【作者】 赵颖；

【导师】 黄坚；

【作者基本信息】 上海交通大学 ， 材料工程， 2012， 硕士

【摘要】 本文以钛合金蒙皮骨架结构为主要应用背景,以钛合金板材的激光搭接焊为主要研究对象,利用大功率YAG激光器和快速轴流型CO₂激光器,对钛合金的激光焊接工艺进行了系统的研究,并对接头的组织和性能进行了分析。采用YAG激光器在钛合金板材上进行表面堆焊试验表明:激光头后倾时,焊缝熔深和熔宽都较激光头前倾时大,轴向保护气体对等离子体的抑制作用更明显;激光头倾角在6¹0度范围内变化对熔深和熔宽的影响不明显;轴向保护气体流量对熔深、熔宽等没有显著的影响,但对焊缝表面的保护效果影响显著;随着离焦量的变化,熔深和熔宽在较小范围内产生变化;随着激光功率的升高,焊接速度的降低,焊缝热输入量增大,焊接时熔化的金属量增多,导致熔深、熔宽均增大。搭接工艺试验表明:骨架的宽度、厚度,蒙皮与骨架间的间隙量以及激光横向偏移量对熔深、熔宽的影响不大,间隙量和激光横向偏移量增大会使焊缝表面咬边、凹陷明显。TC4钛合金CO₂激光搭接焊试验表明:随着焊接热输入减小,焊缝的熔深、熔宽及两板连接宽度均减小;当焊接热输入保持不变,随着激光功率的增加,焊缝的熔深、熔宽及两板连接宽度均明显增大,这说明激光功率在增大焊缝尺寸方面起了更为重要的作用。钛合金YAG激光焊和CO₂激光焊后,熔透和非熔透焊缝均呈典型的钉形形状,钉形焊缝的上下两部分区域的β柱状晶具有不同的生长方向,且上部β柱状晶尺寸大于下部。YAG激光焊非熔透焊缝底端过渡圆滑,而CO₂激光焊底端有尖端凸起的现象。在相同的热输入情况下,YAG激光焊较CO₂激光焊焊缝要宽。焊缝组织为针状马氏体αˊ组成的篮网组织,热影响区中存在少量细小马氏体组织,且呈梯度分布。随着热输入量的增大,β相柱状晶有增大的趋势且内部针状马氏体更加密集。焊接热输入量保持不变,但随着激光功率的增大,焊缝中心的组织也随之变的粗大。钛合金激光焊易产生气孔,气孔趋于分布在熔合线附近;焊接工艺参数相同,未熔透焊缝内部存在大量气孔,熔透焊缝内部气孔缺陷不明显。由于马氏体的界面增强效应,焊缝横向显微硬度沿焊缝中心向母材逐渐降低,焊缝上部的显微硬度值略高于下部。随着焊接热输入的增大,接头的显微硬度值和抗剪强度均增大,但当热输入过大时,焊缝的组织变的粗大,反而降低了接头的显微硬度值和抗剪强度;当焊接热输入相同时,较大的激光功率会得到较粗大的柱状晶组织,从而降低焊接接头的硬度和抗剪强度。更多还原

【Abstract】 The main application backgrounds of this paper are frame-covering structures of titanium alloy, and its research subjects are titanium alloy laser welded lap joints. By using high power YAG laser beam and CO₂ laser beam, the influence of laser welding parameters were studied, and the microstructure and mechanical properties of the joints were also analyzed.By using YAG laser beam, the surfacing welding of titanium alloy sheets with 6 mm thickness is carried out. It is found that when the laser head lean backward, both weld penetration and width increased compared to the laser head lean forward. Within 6¹0 degree range, the angle of inclination of laser head had little influence on variation of weld penetration and width. The flowrate of axial protection gas have significant effect on weld protection. With the defocusing value changing, the fusion width and depth in a small scope changes. Both weld penetration and width increased with the laser power increasing and welding speed decreasing. The laser welding experiments of TC4 titanium lap joints indicates that the width and thickness of the skeleton, the gap between covering and skeleton and laser transverse offset had little influence on variation of weld penetration and width. Gap and transverse offset increase made bite side and hollow obvious.The CO₂ laser welding experiments of TC4 titanium lap joints show that the weld depth, width and two board connection length decreased with reducing welding heat input. When welding heat input was fixed for a value, with increasing laser power and welding speed by the same proportion, the weld depth, weld width and two board connection length all increased obviously. It manifested that laser power variation played a more important role in increase of weld size.The weld shape both of penetration weld and partial penetration weld are like nails form.βcolumnar crystals in different zones have different direction of growth, and the upperβcolumnar crystals dimensions is greater than the lower. The root of partial penetration weld has smooth transition by YAG laser welding, however, the root has cusp by CO₂ laser welding. The microstructure of fusion zone is like network made up of acicularαˊ martensite, the base metal is composed of darkβphase in the brightαmatrix, the HAZ is a mixture of primaryαandαˊ. With the weld heat input increased, the grains in weld joints became coarse. When weld heat input was fixed for a value, larger laser power has resulted in grain growth.The microhardness gradually reduced from fusion zone to the HAZ and base metal due to the interface enhancement effect of martensites. Covering have higher microhardness in the FZ and HAZ than the skeleton has.Both shear strength and microhardness increased with the weld heat input increasing. However, when the weld heat input is excessive, microstructure of weld joint became coarse and microhardness and shear strength decreased.When the weld heat input was fixed for a value, larger laser power has resulted in grain growth and decrease of shear strength and microhardness of lap joints.更多还原