GH909合金激光焊接接头的微观组织特征

Microstructural Characteristics of Laser Welded Joints of GH909 Alloy

采用光纤激光器对2mm厚GH909合金进行焊接实验,探究了焊缝横截面宏/微观成形、外延生长组织演变和接头典型位置的晶体分布特征。研究结果表明:GH909合金焊缝呈典型的沙漏形,焊缝区晶粒尺寸明显大于母材和热影响区的晶粒尺寸;典型的柱状晶析出在焊缝中部和下部,少量等轴晶在焊缝顶部形成;平面生长的针状晶优先在上部近焊缝区形成,并通过竞争淘汰转变为稳定生长的细长柱状树枝晶,与此同时,中部近缝区发生胞状晶、胞状树枝晶向柱状树枝晶的转变;迁移晶界出现在热影响区,焊缝中靠近熔合线的位置存在外延生长和非外延生长两种生长模式;焊缝中部晶粒呈现为典型的非连续生长特征,晶粒宽度由小变大,但立即被新晶粒代替;焊缝上部晶粒的生长方向一致,呈现聚集特征,连续生长的γ柱状晶最终形成贯穿的巨大晶粒。

Objective Owing to the excellent performance and the low coefficient of thermal expansion at high temperatures,Fe-Co-Ni-based superalloys have been widely used in the precision structural components of aerospace.As these alloys lack a Cr element,there is no volume expansion effect caused due to solid-phase transformation.Meanwhile,the presence of Nb,Ti,and Al makes these alloys prone to element segregation and cracking during solidification.Certain products will be discontinued,and flight safety may be seriously affected.Based on this,laser welding was performed on GH909 alloy to investigate the microstructure evolution and the orientation characteristics of the joints.Many studies have been published on the relationship between the weldability,the microstructure,and the joint properties of 900 series superalloys.However,there have been few studies on the joint evolution of microstructures and crystal orientation properties.As a result,we investigated the macroscopic forming,microstructure,and crystal orientation properties of laser-welded joints made of the GH909 alloy to rich related therory of GH909 superalloy welding metallurgy and expand application field of laser welding.Methods This study conducted experimental investigations on 2 mm thickness GH909 alloy plates.The welded article was transversely sectioned with a line cut machine into three samples for microstructural examination.Samples were fabricated via a series of rough grinding,fine grinding,and mechanical polishing.The electrolyte and aqua regia diluent were used to remove the stress on the surface and display the partition and the microstructural characteristics of the joints using the scanning electron microscopy in backscattered electron mode and electron backscattered diffraction technique.Results and Discussions The weld of GH909 alloy presented a typical“hourglass shape”and the weld-zone grain size was larger than that of the base metal and heat-affected zone(Fig.3).Typical columnar grains precipitated in the middle and the lower parts of the weld,while a small number of equiaxed grains formed on the top of the weld(Fig.4).Due to the similar shape of the fusion line,the growth of grains at the top and the bottom of the weld demonstrated horizontal symmetry.Planar acicular grains formed preferentially in the upper near-weld zone and were transformed into elongated columnar dendrites with stable growth via competition selection.Moreover,a change occurred in the middle near-weld zone involving cells,cellular dendritic,and columnar dendritic.Migrated grain boundaries were observed in the heat-affected zone,and two growth modes,including epitaxial and nonepitaxial growths,were observed near the fusion line of the weld(Fig.5).The grains in the middle weld exhibited typical discontinuous growth patterns.The grain changed from coarse to fine,and it was quickly replaced by new grains.The grain boundary expansion and liquefaction occurred while the grains coarsened in the heat-affected zone,resulting in forming the migrated grain boundary(Fig.7).Two growth modes,including epitaxial and nonepitaxial growths,existed in the position near the fusion line of the weld.The analysis of grain boundary orientation difference showed that the heat-affected zone(average value:43.45°)and weld(average value:38.16°)near the fusion line were primarily composed of large-angle grain boundaries(>15°--50°),with more grain boundaries of orientation difference between>50°and 63°formed in the nonepitaxial growth mode of the weld.The grain growth direction was consistent in the upper weld,indicating an agminated feature.γcolumnar grains continuously grew and eventually formed a large perforated grain(Fig.8).The grain boundary orientation decreased gradually from the heat-affected zone to the center of the weld.Conclusions The joint’s weld cross-section had a typical“hourglass shape”.Typical columnar crystals appeared in the middle and lower welds,with a few equiaxed crystal zones at the top of weld.The grain in the weld zone coarsened noticeably when compared to that in the base metal and heat-affected zone.Near the weld zone of the upper fusion line,needle-like crystals in a plane form existed,which formed elongated columnar dendritic crystals through competition and elimination mechanism.Near the weld zone of the middle fusion line,the transition from cellular crystals and cellular-like dendrites to columnar dendrites happened.There were migrating grain boundaries in the heataffected zone.Moreover,two modes,including epitaxial and nonepitaxial growths,emerged near the fusion line of the weld seam.The grains in the middle of weld grew in a typical discontinuous pattern,with the grain increasing from fine to coarse.However,these grains were quickly replaced by new grains.The grain growth direction was consistent at the top of the weld,indicating aggregative characteristics.The continuous growth of the columnar crystals resulted in a large perforated grain.