【作者】 孟宇；

【导师】 谷晓燕；

【作者基本信息】 吉林大学 ， 材料与化工（专业学位）， 2024， 硕士

【摘要】 为了应对日益严重的环境污染和能源短缺问题,轻量化成为了工业发展的主要研究方向之一。镁合金是最轻的金属结构材料,具有很高的比强度、比刚度、铸造性能,而且资源储量非常丰富。而钢具有较高的绝对强度,在重要的承载结构中具有不可替代性。镁合金与钢的连接符合轻量化理念,具有重要的研究意义。由于镁合金和钢的物理和化学性能差异较大,几乎不能形成冶金结合,导致二者的直接熔化焊接非常困难。超声波焊接是一种典型的、不引起材料熔化的固相焊接技术,具有焊接时间短、能耗低等特点,对于镁合金/钢异种材料的连接具有独特优势。本文进行了AZ31B镁合金与Q235钢的超声波点焊,研究了不同焊接参数下接头的界面成形、微观结构和力学性能,建立微观组织与接头力学性能的相关性。实验结果表明:对于镁/非镀锌钢超声波焊接接头,随着焊接时间、焊接振幅的增加,焊接能量和界面峰值温度逐渐提高,母材表面会先由初始凹凸不平的状态转变为平直状态,之后界面处形成波浪状起伏,增大了接头的结合面积,界面扩散层的厚度逐渐增加;当焊接压力过小时,界面处没有形成波浪状起伏,接头结合面积较小,当焊接压力过大时,阻碍了工件的相对摩擦,界面处也没有形成波浪状起伏,0.3MPa～0.4MPa为合适的工艺窗口。对于镁/镀锌钢超声波焊接接头,在焊接时间为0.4s时,界面处开始发生Mg-Zn共晶反应;在焊接时间为0.6s时,界面处Zn层全部发生共晶反应,Zn层消失,Mg-Zn共晶组织均匀分布于整个焊接区,其面积较大;在焊接时间为0.8s时,共晶液相在焊接压力的作用下被挤出焊头作用区域,在焊头作用区外的两板接触面之间铺展,起到了类似钎料的作用,这会使得接头的有效结合面积增大;当焊接时间为1.0s时,界面处出现了起伏的现象,此时接头界面成形机制与镁/非镀锌钢类似;当焊接时间为1.2s时,界面处镁侧出现了沿晶界分布的的网状共晶组织。随着焊接时间的增加,镁/非镀锌钢接头的强度呈先增大后减小的趋势,当焊接时间为0.4s时,接头断口处出现了大量的未结合区域;当焊接时间为1.0s时,接头强度达到最大,剪切力为2298N,此时接头的断裂形式为解理断裂;当焊接时间为1.2s时,由于压痕深度过大,镁板的有效承载面积减小,接头强度开始降低。对于镁/镀锌钢接头,在焊接时间为0.4s时,界面处生成的共晶液相能够有效去除镁合金表面的氧化膜,促进了镁合金与钢的紧密接触,使得接头的强度得到显著提高;当焊接时间为0.8s时,共晶液相在焊接压力的作用下,被完全挤出焊头直接作用的焊接区域,在焊头作用区外的两板接触面之间铺展,起到了类似钎料的作用,这会使得接头的有效结合面积增大;当焊接时间为1.0s时,接头强度达到最大,剪切力为3903N,断口处发现大量的撕裂棱和韧窝,此时接头的断裂形式为准解理断裂;当焊接时间为1.2s时,因镁板的有效承载面积减小,接头强度出现下降,断口处出现明显的孔洞,在孔洞附近出现沿晶断裂的特征。更多还原

【Abstract】 In response to the escalating environmental pollution and energy shortages,lightweight has become one of the primary research directions in industrial development.Magnesium alloys,as the lightest metallic structural materials,possess exceptionally high specific strength,specific stiffness,and casting properties,coupled with abundant resource reserves.Steel,with its high absolute strength,plays an irreplaceable role in critical load-bearing structures.The connection between magnesium alloys and steel aligns with the lightweight concept and holds significant research significance.However,due to substantial differences in the physical and chemical properties between magnesium alloys and steel,it is hard for them to form metallurgical bonding,making direct fusion welding of the two extremely challenging.Ultrasonic welding is a typical solid-phase welding technique that does not induce material melting.It features short welding times and low energy consumption,offering unique advantages for joining dissimilar materials such as magnesium alloys and steel.In this paper,ultrasonic spot welding of AZ31 B magnesium alloy and Q235 steel is investigated.The interface formation,microstructure,and mechanical properties of the joints with various welding parameters are examined.Correlations between microstructure and joint mechanical properties are established.The experimental results indicate that,for magnesium/non-galvanized steel ultrasonic welded joints,with an increase in welding time and amplitude,welding energy and peak interface temperature gradually rise.The surface of the base material transitions from its initial uneven state to a flat state,followed by the formation of undulations at the interface,thereby increasing the bonding area of the joint.The thickness of the interface diffusion layer gradually increases.When the welding pressure is too low,undulations do not form at the interface,resulting in a smaller bonding area.Conversely,excessive welding pressure impedes relative friction between the workpieces,also preventing the formation of undulations at the interface.An appropriate process window lies between 0.3 MPa and 0.4 MPa.For magnesium/galvanized steel ultrasonic welded joints,at a welding time of0.4 seconds,the Mg-Zn eutectic reaction begins at the interface.At 0.6 seconds,the entire Zn layer disappears due to the Mg-Zn eutectic reaction.The Mg-Zn eutectic structure is uniformly distributed throughout the entire weld area with larger area coverage.By 0.8 seconds,the eutectic liquid phase is squeezed out of the weld nugget area under the action of welding pressure,spreading between the contact surfaces of the two plates outside the weld nugget area.It plays a role similar to solder,thereby increasing the effective bonding area of the joint.At 1.0 second,undulations appear at the interface,resembling the formation mechanism of magnesium/non-galvanized steel joints.By 1.2 seconds,a network-like eutectic structure distributed along grain boundaries emerges on the magnesium side of the interface.With increasing welding time,it shows a trend of initially increasing and then decreasing for the strength of magnesium/non-galvanized steel joints.At a welding time of 0.4 seconds,numerous unbounded regions appear at the joint fracture surface.When the welding time reaches 1.0 second,the joint strength peaks at 2298 N shear force,with the fracture mode being cleavage fracture.However,at 1.2 seconds,due to excessive indentation depth,the effective load-bearing area of the magnesium plate diminishes,resulting in a decrease in joint strength.For magnesium/galvanized steel joints,at a welding time of 0.4 seconds,the eutectic liquid phase formed at the interface effectively removes the oxide film on the surface of the magnesium alloy,facilitating close contact between the magnesium alloy and the steel.This significantly enhances the strength of the joint.At 0.8 seconds,under the influence of welding pressure,the eutectic liquid phase is completely extruded from the welding area directly affected by the welding tip.It spreads between the contact surfaces of the two plates outside the welding tip area,acting similarly to solder.This results in an increase in the effective bonding area of the joint.When the welding time reaches 1.0second,the joint strength peaks at 3903 N shear force,with the fracture surface showing abundant tear edges and dimples.At this point,the fracture mode of the joint is quasi-cleavage fracture.However,at 1.2 seconds,due to the reduced effective loadbearing area of the magnesium plate,the joint strength decreases.Significant holes appear at the fracture surface,with characteristics of trans-granular fracture observed near the holes.更多还原