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.     

Gas tungsten arc welding of fine-grained AZ31B magnesium alloys made by powder metallurgy

AZ31B magnesium alloys with various grain sizes and oxygen contents were prepared by powder metallurgy (P/M) combined with hot extrusion, and the P/M magnesium alloys were subjected to gas tungsten arc welding (GTAW). Porosities are observed in weld joints of the P/M AZ31B alloys with high oxygen contents. Gas composition analysis of porosity shows that the porosity originates mainly from the decomposition of magnesium hydroxide formed during the fabrication of magnesium alloys. The porosity can be decreased or prevented by reducing the amount of magnesium hydroxide (which is expressed as oxygen content in the present study) in the base materials, through controlling the P/M processing time. Use of a filler rod and/or an insert sheet containing rare earth element La tends to decrease the porosity in weld joints. When oxygen content in P/M AZ31B alloys is reduced to 440 ppm or less, sound weld joints without porosity are obtained. Mechanical test demonstrates that tensile strength of the sound weld joints of P/M AZ31B alloys is at the same level as that of weld joint of a commonly hot-extruded AZ31B alloy.     

Laser welding of AZ31B magnesium alloy to Zn-coated steel

The characteristics of laser lap welding of AZ31B magnesium alloy to Zn-coated steel were investigated. Welding was difficult when the laser beam was irradiated onto the AZ31B alloy and the processing parameters were set to obtain a keyhole welding mode. The difference in the physical properties between the two materials resulted in unstable welding process particularly when the laser beam penetrated into the steel specimen and a keyhole was formed therein. By switching to a conduction mode, the process stability was improved and successful welding could be achieved because the liquid metal film remained unbroken and the laser beam did not penetrate into the material. A 25 mm wide joint failed in tensile shear testing at loads exceeding 6000 N. This high joint strength was attributed to the formation of a 450 nm thick layer of Fe₃Al intermetallic compound on the steel surface as a result of the interaction between Al from the AZ31B alloy and Fe. The presence of Zn-coating layer was essential to eliminate the negative effects of oxides on the joining process.     

镁合金AZ31B钨极氩弧焊焊接过程热效率研究

以镁合金AZ31B为研究对象,采用钨极氩弧焊(TIG)填丝工艺,研究在不同焊接工艺参数下(I=120～200A,U=11～15V,v=3～11mm/s)的焊接电弧热效率和熔化效率,并探讨焊接工艺参数、焊接热效率和熔化效率对焊接接头微观组织的影响.结果表明:焊接工艺条件下,镁合金AZ31B的焊接电弧热效率值在0.56～0.82之间;熔化效率随着焊接电流和焊接速率的增加而增大;不同的焊接工艺对焊接接头的横截面形状(熔深B和熔宽H)和微观组织都有较大的影响.     

High power fiber laser arc hybrid welding of AZ31B magnesium alloy

High power fiber laser–metal inert gas arc hybrid welding of AZ31B magnesium alloy was studied. The fusion zone consisted of hexagonal dendrites, where the secondary particle of Al₈Mn₅ was found at the center of dendrite as a nucleus. Within hybrid weld, the arc zone had coarser grain size and wider partial melted zone compared with the laser zone. The tensile results showed the maximum strength efficiency of 5 mm thick welds was up to 109%, while that of 8 mm thick welds was only 88%. The fracture surface represented a ductile–brittle mixed pattern characterized by dimples and quasi-cleavages. On the fracture surface some metallurgical defects of porosity and MgO inclusions around with secondary cracks were observed. Meanwhile, a strong link between the joint strength and weld porosity were demonstrated by experimental results, whose relevant mechanism was discussed by the laser–arc interaction during hybrid welding.     

Feasibility of joining AZ31B magnesium metal matrix composite by friction welding

The present study investigates the feasibility of joining AZ31B magnesium nanocomposites by rotational friction welding. The AZ31B magnesium metal matrix composite, which was reinforced with alumina (Al₂O₃, average particle size of 50 nm) and calcium (Ca), was produced by a hybrid casting process and hot extruded at 350 °C. Joining processes were done by varying the friction welding parameters. The integrity of the joints was investigated by optical microscopy. The mechanical properties of the joints were examined by tension tests and micro hardness tests. The efficiencies of the joints were analysed using statistical analysis. The fracture mode was studied using a scanning electron microscope. It was observed that as the friction pressure and forging pressure increased, the joint efficiency increased. Also, as the friction time increased, the efficiency of the joint decreased.     

Gas tungsten-arc filler welding of AZ31 magnesium alloy

Optical microscopy and scanning electron microscopy (SEM) were used to examine the microstructure and fracture of AZ31 magnesium alloy joint welded by automatic gas tungsten-arc filler (GTAF) welding process with AZ61 wire. More endeavors have been put on investigating the effect of filler wire on microstructure and performance of welded joint. The result showed that the grain size in heat-affected zone (HAZ) of GTAF welded joint holds significant variety compared with that of GTA welded joint (without filler wire). The modification of microstructure in HAZ has changed the fracture location as well as the ultimate tensile strength (UTS) of joint in tensile test.     

Tensile and fatigue behaviour of wrought magnesium alloys AZ31 and AZ61

The mechanical behaviour of two hot rolled magnesium alloys, namely the AZ31 and AZ61, has been evaluated experimentally under both monotonic and cyclic loading. Both longitudinal (L) and long transverse (LT) directions were evaluated. The tensile behaviour of the L and LT directions is similar and differs only in the offset 0.2% yield strength for both materials. This difference is attributed to the angular spread of basal poles toward the rolling direction and is more pronounced for the case of AZ31. A distinct hardening response is obvious in both directions. Twinning formation was observed; it is more pronounced in the longitudinal direction while the fracture mode is intergranular and equiaxed facets are present in the fracture surfaces of the specimens. The S–N curves exhibit a smooth transition from the low to high cycle fatigue regime. AZ61 exhibits an overall better fatigue behaviour compared to AZ31. A transgranular crack initiation mode is observed in all tested specimens while the propagation of the cracks is characterized as intergranular.     

Cyclic oxidation behaviour of cerium implanted AZ31 magnesium alloys

AZ31 samples were implanted with 90 keV cerium ions with a dose of 1 × 10¹⁷ ions/cm². Auger electron spectroscopy (AES), X-ray photoelectron spectroscopy (XPS) and Glancing angle X-ray diffraction (GAXRD) were used in order to investigate the characterization of elements in the implanted surface. The results indicate that after cerium implantation a pre-oxidation layer with double structure was formed. The influence of cerium implantation on the cyclic oxidation behaviour of AZ31 samples was studied at 773 K in air for 96 h, and the morphologies of the oxide scales were examined by scanning electron microscopy (SEM). It was found that the oxidation resistance of the implanted sample has been improved. The mechanisms to explain the experimental results were also proposed.     

Application of T-shape friction test for AZ31 and AZ80 magnesium alloys at elevated temperatures

T-shape side pressing experiment is a sort of friction test which, recently, is employed for evaluation of friction for bulk metal forming processes. One of important advantages of this experiment, compared with other friction tests such as the ring compression test, is the occurrence of appropriate surface enlargement during the deformation of the specimen. This paper is concerned with experimental and numerical studies on this test, when it is used for some magnesium alloys such as AZ31 and AZ80. Based on the experimental results, it was found that the friction sensitivity of T-shape experiment increased when the die edge radius decreased or the test temperature or ram velocity increased. Good repeatability of this test was also observed during experimental part of this research work. Finally, employing the flow curves gained from the compression tests and friction factors obtained from the T-shape experiments for the finite-element simulations of this test, resulted in a very good agreement between the numerical and experimental load curves.     

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.     

Study on the temperature measurement of AZ31B magnesium alloy in gas tungsten arc welding

This paper is based on the Finite Element Analysis (FEA) to study the AZ31B Magnesium Alloy welding temperature filed, using a convenient, non-contact and fast response measured temperature method—Infrared Radiation (IR), the welding temperature field of AZ31B magnesium alloy plate in Gas Tungsten Arc Welding (GTAW) is measured by IR, the isothermal map of magnesium alloy plate is measured using IR device. The cooling curves are measured by thermocouple. Experiments and simulations by FEA are carried out to investigate the welding temperature field. The simulated results showed good agreement with the experiment ones.     

Hot working behavior of AZ31 and ME21 magnesium alloys

The plastic deformation and recrystallization behavior of the commercial magnesium alloys AZ31 and ME21 were analyzed in a wide temperature range. Using the conventional hyperbolic sine equation the flow stress dependence on temperature and strain rate was modeled. The activation energy for plastic deformation significantly increased with increasing temperature and delivered values above 180 kJmol^?1 for both alloys in the very high-temperature regime (400–550 °C). At lower temperatures (250–400 °C) the activation energy of the AZ31 alloy was approximately 108 kJmol^?1 considering the peak stress as well as 120 kJmol^?1 considering the flow stress at a strain of 0.5. The stress exponent varied in a range between 4.5 and 6.5. During the high-temperature compression tests a partial recrystallized microstructure was formed, which was distinctly different in AZ31 compared to ME21 due to the different onset of dynamic recrystallization (DRX) mechanisms.     

Laser-tungsten inert gas hybrid welding of dissimilar metals AZ31B Mg alloys to Zn coated steel

Laser-tungsten inert gas (TIG) hybrid welding has been developed for joining Mg alloys to Zn coated steel in a lap joint configuration. The joint could not be produced in laser or arc welding only, while acceptable joints without obvious defects were obtained with a relatively wide processing window in the hybrid process. Two reaction layers were observed to form at the interface and were identified as Mg–Zn eutectic structure (α-Mg + MgZn) and Fe₃Al phase by TEM analysis. In some cases, Al₆Mn phase also formed adjacent to the Fe–Al reaction layer. The tensile-shear strength attained the maximum value of 68 MPa, representing 52.3% joint efficiency relative to Mg base metal. The element Al from AZ31B Mg alloys diffused to the liquid/solid interface and then reacted with the elements from steel, such as Fe and Mn, contributing to the metallurgical bonding at the interface. The weak bonding between Mg–Zn reaction layer and newly formed Fe–Al layer resulted in the interfacial failure.     

Preliminary study of biodegradation of AZ31B magnesium alloy

Magnesium alloys are potential to be developed as a new type of biodegradable implant material by use of their active corrosion behavior. Both in vitro and in vivo biodegradation properties of an AZ31B magnesium alloy were investigated in this work. The results showed that AZ31B alloy has a proper degradation rate and much lower hydrogen release in Hank’s solution, with a degradation rate of about 0.3 mm/year and hydrogen release below 0.15 mL/cm². The animal implantation test showed that the AZ31B alloy could slowly biodegrade in femur of the rabbit and form calcium phosphate around the alloy sample, with the Ca/P ratio close to the natural bone.     

Interface strengthening in dissimilar double-sided friction stir spot welding of AZ31/ZK60 magnesium alloys by adjustable probes

Dissimilar welding of AZ31/ZK60 magnesium alloys with a thickness of 2 mm was successfully carried out by the double-sided friction stir spot welding with adjustable probes.A dissimilar joint bearing flat surfaces on both sides without a keyhole was obtained and the shear failure load of 8.7±0.5 kN was reached.The role of the adjustable probes has been revealed in detail.In the center of the stir zone,the welding interface structure was heterogeneous around which some distinct oxides still remained,leading to a weak interface strength.On the contrary,the welding interface structure around the shoul-der/probe interface was homogeneous with no oxides giving rise to a strong interface strength,which is attributed to the severe material flow introduced by the adjustable probes.In addition,the vicinity outside the shoulder/probe interface,where the fracture occurred during the shear tensile tests,was also strengthened owing to the shearing and torsion by the adjustable probes.Therefore,a stable plug failure can be obtained and the joint properties can be improved.     

An investigation of second phases in as-cast AZ31 magnesium alloys with different Sr contents

Second phases in the AZ31 as-cast magnesium alloys with different Sr contents (0, 0.1, 0.5, 1.0, 2.0, and 5.0 wt%) were investigated using scanning electron microscopy, energy dispersive spectrometry, differential scanning calorimetry, X-ray diffraction, and transmission electron microscopy. The results indicated that the Mg₂₁(Zn, Al)₁₇ phase with small amount was formed in the AZ31 as-cast alloy without Sr addition, in addition to the Mg₁₇Al₁₂ phase. At the same time, the alloy with the addition of 0.1 wt% Sr mainly consisted of the α-Mg, Mg₁₇Al₁₂, Mg₂₁(Zn, Al)₁₇, and Al₄Sr phases. In addition, the α-Mg, Mg₂₁(Zn, Al)₁₇ and Al₄Sr phases were found to be the main second phases for the alloy with the addition of 0.5 wt% Sr. However, only the α-Mg, Al₄Sr and (Mg, Al)₁₇Sr₂ phases were mainly formed in the AZ31 alloy with the addition of 1.0 wt% Sr. As for the alloys with the additions of 2 and 5 wt% Sr, their as-cast microstructures were mainly composed of the α-Mg and (Mg, Al)₁₇Sr₂ phases.     

AZ31B镁合金管材热态内压成形性能的研究

为了研究变形镁合金AZ31B管材的热态内压成形性能,通过单向拉伸测试了不同温度和应变速率下其力学性能的变化,通过胀形实验研究了温度对内高压成形性能的影响,以及相应变形条件下微观组织的变化.实验结果表明:在20～300℃时,AZ31B的屈服强度和抗拉强度随着温度的升高而降低,总伸长率随着温度的升高而提高,均匀伸长率随着温度的升高先增大后减小;当应变速率在0.001～0.1s-1时,屈服强度和抗拉强度随应变速率的增大而升高,总伸长率随着应变速率的增大而减小,均匀伸长率随着应变速率的增大先增加后减小;当温度在20～250℃时,镁合金管材的极限胀形率随温度的升高先增大后减小,在175℃时达到最大值.微观组织观察表明,175℃下不完全动态再结晶和孪晶两种微观组织的出现是使镁合金管材极限胀形率提高的主要原因.     

Microstructure and interfacial reactions of soldering magnesium alloy AZ31B

In this paper, economic and innoxious solder alloys with low melting temperature were designed for AZ31B. Their chemical composition and relevant parameters were investigated for a high-performance structure of bonding region. Results of microstructure observation showed that Zn-enriched phases disappeared and α-Mg existed in the joints in the form of coarse dendrites by increasing the concentration of Mg in the solder alloys. Water cooling with a high cooling rate was adopted in experiments. Experimental research showed that high cooling rate restricted the grains of α-Mg as the equiaxed dendrites, which was about 1/5 of the coarse dendrite but their number was more than 40–50 times. Both morphology with typical fracture and the analysis on X-ray diffraction fracture indicated that equiaxed dendrites significantly improved the mechanical property of the joints. Necking phenomenon occurred in the bonding region was in favor of the improvement of joint shear strength.