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.     

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.     

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.     

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.     

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.     

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.     

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.     

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.     

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.     

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.     

AZ31B变形镁合金TIG焊接头组织及断口形貌分析

以AZ31变形镁合金为研究对象,采用光学显微镜和扫描电子显微镜等测试手段对其TIG交流自动焊接头的微观组织和接头的断口形貌进行了观察分析,结果表明,在TIG自动焊条件下,AZ31B镁合金接头的焊缝区主要为细小的等轴晶,热影响区为粗大的过热组织,晶粒比较粗大,通过对接头断口形貌观察发现其接头断裂特征为韧-脆混合断裂.     

Pore formation mechanism and its mitigation in laser welding of AZ31B magnesium alloy in lap joint configuration

Magnesium is one of the lightest structural metals that has been used in different industries such as automobile, aerospace and electronics. However, in fusion joining of magnesium alloys, porosity is one of the main drawbacks to achieve a weld with desirable properties. The oxide layer existing on the surface of magnesium alloy is one of the causes of pore formation in the weld bead. In the current study, a fiber laser with a power of up to 4 kW is used to weld samples in a zero-gap lap joint configuration. Two groups of samples are studied: as-received (AR) surfaces (where an oxide layer remains on the surface) and treated surfaces. The surface treatment includes two techniques: mechanically removed (MR) and the use of a plasma arc (PA) as a preheating source. Also, a separate set of experiments are designed for preheating samples in a furnace for comparison with the PA-treated results. To reveal the chemical compositions of the welds and metal sheet surfaces, an energy dispersive spectroscopy (EDS) is performed. Surface chemical compositions are tested by X-ray photoelectron spectroscopy-reflected electron energy loss spectroscopy (XPS-REELS) to characterize the surface composition on AR and PA-treated samples. The dynamic behavior of the weld pool and laser-induced plasma plume is monitored in real-time using a high speed CCD camera to investigate the stability of the laser welding process. The presence of the oxide layer at the faying surface of two overlapped sheets results in an unstable process. The obtained results reveal that the preheating procedure can effectively mitigate pore formation at the interface of the two overlapped sheets.     

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.     

AZ31B可降解镁合金的研究

研究了铸态、挤压态及热处理态AZ31B镁合金的力学性能和耐蚀性能，选出性能最优的AZ31B镁合金，植入动物下颌骨处进一步研究其在体内的降解行为及其降解产物对动物体的影响。研究结果表明，AZ31B镁合金经过挤压和固溶时效处理可以提高其力学性能和耐腐蚀性能．将处理后的AZ31B镁合金植入兔下颌骨后发现，材料降解未对动物体造成不良影响，并且降解过程不会影响下颌骨骨折固定的稳定性。因此，可降解AZ31B镁合金有望用于制作下颌骨骨折后的内固定系统。     

Organo-silane coatings for AZ31 magnesium alloy corrosion protection

The protective performances of coatings formed by organo-silanes with a long alkyl chain for AZ31 magnesium alloy corrosion protection were evaluated by electrochemical techniques. The coatings were formed in hydroalcoholic baths of octyl- or octadecyl-trimethoxy-silane, and hydrolyzed at different pH values.

The coatings formed at all the pH by the lower homologue were always porous and scarcely protective. Those built by the higher homologue were more corrosion resistant and the best results were observed when octadecyl-trimethoxy-silane was hydrolyzed at pH 5: in this case a thick, scarcely defective, layer was formed and no evident corrosion attack was observed after 1000 h immersion in 0.05 M Na₂SO₄ solution.