超声冲击对MB8镁合金对接接头疲劳性能的影响

Effect of ultrasonic impact treating on the fatigue properties of MB8 magnesium alloy butt joint

采用HJ-Ⅲ型超声冲击设备对MB8镁合金对接接头焊趾区域进行冲击处理.利用EHF-EM200K2-070-1A电液伺服疲劳试验机对超声冲击前后的MB8镁合金对接接头进行疲劳对比试验.采用光学显微镜和高分辨率的透射电镜对超声冲击后的MB8镁合金对接接头的表层组织进行了分析.结果表明,在试验循环基数为2×106周次的条件下,超声冲击态接头的疲劳强度为52.8 MPa,比焊态试样的疲劳强度提高了37.5%,在同等应力水平下,接头的疲劳寿命提高了58~65倍.超声冲击后,焊趾处的应力集中程度降低,焊址及附近区域发生明显塑性变形,变形层厚度大约为70μm,并在焊趾表面获得了纳米晶组织,同时把焊接残余拉应力转变为压缩应力.超声冲击可以大幅度地提高MB8镁合金焊接接头的疲劳寿命.

Surface treatment was carried out on the weld toe of welded butt joint of MB8 magnesium alloy with HJ-Ⅲ type ultrasonic impact machine. The comparative fatigue test both for treated and un-treated specimens were carried out using EHFEM200K2-070-1A type electro-hydraulic servo fatigue machine.The microstructure in the top surface layer of weld toe was characterized by means of optical microscope( OM) and transmission electron microscopy( TEM). Experimental results indicate that when the fatigue life is 2 × 106,the fatigue strength of treated specimen is 52. 8 MPa,the strength has been increased about37. 5%. Compared to the un-treated specimen,the fatigue life of treated specimen has been increased 58-65 times. After impact treating,the stress concentration has been decreased,the severe plastic deformation in the surface of weld toe was formed,the thickness of the plastic deformation layer is about 70 μm,and the nano-grains were obtained on the treated surface. The tensile welding residual stresses can be reduced significantly, even change to compressive stresses. The fatigue life of MB8 magnesium alloy can be greatly improved by the ultrasonic impact treatment.