Pulsed MIG welding of AZ31B magnesium alloy

Abstract

Pulsed metal inert gas welding of AZ31B magnesium alloy is carried out, and continuous butt joints of high quality are obtained at optimised parameters. The effects of parameters on weld formation and welding stability are studied. The microstructure, mechanical properties and fracture of weld beads with different filler wires are investigated. The results show that it is a stable drop transition process with optimised parameters, which belongs to globular transfer. The precipitates in fusion zone and heat affected zone (HAZ) are uniform, dispersive and almost granular. The grain size in fusion zone is fine, and the grain size does not grow too large in HAZ compared with the base metal. The ultimate tensile strength of weld beads can be 94% of base metal, and the average elongation is 11%. Dimples and coarse tearing ridges can be observed on the fracture of the weld bead.     

Ultrafine grain structure features in spray-formed AZ31 magnesium alloy

The ultrafine grain structure was developed in spray-formed AZ31 magnesium alloy by optimizing delivery tube orifice diameter. A significant refinement of grain size ∼1 μm in ultrafine level was achieved by using 2.25 mm delivery tube orifice diameter. The tensile strength value was increased from 145 MPa of as-cast alloy to 250 MPa of spray-formed alloy, registering an enhancement of ∼72%. On the other hand, elongation was increased from 6% to 13% using 2.25 mm orifice, registering more than onefold increase in elongation. Also, hardness enhancement of ∼49% was observed in spray-formed AZ31 alloy compared to as-cast alloy. The fracture surface of spray-formed AZ31 Mg alloy evidences the mixed type of ductile and brittle fracture.     

Development of ternary Mg based alloy for soldering of AZ31B magnesium alloy

Abstract

Two kinds of ternary Mg based alloys were designed to join the AZ31B magnesium alloy plates by high frequency induction soldering with argon shielding gas. The microstructures and properties of the filler metals and joints were investigated by SEM, X-ray diffraction, differential scanning calorimetry, spreading test and tensile test. The results have shown that the microstructures of Mg–31·5Al–10Sn filler metal mainly consist of Mg₁₇Al₁₂, Mg₂Sn and a trace amount of α-Mg phases, while the microstructures of Mg–29·5Zn–1Sn filler metal include α-Mg phase and Mg₇Zn₃ with a trace of α-Mg and Mg₂Sn phases. Both of the filler metals have narrow melting zones; however, the spreading area of the Mg–31·5Al–10Sn filler metal is much larger than that of the Mg–29·5Zn–1Sn filler metal on the AZ31B base metal. The average tensile strength of solder joints with Mg–31·5Al–10Sn filler metal is a little higher than that of the latter solder joints with Mg–29·5Zn–1Sn filler metal.     

Microstructural evolution in AZ91D magnesium alloy during electron beam welding

Vacuum electron beam welding can have a low heat input, which means there is a minimum heat affected zone during welding of AZ91D magnesium alloy. From the observed microstructure, the weld of the AZ91D magnesium alloy can be divided into four regions, which are the weld metal zone, a partially-melted zone adjacent to the fusion boundary, a partially-melted zone adjacent to the base metal and the base metal zone. A sharp transition from the fusion zone to the non-melted zone, especially the characteristic partial melting microstructure and nature of the alloy elements, was observed. It was found that significant partial melting had taken place in the very narrow region around the weld metal of the AZ91D magnesium alloy. The Al content of eutectic β-Mg₁₇Al₁₂ in the partially-melted zone adjacent to the fusion boundary was close to the content in the continuously precipitated eutectic β particles in the fusion zone and much lower than the eutectic β in the base metal. The fully melted eutectic β-phase coexisted with the partially melted eutectic β phase in the partially-melted zone adjacent to the base metal.     

Microstructures and tensile properties of AZ31 magnesium alloy by continuous extrusion forming process

The objective of this study is to investigate the possibility of continuous extrusion forming (Conform process) of AZ31 magnesium alloy. The results indicate that continuous extrusion forming can refine the structure, improve the degree of the structure homogeneity and change the crystal orientation of basal plane and hence enhance the ductility but decrease tensile strength at room temperature. The fracture mechanisms of the material prepared by Conform process change from the mixture of ductile and brittle to the full dimpled rupture compared with the conventional extrudate.     

Growth and corrosion of aluminum PVD-coating on AZ31 magnesium alloy

Magnetron sputtering was applied to prepare aluminum coating on a mechanically polished AZ31 magnesium alloy. A loose oxide film was spontaneously formed on the surface of AZ31 magnesium alloy during polishing process. The aluminum coating, which was subsequently deposited on this oxide layer, presented a developed columnar microstructure. Attributed to the barrier effect of Al coating, the Al coated AZ31 showed a higher corrosion resistance than bare AZ31 in corrosion tests. Generally, Al coating is cathodically protected by magnesium alloy substrate. But it is interesting in this study that Al coating still suffered from severe corrosion due to the occurrence of the alkalization effect.     

Improving low-cycle fatigue properties of rolled AZ31 magnesium alloy by pre-compression deformation

The effect of pre-compression deformation on the low-cycle fatigue properties and cyclic deformation behavior of as-rolled AZ31 alloy was investigated by performing the stress-controlled low-cycle fatigue tests at room temperature. Fatigue properties and cyclic damage process should be closely related to the twins. The present work aimed to investigate the deformation mechanism and fatigue life caused by the introduced {1 0−1 2} twinning–detwinning from the viewpoint of stress amplitude. The results reveal that the twins contribute to the fatigue properties and cyclic damage process of AZ31 alloy. There were noticeable changes in hysteresis loops, microstructures and fatigue lives when the stress amplitude increased from 120 to 150 MPa. The fatigue life of pre-compressed samples was more superior to that of the as-rolled sample under different stress amplitudes, especially under the stress amplitude close to the tensile yield strength of the as-rolled sample.     

Optimum hot forming temperature of AZ61 magnesium alloy

A new concept of stability of materials is introduced by defining the optimum hot forming temperature for any given strain rate. This temperature is obtained through forming maps that are based on Lyapunov concepts and the introduction of a Garofalo equation in the Lyapunov criterion. This new approach is applied to a magnesium alloy AZ61. Torsion tests were carried out in the temperature range 574–734?K and strain rate range 0.7–8.7?s^?1 and the microstructures were determined using optical microscopy. Using the peak stress, optimum workability at 630?K is obtained at 12?s^?1. The results and the maps are compared with data and maps of other authors for AZ61 alloys in various states.     

EBSD analysis of fatigue crack initiation behavior in coarse-grained AZ31 magnesium alloy

Plane bending fatigue tests had been conducted to investigate fatigue crack initiation mechanism in coarse-grained magnesium alloy, AZ31, with hexagonal close-packed (hcp) crystallographic structure. The initial crystallographic structure was analyzed by an electron backscatter diffraction (EBSD) method. Subsequently, a fatigue test was periodically terminated and time-series EBSD analyses were performed. Basal slip and primary twin operated predominantly. In a twin band, secondary twin operated, and resulted in the fatigue crack initiation. The crack initiation was strongly affected by Schmid factors in the grains and twin bands.     

Dynamic recrystallization behavior and microstructural evolution of Mg alloy AZ31 through high-speed rolling

High-speed rolling (HSR) is known to improve the workability of Mg alloys significantly, which makes it possible to impose a large reduction in a single pass without fracture. In the present study, dynamic recrystallization (DRX) behavior and microstructural and textural variations of Mg alloy AZ31 during a HSR process were investigated by conducting rolling with different imposed reductions in the range of 20%–80% at a high rolling speed of 470 m/min and 400 °C. High-strain-rate deformation during HSR suppresses dislocation slips but promotes twinning, which results in the formation of numerous twins of several types, i.e., {10–12} extension twins, {10–11} and {10–13} contraction twins, and {10–11}–{10–12} double twins. After twinning, high strain energy is accumulated in twin bands because their crystallographic orientations are favorable for basal slips, leading to subsequent DRX at the twin bands. Accordingly, twinning activation and twinning-induced DRX behavior play crucial roles in accommodating plastic deformation during HSR and in varying microstructure and texture of the high-speed-rolled (HSRed) sheets. Area fraction of fine DRXed grains formed at the twin bands increases with increasing rolling reduction, which is attributed to the combined effects of increased strain, strain rate, and deformation temperature and a decreased critical strain for DRX. Size, internal strain, and texture intensity of the DRXed grains are smaller than those of unDRXed grains. Therefore, as rolling reduction increases, average grain size, stored internal energy, microstructural inhomogeneity, and basal texture intensity of the HSRed sheets gradually decrease owing to an increase in the area fraction of the DRXed grains.     

Corrosion fatigue behavior of extruded magnesium alloy AZ31 in sodium chloride solution

In the present study, corrosion fatigue experiments were done using the extruded magnesium alloy AZ31 in the 3% sodium chloride solution to clarify the corrosion fatigue characteristics of the material. Corrosion fatigue lives greatly decreased as compared with those in laboratory air. It was also clarified that most of the corrosion fatigue life (70–80%) at the lower stress amplitude is occupied with the period of the corrosion pit growth. Corrosion fatigue lives were evaluated quantitatively by dividing the corrosion fatigue process into the following two periods, i.e. (1) the corrosion pit growth period preceding the crack initiation from the pit and (2) the crack growth period before the specimen failure. In the analysis, the law of the corrosion pit growth proposed by authors was used to deal with the above first period. The evaluated results corresponded well to the experimental results.     

Modeling and Simulation of the Microstructure Evolution of the Gas-atomized Alloy Droplets during Spray Forming

In order to understand the solidification process of an atomized droplet and predict the fraction solidification of droplets with flight distance during spray forming, a numerical model based on thepopulation dynamics approach is developed to describe the microstructure evolution under the common action of the nucleation and growth of grains.The model is coupled with droplets heat transfer controlling equations and solved for Al-4.5 wt pct Cu alloy. It is demonstrated that the numerical results describe the solidification process well.     

Dissimilar material laser welding between magnesium alloy AZ31B and aluminum alloy A5052-O

Joining technology of lightweight dissimilar metals between magnesium and aluminum alloys is essential for realizing hybrid structure cars and other engineering applications. In the present study, the normal center-line welding of lap joint was carried out by laser welding. It was found that the intermetallic layer formed near interface between two metals significantly degraded the joining strength. FEM heat transfer analysis was carried out to find out an available method to control penetration depth and width of molten metal, which contributes to control thickness of intermetallic compound layer. Based on the results of FEM analysis, the edge-line welding of lap joint was carried out, which could easily control the thickness of intermetallic layer and successfully obtained high joining 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.     

In situ observation of ageing process and new morphologies of continuous precipitates in AZ91 magnesium alloy

An in situ observation of the precipitating process of γ-Mg₁₇Al₁₂ phase in die-cast AZ91 magnesium alloy, was carried out with a transmission electron microscope equipped with a heating stage maintained at 473 K for 8 h. In addition to the thin plate-shaped continuous precipitates, continuous precipitates with rod-shaped and the Potter orientation relationship were observed and analyzed with transmission electron microscopy including high-resolution transmission electron microscopy techniques. It was also observed firstly that there exist plate-shaped continuous precipitates with the Pitsch-Schrader orientation relationship in the die-cast AZ91 magnesium alloy.     

铸态AZ81镁合金ECAP态组织与性能研究

采用自制的90°模具,经Bc路径在温度为300℃下研究对比了铸态及不同道次的等通道挤压(ECAP)态AZ81镁合金微观组织和力学性能.结果表明ECAP随着挤压道次的增加,AZ81镁合金显微组织和力学性能发生显著变化.当挤压到4道次,平均晶粒尺寸由原来铸态的145um细化为9.6um,拉伸断口韧窝明显增多;抗拉强度从180 MPa提高到306 MPa,延伸率和硬度分别达到15.8%和142HL.分析表明,AZ81镁合金在高温挤压过程中Mg17Al12相粒子被破碎,并部分溶入基体,$-Mg基体与%-Mg17Al12相互相阻碍其晶粒长大,获得细小晶粒组织.     

Cyclic hardening/softening behaviour in AZ31B magnesium alloy based on infrared thermography

The work-hardening/softening behaviour of AZ31B magnesium alloy during high cycle fatigue was investigated. The superficial temperature evolution during fatigue tests was used as a criterion for the different levels of work-hardening/softening. The microstructures under different cycles were observed by transmission electron microscope. Tensile test (with post-fatigue) was conducted to quantify the work-hardening/softening behaviour which showed that high dislocation density after cyclic loading lead to high tensile strength. The temperature evolution of the specimens with different levels of work-hardening/softening during tensile tests is related to the microstructures; the results indicated that the temperature rise of the specimen with high density dislocation was lower. Microstructures after tensile tests showed that high dislocation density after cyclic loading would lead to high twinning density.     

AZ31镁合金非等温拉深性能的研究

针对AZ31镁合金等温拉深性能差的问题,提出了AZ31镁合金的非等温拉深工艺.通过平底杯形冲头拉深试验研究了不同冲头温度和板料温度对AZ31镁合金非等温拉深性能的影响,确定了使AZ31镁合金具有最佳拉深性能的板料和冲头温度范围.实验结果表明,除了板料和冲头温度之外,拉深速度和润滑条件对AZ31镁合金的非等温拉深性能也有重要影响.     

Metadynamic recrystallization behavior of AZ61 magnesium alloy

Metadynamic recrystallization (MDRX) behavior of AZ61 magnesium alloy and its effects on flow behavior and microstructure evolution have been investigated in this study. Towards this end, a set of double-hit hot compression tests was conducted under strain rate of 0.1 s⁻¹ at 400 °C. To differentiate the static and metadynamic recrystallization dominant strain regions, the first stage of deformation was carried out up to the different pre-strains with a constant inter-pass annealing time of 200 s. The results indicated that the MDRX is predominant recrystallization mechanism where the pre-strains are higher than 0.35. Furthermore, to investigate the influence of MDRX on subsequent flow behavior and the related microstructure, an elaborated inter-pass annealing treatment was executed employing a range of inter-pass annealing time (2–500 s). The results show that the progress of MDRX leads to an increase in the flow stress as well as the rate of work hardening encountered in the subsequent deformation. Additionally, the microstructural examinations confirm that the observed hardening phenomenon is a consequence of grain growth evolved from MDRX and its direct effect on the onset of dynamic recrystallization at the second stage of deformation.     

Improved properties of friction stir-welded AZ31 magnesium alloy by post-weld heat treatment

Mechanical properties and microstructure of friction stir-welded AZ31 based on variety post-weld heat treatment (PWHT) temperatures were evaluated, and an optimal PWHT condition was identified. At rotational speed of 1200?rev?min^?1 and welding speed of 300?mm?min^?1, the average yield tensile, tensile strength and elongation of friction stir-welded joints was 92.5?MPa, 199.1?MPa and 7.3%, respectively. It was found that (300°C – 1?h) heat treatment after welding was more beneficial than other heat treatments in enhancing the mechanical properties and homogenising grain size. The maximum yield and tensile strength was 139.9 and 238.4?MPa, respectively, tensile longitudinal and compressive transverse residual stress could be effectively eliminated, and the fatigue strength increased 34.2% comparing with as-welded joints.