热挤压成形GH3625合金管材组织及裂纹形成机理

通过对比热挤压成形管材和爆裂管材的组织以及对爆裂管材裂纹和断口的分析,研究了热挤压成形GH3625合金管材的组织及裂纹形成机理。结果表明:爆裂管材与成形管材的组织均为等轴晶,但爆裂管材的开裂使晶界处的应力集中得以释放,其组织中并没有形成变形孪晶,在管材径向方向上也不存在晶粒尺寸不均匀的现象。挤压比过高导致管材在热挤压过程中绝热升温严重,使低熔点的Laves相熔化并扩散到周围基体中,是裂纹形成的根本原因。在模具出口处高拉应力的作用下,这些裂纹不断扩展最终连接在一起,导致管材的爆裂现象。由于断口表面冷却速率较高,组织通过奥氏体区的时间较短,再结晶形核核心多且晶粒长大过程受阻,使断口表面形成了一层十分细小的再结晶晶粒。

Microstructure and Crack Forming Mechanism of GH3625 Alloy tube by Hot Extruded Forming Process

By comparing the microstructure of hot extruded forming tube and burst tube as well as the analysis of crack and fracture of burst tube, the microstructure and crack formation mechanism of GH3625 alloy tube during hot extrusion were studied. The results show that the microstructure of the burst tube and the formed tube are equiaxed crystals, but the cracking of the burst tube causes the stress concentration at the grain boundary to be released. Besides, no deformation twins are formed in the microstructure and there is no uneven grain size in the radial direction of the tube. The root cause of crack formation is that the extrusion ratio is too high, which causes the adiabatic heat of the tube to be severely heated during the hot extrusion process, so that the low melting Laves phase melts and diffuses into the surrounding matrix. Under the action of high tensile stress at the exit of the mold, these cracks continue to propagation and eventually join together, causing the tube to burst. The higher cooling rate of the fracture surface leads to a shorter time for the passage of the microstructure through the austenite region, more recrystallized nucleation core and the grain growth process is hindered, so that a very fine recrystallized grain is formed on the fracture surface.