Friction stir welding of AZ61A magnesium alloy

This paper deals with the development of an empirical relationship to predict tensile strength of friction stir welded AZ61A magnesium alloy. The process parameters such as tool rotational speed, welding speed, axial force and tool pin profile play a major role in deciding the tensile strength. The response surface method (RSM) was used to develop the empirical relationship. The four-factor, five-level central composite design was used to minimize the number of experimental conditions. The developed empirical relationship can be effectively used to predict tensile strength of friction stir welded AZ61A magnesium alloy joints at 95 % confidence level.     

Investigation on weldability of magnesium alloy thin sheet T-joints: arc welding, laser welding, and laser-arc hybrid welding

Four welding methods, including laser welding, gas tungsten arc (GTA) welding, laser–GTA hybrid welding, and laser–GTA hybrid welding with cold welding wire, are used to investigate the weldability of T-joints of magnesium alloy thin sheet. Stake welding process is presented in this paper in order to overcome the defects, such as stress concentration and deformation, and improve the accessibility of T-joints in fillet welding process. The effect of heat source type on weldability of T-joints is analyzed. The microstructures and mechanical properties are investigated. Experimental results indicate that comparing with the other three welding methods, laser–GTA hybrid welding with cold welding wire is the most effective process for T-joints of magnesium alloy thin sheet. In this process, T-joints are full penetration and the toes are smooth and round, and besides, reinforcement forms on the upside of weld bead by the filled wire. The mechanical properties of T-joints made with laser–GTA hybrid welding with cold welding wire achieve 90 % of that of base metal and are superior to that without welding wire.     

Developing diffusion bonding windows for joining AZ31B magnesium and copper alloys

In this investigation, Mg–Cu dissimilar materials are joined by diffusion bonding process. The principal difficulty when joining Mg–Cu lies in the existence of hard-to-remove oxide films on the magnesium surfaces and the formation of brittle metallic interlayers and oxide inclusions in the bond region. In this investigation, an attempt was made to develop diffusion bonding windows for effective joining of AZ31B Magnesium and commercial grade copper alloys. Joints were fabricated using different combination of process parameters such as bonding temperature, bonding pressure and holding time. The bonding quality was checked by microstructure analysis and lap shear tensile test. Based on the results, diffusion bonding windows were constructed, and they are presented in this paper. These windows will act as reference maps for selecting appropriate diffusion bonding process parameters to get good quality bonds for Mg–Cu alloys.     

Investigating the characteristics of AZ61 magnesium alloy on the hot and semi-solid compression test

The characteristics of magnesium AZ61 in semi-solid state have been investigated to understand the changes in its stress-strain in the compression process, the effects on its strain under different processing temperatures, and the consequence of the change of peak stress. First, the AZ61 magnesium alloy is heated to 660 °C and sustained at that temperature for 5 min. Then by natural cooling, the cooling curve is acquired. This natural cooling experiment immediately reveals the semi-solid temperature range of AZ61 magnesium alloy. On this temperature range, the study of its semi-solid characteristics and the observation of microstructure can be carried out. After that, the observed results determine the experimental conditions of semi-solid compression. The compression experiment primarily has two parts. One is to understand the changes of AZ61 magnesium alloy from normal temperature to 400 °C, and the other is to carry out the compression experiment of AZ61 magnesium alloy under semi-solid temperature. Upon completion of the experiment, this research explores their microstructures, and compares the structural change of AZ61 magnesium alloy in a hot chamber within the semi-solid temperature range.     

Study of plasma in laser welding of magnesium alloy

Magnesium-rare earth alloy was welded using the CO₂ laser. The surface plasma of laser welding was studied by the spectra and also, the plasma temperature was calculated using the relative intensity of spectral line. The behavior of the plasma was observed with a high-speed camera. The relationship between the welding parameters and plasma temperature was discussed. The results show that laser power has little effect on plasma temperature. With increasing welding speed, plasma temperature decreases first, and then, increases. The cooling effect of the plasma using helium is significant. The plasma recoil is about 7,500 Pa during laser-welded NZ30K because of the lower boiling point of magnesium.     

Effects of humidity and contact material on fretting fatigue behavior of an extruded AZ61 magnesium alloy

Fretting fatigue tests of the extruded AZ61 magnesium alloy with the same contact material under low and high humidity were carried out to investigate basic fretting fatigue characteristics and effect of humidity on fretting fatigue behavior. Influence of contact material was also studied by using JIS S45C carbon steel contact material. Degradation of fatigue strength due to fretting was much more significant than that due to corrosion under high humidity condition. Therefore, no effect of humidity on fretting fatigue strength was found. Reduction rate of fatigue strength due to fretting for the magnesium alloy was between those of aluminum alloys and titanium alloys. Tangential force coefficient of the magnesium alloy was rather low compared to other materials such as steels, aluminum alloys and titanium alloys. Fretting fatigue strength with the S45C contact material was inferior compared to that with the same contact material. This is mainly due to higher tangential force in AZ61/S45C contact. Fretting fatigue cracks at the edge of fretting contact region were observed to nucleate in the very early stage of fatigue life, similar to other structural materials.     

AZ31B镁合金表面激光合金化Al-SiC涂层制备及其性能研究

针对镁合金表面耐磨性差,采用预置粉末法对AZ31B表面进行激光合金化Al-SiC粉末实验。使用光学显微镜和扫描电子显微镜(SEM)、能谱分析仪(EDS)、摩擦磨损试验机、显微硬度计对合金化涂层的微结构、相组成及性能进行了分析研究。结果表明,强化层与基体呈冶金结合、组织均匀致密;合金化层主要由Mg17Al12、SiC、Mg2Si、Al4C5、Al2O3等相组成。涂层的显微硬度、耐磨损性能都明显高于基体。     

Eutectic bonding of austenitic stainless steel 316L to magnesium alloy AZ31 using copper interlayer

The eutectic bonding of magnesium alloy (AZ31) to austenitic stainless steel alloy (316L) was performed using pure Cu interlayers. The effect of hold time on the microstructural developments across the joint region and the related effect on bond shear strength were studied at a bonding temperature of 530°C. The bonding process took place through a sequential occurrence of solid-state diffusion of Cu into the magnesium alloy, eutectic phase formation, interlayer dissolution, and isothermal solidification. A (Mg–Cu–Al) ternary intermetallic phase formed within the joint and concentrated into the center of the bond during the solidification stage increasing the hardness value to a maximum average of VHN313 while the maximum recorded bond shear strength was 57 MPa achieving 69% of the AZ31 shear strength and about three to four times of the adhesive joints.     

AZ31B镁合金激光喷丸强化后疲劳裂纹扩展的数值模拟研究

激光喷丸(LSP)是一种先进的材料表面强化工艺,能有效提高零件的机械性能及其使角寿命.建立了以有限元软件ABAQUS和MSC.Fatigue为平台,面向抗疲劳制造的激光喷丸工艺有限元分析模型.结果表日月激光喷丸可以有效抑制疲劳裂纹扩展,延长疲劳寿命,喷丸次数的增加在一定程度上增大残余应力及疲劳寿命.残余应力抑制疲劳裂纹扩展的原因归结为最终断裂尺寸的增大以及裂纹扩展速度的减小.研究结果为LSP抗疲劳效果的预测提供了有效的方法,对于优化工艺参数,减少试验次数,降低成本具有指导意义.     

长焊缝激光拼焊焊缝碾压预成型技术研究

为降低长焊缝激光拼焊对板材边缘直线度的要求,提出了焊缝碾压预成形技术,其通过利用碾压轮碾压板材边缘使其发生属性变形来消除焊缝间隙,并且能够实现碾压量随间隙量大小而实时调整以保证板材碾压变形后的延展量正好填补两板间的间隙量.介绍了其机构原理及控制原理:通过实验测量与数学拟合的方法建立了碾压过程的数学模型.碾压和焊接实验表明长焊缝激光拼焊焊缝碾压预成形技术能方便消除焊前两板间间隙,大大降低激光焊接对板材边缘直线度的要求,保证焊接质量和提高焊接速度.     

Effects of welding parameters on weld geometry during high-power laser welding of thick plate

The International Journal of Advanced Manufacturing Technology - During high-power deep-penetration laser welding of thick plate, the geometrical characteristics of weld cross section were...     

脉冲激光焊接Hastelloy C-276合金的熔池流动传热特性分析

基于流体动力学方程和传热方程建立了三维瞬态模型,用于研究脉冲激光焊接0.5 mm厚Hastelloy薄板时熔池的流动行为及传热特性.应用Fluent软件,采用有限容积法(FVM)求解控制方程,用SIMPLE算法处理速度与压力的耦合.引入Pe来衡量焊接熔池中对流传热与传导传热的相对强弱,并以此分析焊接熔池的传热特性.结果表明:沿焊接方向,焊接熔池的流动速度随着离熔池中心距离的增加先增加后减小;在给定试验条件下,熔池流动速度在离熔池中心0.2 mm左右时出现最大值,且沿焊接方向前方稍大于后方,而后迅速减小为零;焊接熔池中对流的存在使得焊接熔池熔深较小而熔宽较大;最终的焊接形貌由对流传热与传导传热相互作用而成.对焊缝形貌的数值模拟结果与实验结果进行了比较,计算结果与实验结果吻合较好.此模型可为脉冲激光焊接Hastelloy C-276薄板时熔池流体流动行为的分析提供理论依据.     

Evaluation of the strain behaviour of butt joints on AZ31 magnesium alloy thin sheets welded by Nd:YAG laser

In this work, the mechanical and technological behaviour of AZ31 magnesium alloy laser welded joints is investigated. The forming behaviour of the joints is analysed by both tensile and biaxial stretch tests. Each test is monitored using a digital image correlation system in order to acquire the complete strain field during the whole test. Both in tensile and biaxial stretching tests, the strain maps reveal that the weld bead makes the strain path experienced by the welded specimen more critical than the one experienced by the base material, and this can be related to morphological defects of the weld bead.     

Tribology characteristics of magnesium alloy AZ31B and its composites

In this paper, wear characteristics of magnesium alloy, AZ31B, and its nano-composites, AZ31B/nano-Al₂O_3, processed by the disintegrated melt deposition technique are investigated. The experiments were carried out using a pin-on-disk configuration against a steel disk counterface under different sliding speeds of 1, 3, 5, 7 and 10 m/s for 10 N normal load, and 1, 3 and 5 m/s for 30 N normal load. The worn samples and wear debris were then examined under a field emission scanning electron microscopy equipped with an energy dispersive spectrometer to reveal its wear features. The wear test results show that the wear rates of the composites are gradually reduced over the sliding speed range for both normal loads. The composite wear rates are higher than that of the alloy at low speeds and lower when sliding speed further increased. The coefficient of friction results of both the alloy and composites are in the range of 0.25–0.45 and reaches minimums at 5 m/s under 10 N and 3 m/s under 30 N load. Microstructural characterization results established different dominant mechanisms at different sliding speeds, namely, abrasion, delamination, oxidation, adhesion and thermal softening and melting. An experimental wear map was then constructed.     

Effects of Mn content and texture on fatigue properties of as-cast and extruded AZ61 magnesium alloys

Fatigue behavior of as-cast and extruded AZ61 magnesium alloys in ambient air (20 °C–55%RH) was investigated. It was found that size and distribution of cast defect influenced tensile and fatigue performance of the as-cast alloy. Fatigue limit of the as-cast alloy was significantly low compared to the extruded alloy. The casting defects served as stress concentration sites for fatigue crack nucleation. Fatigue tests were also carried out on a high Mn content alloy. All of the specimens failed from an inclusion near the specimen surface. Fatigue limit of Mg alloy with high Mn content was lower compared to that of the low Mn content alloy. Further, investigation on the effect of texture on fatigue and fatigue crack growth behavior of the extruded AZ61 magnesium alloy plate was carried out. The results showed that fatigue strength in the longitudinal direction to the extruded direction was higher compared to those in the transverse and 45° directions. Significant effect of specimen orientation on fatigue crack growth behavior for both short and long cracks was found near the threshold region. However, regardless of specimen orientation, the da/dN–ΔK_eff curves for all three kinds of specimens were in a narrow band. It is suggested that the difference in the fatigue life among the specimen orientations will be mainly due to the difference in the crack closure behavior. A transition of fracture mechanism was found for a long crack. Slip fracture mechanism was dominant above the transition point, whereas below the transition point, slip fracture mechanism was associated with cleavage fracture.