling friction stir spot welding for AZ31 magnesium alloy joints

A liquid-nitrogen-cooling friction stir spot welding (C-FSSW) technology was developed for welding AZ31 magnesium alloy sheets. The liquid-nitrogen cooling degraded the deformability of the welded materials such that the width of interfacial cracks increased with increasing cooling time. The grain size of the stirred zone (SZ) and the heat-affected zone (HAZ) of the C-FSSW-welded joints decreased, whereas that of the thermomechanically affected zone (TMAZ) increased with increasing cooling time. The maximum tensile shear load of the C-FSSW-welded joints welded with a cooling time of 5 or 7 s was larger than that of the friction stir spot welding (FSSW)-welded joint, and the tensile shear load decreased with increasing cooling time. The microhardness of the C-FSSW-welded joints was greater than that of the FSSW-welded joint. Moreover, the microhardness of the SZ and the HAZ of the C-FSSW-welded joints increased, whereas that of the TMAZ decreased, with increasing cooling time.

Effects of graphene nanoplates on microstructures and mechanical properties of NSA-TIG welded AZ31 magnesium alloy joints

The effects of graphene nanoplates (GNPs) on the microstructures and mechanical properties of nanoparticles strengthening activating tungsten inert gas arc welding (NSA-TIG) welded AZ31 magnesium alloy joints were investigated. It was found that compared with those of activating TIG (A-TIG), and obvious refinement of α-Mg grains was achieved and the finest α-Mg grains of fusion zone of NSA-TIG joints were obtained in the welded joints with TiO2+GNPs flux coating. In addition, the penetrations of joints coated by TiO2+GNPs flux were similar to those coated by the TiO2+SiCp flux. However, the welded joints with TiO2+GNPs flux coating showed better mechanical properties (i.e., ultimate tensile strength and microhardness) than those with TiO2+SiCp flux coating. Moreover, the generation of necking only occurred in the welded joints with TiO2+GNPs flux.

Effects of heat treatment on the activated flux TIG-welded AZ31 magnesium alloy joints

The effects of heat treatment on the microstructures and mechanical properties of tungsten inert gas-welded AZ31 magnesium alloy joints with Cr2O3 flux coating were investigated by microstructural observations, microhardness, and tensile tests. The results showed that the activating flux of Cr2O3 improved the weld penetration and the depth/width ratio of the tungsten inert gas-welded AZ31 magnesium alloy joints markedly. Heat treatment (1) eliminated the coarsened network-like β-Mg17(Al, Zn)12 particles which formed in activating tungsten inert gas and (2) resulted in more uniform grain size distribution of partially melted zone. Apart from that, either the microhardness of the seams, the ultimate tensile strength, or the elongation of the welded joints was improved by aging treatment, while too high aging temperature would give rise to a sharply decreased ultimate tensile strength.

Effects of nano-SiC particles on the FSSW welded AZ31 magnesium alloy joints

The microstructure of nano-SiC enhancing friction stir spot welding (FSSW) joint with dwell time of 3 s was characterised by an onion ring structure which consisted of alternate SiC-free zones and SiC-rich zones where SiC particles refined the grains. However, onion ring structure disappeared and SiC particles dispersed homogeneously when dwell time was 5 s. The microhardness of stir zone (SZ) and tensile shear load of SiC enhancing FSSW joint were higher than those of conventional FSSW joints. After heat treatment at 200°C for an hour, grains of the SZ grew substantially and coursed reduction in mechanical properties of joints, while grain size of SZ and tensile shear load of SiC enhancing joint was invariant but the microhardness of SZ increased.

The effects of interface gap on weld strength during overlapping fiber laser welding of AISI 304 stainless steel and AZ31 magnesium alloys

This paper reports on an experimental study involving examination of the effects of an interfacial gap in the range of 0–0.3 mm on the weld strength and weld appearance of lap welds produced by laser welding. AISI 304 stainless steel and AZ31 magnesium alloy joints were processed separately using an IPG YLR-1000 fiber laser. The weld joint strengths were mechanically tested by both tensile shear and T-peel testing. The test results indicate that an adequate amount of interface gap significantly improves weld strength and surface appearance for both AISI 304 stainless steel and the AZ31 magnesium alloy. In addition, it is shown that porosity formation in laser welding AZ31 magnesium alloy can be mitigated by adding an optimal interface gap. Finding the optimum gap size can maximize the quality of lap welds within the constraints of the laser equipment.

Effects of current on nanoparticle reinforced magnesium alloy welding joints

Effects of welding current on macromorphologies, microstructures and mechanical properties of nano-SiC particles strengthening activating flux tungsten inert gas welded AZ31 magnesium alloy joints were investigated by scanning electron microscope and energy dispersive X-ray spectrometer observations and microhardness and tensile tests. The results showed that SiC particles mainly dwelled in the centre and bottom zone of welding pool. Moreover, with the welding current increasing, the mechanical properties of the welding joints were improved by the increased SiC particles in welding pool, the depth/width ratios of weld pool and the refined α-Mg grain size. However, the α-Mg grains coarsened and the mechanical properties of joints decreased due to excessive heat input in welding process when welding current over 115 A.

Effects of dwell time on the microstructures and mechanical properties of water bath friction stir spot-welded AZ31 magnesium alloy joints

In this work, a new water bath friction stir spot welding (WB-FSSW) process was developed. The effects of dwell time on the microstructures and mechanical properties of the WB-FSSW-welded AZ31 magnesium alloy joints were investigated by microstructural observation, tensile tests, and microhardness tests. It was found that compared with those of ordinary FSSW welds, the WB-FSSW welds have poorer tensile shear force, almost equal tensile shear strength per unit area τ and higher microhardness mainly due to the narrower bonded zone width of the hooks and microstructures in the welded joints, respectively. Moreover, with the increase in the dwell time, the microhardness and the tensile shear strengthen per unit area τ of the WB-FSSW-welded joints increased firstly and then decreased because of the fluctuant evolution of grain size of α-Mg and the volume fraction of the β-Mg17All2. The optimal dwell time for the WB-FSSW welding was from 10 to 15 s whereby the finest grains and the highest mechanical properties of the welded joints can be achieved.

Effects of nano-particles strengthening activating flux on the microstructures and mechanical properties of TIG welded AZ31 magnesium alloy joints

Abstract In this paper, AZ31 magnesium alloy joints were processed by nano-particles strengthening activating flux tungsten inert gas (NSA-TIG) welding, which was achieved by the mixed TiO2 and nano-SiC particles coated on the samples before welding tests. The macro/micro structural observation and mechanical properties evaluation of the welding joints were conducted by using optical microscope, scanning electron microscope, energy dispersive X-ray spectroscopy, X-ray diffraction and tension and microhardness tests. The results showed that nano-particles strengthening activating flux effective improved the microstructure, microhardness in fusion zone, ultimate tensile strength of the TIG welding joints. In addition, the chemical reaction between part of SiC particles and AZ31 magnesium alloy produced Al4C3 and Mg2Si in the joints. The Al4C3 performed as nucleating agents for α-Mg and the dispersed Mg2Si and SiC particles enhanced the mechanical properties of the NSA-TIG welding joints. However, large heat input induced by the increase of the surface coating density of the nano-particles strengthening activating flux, increased the α-Mg grain sizes and weakened the mechanical properties of the welded joints. Therefore, the grain size of α-Mg, distribution of β-Mg17Al12, Mg2Si and SiC particles together influenced the evolution of the mechanical properties of the NSA-TIG welded AZ31 magnesium alloy joints.

Effects of welding current on properties of A-TIG welded AZ31 magnesium alloy joints with TiO2 coating

The effects of welding current on the macro-morphology, microstructure and mechanical properties of tungsten inert gas (TIG) welded AZ31 magnesium alloy joints with TiO2 coating were investigated. The results showed that the increase of welding current led to the increase in the depth/width ratio and deteriorated the surface appearance of the welded seams with TiO2 coating. The grain size of α-Mg and the amount of granular β-Mg17Al12 particles in the welded seams also increased. The welded joints with TiO2 coating exhibited a deeper weld penetration and larger grain size compared with the welded joint without TiO2 coating. When the welding current was less than 130 A, the ultimate tensile strength of the welded joints with TiO2 coating increased with the increase in welding current and then decreased when the welding current was greater than 130 A. The average microhardness of the heat-affected zone and fusion zone decreased gradually with the increase of welding current.

Development of novel heating tool friction stir spot welding (HT-FSSW) for AZ31 magnesium alloy

In this work, a new heating tool friction stir spot welding (HT-FSSW) process was developed, and its impacts on the microstructure and mechanical properties of the welded AZ31 magnesium alloy joints were investigated by microstructure observation, tensile tests and microhardness tests. An increase in the heating tool temperature resulted in a decrease in the grain size of the stir zone (SZ) and an increase in the grain size of the thermomechanically affected zone (TMAZ). The rising heating tool temperature also aggrandised the bonded zone width and enhanced the tensile shear load strength per unit area of the HT-FSSW welded joints. With an increase in the heating tool temperature, the microhardness of SZ increased while that of the TMAZ decreased. Moreover, the slope of the Hall–Petch relationship between microhardness and grain size of the TMAZ is larger than that of the SZ.

Effects of SiC on the Strengthening Activated Tungsten Inert Gas (SA-TIG) Welded of Magnesium Alloy

The effects of SiC particles on the morphologies, microstructures, and mechanical properties of tungsten inert gas welded AZ31 magnesium alloy joints with TiO2 flux coating were studied. The results showed that an increase in the amount of the TiO2 coating improved the weld penetration and the depth/width (D/W) ratio of the TIG welded AZ31 magnesium alloy seams. The α-Mg grains of FZ of welded joints refined gradually with the addition of micro-sized SiC (<30%) /nano-sized SiC (<40%) particles in coatings, and then coarsened when more SiC particles added. In addition, the microhardness of the FZ and ultimate tensile strength of the welded joints increased with the addition of SiC particles, and nano-sized particles have more strong efficiency on the improvement of mechanical properties compared with micro-sized SiC particles.

Effects of fluxes on distribution of SiC particles and microstructures and mechanical properties of nanoparticles strengthening A-TIG (NSA-TIG) welded magnesium alloy joints

The effects of fluxes and the size of SiC particles on the morphologies, microstructures and mechanical properties of tungsten inert gas welded AZ31 magnesium alloy joints were investigated. The results showed that addition of SiC particles in the activating fluxes led to the decrease in all the weld penetrations while the surface appearance of welded joints improved in the case of TiO2 activating flux was adopted. Grain refinement was obtained in the welded seams with the addition of SiC particles in TiO2 flux and the effect of nanosized SiC particles on grain refinement is found to be more impressive than that of microsized SiC particles. The microhardness of the fusion zone and ultimate tensile strength of the welded joints increased with the addition of SiC particles in TiO2 activating flux and nanosized particles have more strong efficiency on the improvement of mechanical properties compared with microsized SiC particles. SiC particles did not improve the mechanical properties of A-TIG welded joints with CaF2 activating flux.

Effects of SiC ceramic particles on microstructures and mechanical properties of gas tungsten arc welded AZ31magnesium alloy with Cr2O3 flux coating

The effects of SiC ceramic particles on the macromorphologies, microstructures and mechanical properties of tungsten inert gas welded AZ31 magnesium alloy joints with Cr2O3 flux coating were investigated. The results showed that an increase in the amount of SiC ceramic particles contained in the Cr2O3 flux coating led to the gradual decrease in weld penetrations. The weld penetrations of all welded seams with flux coatings were obviously higher than those without flux coating. Grain refinement was obtained in the welded seams when SiC ceramic particles were added in the Cr2O3 flux coating. With an increase in the amount of SiC ceramic particles, the ultimate tensile strength value of the welded joints increased at first and then decreased. Moreover, the microhardness of the fusion zone increased gradually with an increase in the addition of SiC ceramic particles in the Cr2O3 flux coating.

Effects of activating fluxes on pulsed laser beam welded AZ31 magnesium alloy

Effects of activating fluxes on macromorphologies, microstructures and mechanical properties of low power pulse laser beam welding of AZ31 magnesium alloy joints were investigated by microstructural observations and mechanical properties tests (including the ultimate tensile strength and hardness). The results showed that activating flux (SiO2) is more effective in improving the laser welded penetrations when the heat input is at a critical value (E = 53·42 J mm−1). Five typical fluxes (SiO2, TiO2, ZnCl2, CaCl2 and CaF2) all increased the weld penetrations and D/W ratios of the welded joints. The morphologies of grains in the band zones with fluxes are characterised by dendritic crystals. Moreover, the widths of the band zones with oxide and chloride fluxes are wider than that with fluorine flux. The mechanical properties of the welded joints with fluxes were lower than those without flux. The fracture surface of the welded joints with fluxes exhibited a character of a mixed fracture.