TC4钛合金活性激光焊焊缝成形及组织性能

Weld Formation, Microstructure and Mechanical Properties of Laser Welded TC4 Titanium Alloy with Activating Fluxes

摘要:

选取CaF₂、NaF和Na₂SiF₆作为活性剂对TC4钛合金进行活性激光焊接，在焊接过程中利用高速摄像机采集焊件上方的光致等离子体图像，焊后借助光学显微镜测量焊缝熔深和熔宽，利用EBSD观察分析焊缝显微组织，采用能谱仪测试焊缝元素组成，测试接头硬度和抗拉强度。结果表明，由于活性剂改变了焊件表面状态和压缩光致等离子体面积从而提高了激光吸收率，这使得焊缝熔化区面积增加；活性剂均可以使未熔透焊缝的熔深增加，正面熔宽减小，熔透焊缝正面熔宽减小，中部和背面熔宽均增加，3种活性剂中Na₂SiF₆对TC4钛合金激光焊焊缝成形的影响最明显；活性剂对焊缝下部显微组织影响不大，涂敷CaF₂和NaF后，焊缝上部β柱状晶的生长方向与未涂敷焊缝β柱状晶生长方向都指向焊缝表面，涂敷Na₂SiF₆焊缝上部β柱状晶的生长方向指向焊缝中心。涂覆CaF₂焊缝针状马氏体α′形态变化不明显，涂敷NaF的针状马氏体α′变得稍细，而涂敷Na₂SiF₆的针状马氏体α′变得短且细，较涂敷CaF₂和NaF的焊缝变化最明显。涂敷Na₂SiF₆的接头抗拉强度提高约12.5%，从改善焊缝成形和提高接头力学性能考虑，Na₂SiF₆可作为TC4钛合金激光焊的首选活性剂。

Abstract:

CaF₂, NaF and Na₂SiF₆ were chosen as activating fluxes for laser welding of Ti6Al4V (TC4) alloy. During the welding process, the laser-induced plasma images above the weldment were captured with a high-speed camera. After welding, the weld penetration and the width of the weld were measured and the microstructure was observed by an optical microscope. Then, the morphology of acicular martensite in the weld was studied by EBSD, and the element composition of the weld was determined by energy spectrometer. The hardness and tensile strength of the joints were tested. The results show that the activating fluxes change the surface state of the weldment and compress the laser-induced plasma area, thereby increasing the laser absorptivity and the area of the weld melting zone. Besides, all the activating fluxes can increase the penetration depth and reduce the top weld width of the partial penetration weld, and reduce the top weld width while increase the middle and bottom weld widths of the penetration weld. It is also found that for the welds coated with CaF₂, NaF and without activating fluxes, the β columnar crystals on the upper part of the welds all grow towards the weld surface, while for the welds coated with Na₂SiF₆, the β columnar crystals on the upper part of the welds grow towards the weld center. The activating fluxes have no obvious effect on the growing direction of the β columnar crystals of the lower part of the weld. Na₂SiF₆ can significantly refine the acicular α′ in the β columnar crystals of the upper part of the welds in the laser welding of TC4 alloy. The tensile strength of the welded joint coated with Na₂SiF₆ is increased by 12.5%. Compared with other activating fluxes, Na₂SiF₆ has the most prominent effect on the weld formation and mechanical property of the laser welded TC4 titanium alloy, and thus it can be used as the most preferred activating flux.