Pore formation during hybrid laser-tungsten inert gas arc welding of magnesium alloy AZ31B—mechanism and remedy

One of the major concerns during high speed welding of magnesium alloys is the presence of porosity in the weld metal that can deteriorate mechanical properties. This study seeks to analyze the presence method and quantity of pore during hybrid laser-tungsten inert gas arc (TIG) welding of magnesium alloy AZ31B by radiography, optical microscopy and electron probe microanalysis (EMPA). At the same time, it identifies both the mechanism of pore formation and a remedy for this problem. The experimental results indicate that lacking of shielding gas for laser beam is the dominant cause of macroporosity formation during the hybrid of laser-TIG welding of magnesium Alloys AZ31B plate, and hydrogen is not main cause to form large pores. A favorable weld without porosity can be obtained by appending lateral shielding gas for laser beam.     

Laser–tungsten inert gas hybrid welding of dissimilar AZ based magnesium alloys

Abstract

The microstructure and mechanical properties of dissimilar AZ based magnesium alloys subjected to laser–tungsten inert gas (TIG) hybrid welding have been investigated. The results show that magnesium alloys can be readily welded as dissimilar joints using this process. The microstructure of the dissimilar magnesium alloy joints is composed of primary α phase (Mg) and β phase (Mg₁₇Al₁₂), based on electron probe microanalysis (EPMA) and X-ray diffraction (XRD) data. In addition, the tensile strength of AZ31B–AZ61 and –AZ91 joints is equal to that of AZ31B base metal. It has also been found that the presence of β phase has a severe influence on the tensile strength and mirohardness of dissimilar magnesium alloy joints.     

Anisotropic property of material arrangement in friction stir welding of dissimilar Mg alloys

Microstructure and mechanical properties of friction stir weld joints of dissimilar Mg alloys AZ31 and AZ80 were investigated in the present work. Several different welding parameters were adopted in the study, and the effects of rotation speed and welding speed on the joint quality were discussed comprehensively. In addition, material arrangement which means that AZ31 alloy was at advancing side or at retreating side has significant influence on the joint formation, including the joint microstructure and mechanical properties. A few kinds of defects were observed when the improper parameters were taken in the experiment, and the reasons for generating these defects were revealed in this work. Sound joints with good mechanical properties could be easily obtained when AZ31 was at retreating side, but it was difficult to obtain the sound joint with the contrary material arrangement. These results suggest that the material with inferior plastic deformability should be set at the advancing side and the material with superior one should be set at the retreating side in order to get sound FSW joint of dissimilar Mg alloys.     

电弧电流对AZ31B/DP980激光诱导电弧焊接接头成形及力学性能的影响

采用激光诱导钨极惰性气体保护(tungsten inert gas, TIG)电弧焊接技术,在未添加任何夹层和镀层的条件下,通过优化工艺,获得了AZ31B镁合金和DP980高强钢高质量搭接焊接头,重点研究TIG电弧电流对焊接接头成形和力学性能的影响规律。结果表明：电弧电流的增大会提高镁合金在高强钢的润湿铺展能力,提升焊缝宽度的同时减小润湿角。镁合金/钢焊接接头的最大拉伸载荷随着电弧电流的增大先升高后降低,接头断裂模式由沿界面断裂转变为沿焊缝断裂。当TIG电流为80 A、激光功率为350 W时,焊接接头最大平均拉伸载荷达到279 N/mm。焊缝宽度和界面层厚度的增大以及激光匙孔的钉扎作用共同提升了镁合金/钢的接头性能。     

Mikrostrukturelle und mechanische Eigenschaften von laserstrahlgeschweißtem Magnesiumfeinblech AZ31‐HP mit und ohne Zusatzwerkstoffe

Microstructural and mechanical properties of laser welded sheets of magnesium AZ31‐HP with and without filler wires This paper describes Nd:YAG laser beam welding experiments carried out on rolled 2.5 mm thick magnesium sheet AZ31‐HP. For the butt welds in flat position, filler wires AZ31X and AZ61A‐F were used, diameter 1.2 mm. The microstructure and mechanical properties of the different laser beam welded joints were examined and compared with one another. The obtained results show that the laser beam welding of AZ31‐HP sheet is possible without hot crack formation, both without and with filler wires. The determined tensile strength, ductility, fracture toughness and microhardness of laser beam welded joints without filler wire were not effected by AZ31X nor AZ61A‐F. By use of these filler wires loss of zinc was minimized and the shape of weldments was optimized. The values of fracture strength, yield strength and microhardness of the joints and base material are quite similar. It is found that the ductility of the joints is lower than the base materials due to the heterogeneous microstructure of the fusion zones and geometrical notches of the weld seams. Both, weld and base material of AZ31‐HP, showed stable crack propagation. Furthermore, for base material slightly lower fracture toughness values CTOD than for the joints were determined.     

Verhalten geschweißter Magnesiumlegierungen unter mechanisch‐korrosiver Komplexbeanspruchung

Corrosion and corrosion fatigue of welded magnesium alloys In addition to the prevalent use of magnesium cast alloys a high potential for lightweight constructions is offered by magnesium‐wrought alloys, in particular in the automobile industry. The use of rolled and/or extruded magnesium alloys (profiles and sheet metals) requires suitable and economic join technologies like different welding procedures in order to join semi finished parts. Thus, the realization of lightweight constructions asks for high standards of materials‐ and joining‐technologies. In this context, the mechanical properties as well as the corrosion behaviour of the joints are of large interest. During welding of magnesium alloys, influences concerning the surface, the internal stresses and the microstructure occur. These influences particularly depend on the energy input and thus, on the welding procedure as well as the processing parameters, which all affect the corrosion behaviour of the joints. Sheets of magnesium alloys (AZ31, AZ61, AZ91) were joined with different welding procedures (plasma‐, laser beam‐ and electron‐beam welding in the vacuum and at atmosphere). The corrosion behaviour (with and without cyclic mechanical loading) of the welded joints was investigated by different methods such as corrosion tests, polarisation curves, scanning electron microscopy and metallography. Furthermore, substantial influencing variables on the corrosion behaviour of welded joints of magnesium alloys are pointed out and measures are presented, which contribute to the improvement of the corrosion behaviour.     

AZ31镁合金表面激光熔覆Al-TiC复合涂层微观组织与腐蚀性能

为有效改善AZ31镁合金表面的腐蚀性能，本文采用激光熔覆技术在AZ31镁合金表面成功制备了无缺陷的Al-TiC复合涂层。研究了不同成分含量的Al-TiC复合涂层的相组成、微观组织和耐腐蚀性能的影响。结果表明：在Al-TiC复合涂层内形成了大量的Al₁₂Mg₁₇、Mg₂Al₃和TiC相。复合涂层内微观组织呈现出连续网络状分布特征。随着Al-TiC混合粉末中Al含量的减小，复合涂层中Al₁₂Mg₁₇、Mg₂Al₃和TiC相的含量呈递增趋势，网络状分布的微观组织结构变得更加均匀连续。复合涂层与AZ31基体之间形成了良好的冶金结合界面。激光熔覆制备的Al-TiC复合涂层耐腐蚀性能较AZ31基体显著提升。自腐蚀电位由基体的-1.563 V提升至-1.144 V，自腐蚀电流由基体的1.55×10^-4 A减小至2.63×10^-6 A。     

Metallurgical characterization of pulsed current gas tungsten arc,friction stir and laser beam welded AZ31B magnesium alloy joints

This paper reports the influences of welding processes such as friction stir welding (FSW), laser beam welding (LBW) and pulsed current gas tungsten arc welding (PCGTAW) on mechanical and metallurgical properties of AZ31B magnesium alloy. Optical microscopy, scanning electron microscopy, transmission electron microscopy and X-Ray diffraction technique were used to evaluate the metallurgical characteristics of welded joints. LBW joints exhibited superior tensile properties compared to FSW and PCGTAW joints due to the formation of finer grains in weld region, higher fusion zone hardness, the absence of heat affected zone, presence of uniformly distributed finer precipitates in weld region.     

Numerical and experiment analysis of residual stress on magnesium alloy and steel butt joint by hybrid laser-TIG welding

Hybrid welding technology has received significant attention in the welding of dissimilar materials recently. While, great welding residual stress and deformation often result by the difference of coefficient of thermal expansion This study describes the thermal elastic–plastic analysis using finite element techniques to analyze the thermo mechanical behavior and evaluate the residual stresses and welding distortion on the AZ31B magnesium alloy and 304L steel butt joint in laser-TIG hybrid welding. A new coupled heat source model was developed which combined by double-elliptic planar distribution, double-ellipsoid body distribution and Rotary–Gauss body distribution model. From the results, it can be concluded that the temperature distribution at the hybrid weld region is exposed to faster rate of heating and cooling in hybrid welding than TIG. Furthermore, compared to the welding stress distribution on the TIG weld, residual stress σ_y is found about 20% higher on hybrid weld joints, and the residual stress on the 304L steel plate is lower than that on the AZ31B magnesium plate.     

AZ31镁合金管材挤压成型数值模拟与实验研究

运用基于AZ31镁合金本构方程与ALE算法的HyperXtrude软件对典型AZ31薄壁管材的挤压过程进行数值模拟,并通过调整焊合室高度、焊合室大圆角及焊合室坡度3个结构参数,分析不同条件下应力分布与速率分布的变化情况。结果表明:焊合室内近工作带处压力随焊合室高度增加不断减小,分流孔与焊合室压力的最大值与平均值均随焊合室大圆角增大发生降低,分流孔与焊合室压力随焊合室入口坡度增大不断增大,并在焊合室高度为16mm、大圆角为18mm以及入口坡度为15°时金属流速均方差达到最小值。优化模具结构缓解了应力集中与流速不均等问题,在实验生产中得到合格产品,型材组织均匀细化。     

镁合金体积成形进展

简要介绍了国内外学者、科技人员进行的ZK30,ZK60,AZ31,AZ31B,AZ61,AZ80,MB15等镁合金材料锻造、挤压方面的基础研究进展和一些简单的产品开发,阐明了镁合金从基本研究到镁合金产品产业化和商业化的发展方向.     

Galvanisches Verzinken von Magnesiumlegierungen

Zinc‐Plating of Magnesium Alloys Magnesium alloys are highly susceptible to corrosion that limits their application when exposure to corrosive service conditions is needed. One of the ways to prevent corrosion is to coat the magnesium‐based substrate to avoid a contact with an aggressive environment. Results concerning corrosion behaviour of wrought AZ31 magnesium alloy with electrolytic zinc coatings deposited from different electrolyte solutions are described. Evaluation of corrosion processes in chlorides containing solutions was performed by electrochemical measurements. It was found that thick and dense electrolytic zinc coatings formed on AZ31 significantly improve the corrosion behaviour of magnesium alloy after one hour immersion of zinc coated magnesium alloys in corrosive media. Further increase of immersion time leads to relatively fast decrease of corrosion properties. Electrolytic zinc coatings obtained in consecutive alkaline / acidic process demonstrate an improvement of corrosion resistance of coated AZ31. The time to coating degradation strongly increases.     

Compressive behaviour of SiC/ncsc reinforced Mg composite processed through powder metallurgy route

In this present work nano coconut shell charcoal (ncsc) and silicon carbide (SiC) particulates were reinforced with AZ31B Mg alloy and suitable magnesium composite was developed by using the powder metallurgy technique followed by hot extrusion. Density measurement of the Mg composites revealed that the addition of ncsc significantly improved the density of the composites and porosity measurement showed minimal porosity. The microstructure of the composites showed even distribution of the ncsc in the AZ31B/3SiC Mg composite. The compressive and impact behaviour of the samples were characterized, the results showed that on increasing the weight percentage of ncsc in AZ31B/3SiC/0.5ncsc Mg composites the mechanical properties such as ultimate compressive strength, 0.2% yield strength, ductility and impact strength decreased. The scanning electron microscope (SEM) analysis of fractured surface of AZ31B Mg alloy and AZ31B/3SiC/0.5ncsc Mg composites showed quasi-cleavage fracture. The presence of ncsc above 0.5 wt% composites revealed mixture of quasi cleavage planes and some dimples.     

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.     

Effects of TiO2 coating on the microstructures and mechanical properties of tungsten inert gas welded AZ31 magnesium alloy joints

The effects of TiO₂ coating on the macro-morphologies, microstructures and mechanical properties of tungsten inert gas (TIG) welded AZ31 magnesium alloy joints were investigated by microstructural observations, microhardness tests and tensile tests. The results showed that an increase in the amount of the TiO₂ coating resulted in an increase in the weld penetration and the depth/width (D/W) ratio of the TIG welded AZ31 magnesium alloy seams. Moreover, the average grain size of the α-Mg grains increased and the β-Mg₁₇Al₁₂ intermetallic compound (IMC) was coarser in the case of higher amount of the TiO₂ coating. With an increase in the amount of the TiO₂ coating, the microhardness of the fusion zone (FZ) of the AZ31 magnesium alloy welded joints decreased slightly initially and then decreased sharply. In addition, with an increase in the amount of the TiO₂ coating, the ultimate tensile strength (UTS) value and elongation of the welded joints increased at first and then decreased sharply.     

Effects of extrusion ratio on the ratcheting behavior of extruded AZ31B magnesium alloy under asymmetrical uniaxial cyclic loading

Magnesium alloys are increasingly used in the automotive and aerospace industries for weight reduction and fuel savings. The ratcheting behavior of these alloys is therefore an important consideration. The objective of this investigation was to study the effects of extrusion ratio on the ratcheting behavior of extruded AZ31B magnesium alloy. The experiments have shown that the extruded AZ31B Mg alloy presented the following characteristic behavior with increasing number of loading cycles: first an apparent cyclic softening was observed, then a cyclic hardening occurred, and finally a stable state was reached. This generic behavior can be explained by the fact that the variation trend of the maximum strain with the number of cycles differs from that of the minimum strain. The extrusion ratio did not influence the cyclic softening/hardening behavior or the final ratcheting strain variation trend of the extruded AZ31B Mg alloy with the mean stress and the peak stress. However, the extrusion ratio influenced the final ratcheting strain variation trend of the extruded AZ31B Mg alloy with the stress amplitude. Increasing the extrusion ratio also reduced the ratcheting strain and the effects of the load history on the ratcheting behavior of the extruded AZ31B Mg alloy.     

In vivo study of degradable magnesium and magnesium alloy as bone implant

In order to investigate the in vivo behavior of pure magnesium and AZ31B and the influence of mineralization induction ability, sample rods were implanted intramedullary into the femora of rabbits. After one and nine weeks, six animals from each group were sacrificed, respectively. Undecalcified cross-sections of implant were performed to observe bone-implant by scanning electron microscopy (SEM) and energy dispersive spectromicroscopy (EDS). The SEM/EDS evaluation showed that there is a thin layer of bone around magnesium and its alloy after nine-week implantation. The results further showed that the aluminum-zinc containing magnesium alloys AZ31B provided a slower degradation rate in vivo than the pure magnesium. At the locations where magnesium was resorbed, the deposition of new bone was found. The results indicate that magnesium is biocompatible, osteo-conductive and is a potential material for use as a degradable bone implant.     

Schwingfeste Auslegung von Schweiß‐ verbindungen aus der Magnesiumknetlegierung AZ31(ISO‐MgAl3Zn1) mit dem Konzept der fiktiven Ersatzradien von rf = 1,0 mm und 0,05 mm

Fatigue design of welded joints from the wrought magnesium alloy AZ31 (ISO‐MgAl3Zn1) by the local stress concept with the fictitious notch radius of r_f = 1.0 mm and 0.05 mm The investigations were carried out with three different types of MIG‐ and TIG‐welded magnesium joints of the alloy AZ31. The evaluation of the results showed that the local stress concept using the fictitious notch radius of r_f = 1.0 mm can be applied to magnesium welded joints from plates with thicknesses t ≥ 5 mm independently of the weld geometries (fully or partially penetrated butt welds, transversal stiffeners). Design curves are proposed for different stress ratios, i.e. R = ‐1 as well as 0 and 0.5, which allow the consideration of residual stresses as well as load induced mean stresses. The results permit also the suggestion of Δσ = 28 MPa as FAT‐value for the IIW‐Fatigue Design Recommendations. Further, the FAT‐value Δσ = 73 MPa for the fictitious radius of r_f = 0.05 mm to be applied to welded thin magnesium joints is derived, too. These FAT‐values are compared with already known data for steel and aluminium joints. A linear relationship between the FAT‐values and the Young’s modulus is determined.     

Influence of neodymium on microstructure and mechanical properties of AZ31B wrought magnesium alloy

Abstract

The influences of rare earth neodymium on microstructure and mechanical properties of as cast and hot rolled AZ31B wrought magnesium alloy were investigated. The results show that the mechanical properties of both as cast and hot rolled AZ31B alloys decrease due to Nd addition. Nd reacts with Al to form Al₂Nd phase when Nd is added. Bulky and brittle Al₂Nd intermetallic degrades the mechanical properties. Moreover, the addition of Nd weakens the grain refining effect of Al on as cast AZ31B alloy, resulting in grain coarsening. Coarse grains also cause the decline of the mechanical properties of as cast AZ31B–Nd alloy. The negative influence of the bulky and brittle intermetallics on mechanical properties of AZ31B alloy can be relieved by large deformation because the intermetallics can be sufficiently broken up during the deformation process.     

In vivo study of degradable magnesium and magnesium alloy as bone implant

In order to investigate the in vivo behavior of pure magnesium and AZ31B and the influence of mineralization induction ability, sample rods were implanted intramedullary into the femora of rabbits. After one and nine weeks, six animals from each group were sacrificed, respectively. Undecalcified cross-sections of implant were performed to observe bone-implant by scanning electron microscopy (SEM) and energy dispersive spectromicroscopy (EDS). The SEM/EDS evaluation showed that there is a thin layer of bone around magnesium and its alloy after nine-week implantation. The results further showed that the aluminum-zinc containing magnesium alloys AZ31B provided a slower degradation rate in vivo than the pure magnesium. At the locations where magnesium was resorbed, the deposition of new bone was found. The results indicate that magnesium is biocompatible, osteo-conductive and is a potential material for use as a degradable bone implant.