冶金型氣孔對(duì)熔化焊接7020鋁合金疲勞行為的影響

冶金型氣孔對(duì)熔化焊接7020鋁合金疲勞行為的影響The Influence of Metallurgical Pores on Fatigue Behaviors of Hybrid Fusion AA7020 Joints

基于同步輻射X射線三維高精度原位成像技術(shù)，識(shí)別和統(tǒng)計(jì)出工藝穩(wěn)定的激光復(fù)合焊接7020鋁合金接頭中的氣孔數(shù)量、形貌、尺寸和空間分布特征，結(jié)合氣孔統(tǒng)計(jì)數(shù)據(jù)結(jié)果和焊縫晶粒大小，獲取了7020鋁合金激光復(fù)合焊接頭中影響接頭疲勞性能的氣孔臨界尺寸。同時(shí)，利用同步輻射X射線三維原位疲勞實(shí)驗(yàn)數(shù)據(jù)和疲勞斷口形貌，探討了疲勞試樣裂紋源處氣孔尺寸、應(yīng)力和疲勞壽命之間的定量關(guān)系?；谟邢拊抡娣治觯芯苛瞬煌恢孟職饪滋幍膽?yīng)力場(chǎng)狀態(tài)。最后，通過(guò)疲勞裂紋擴(kuò)展速率實(shí)驗(yàn)，揭示了氣孔對(duì)疲勞裂紋萌生、擴(kuò)展和試樣瞬斷的影響。研究結(jié)果表明，激光復(fù)合焊接頭臨界氣孔尺寸可定為30 μm；同步輻射X射線成像和疲勞斷口結(jié)果顯示，較大的表面氣孔和近表面的氣孔較容易萌生疲勞裂紋。仿真研究也表明，氣孔周圍的應(yīng)力集中程度隨著氣孔位置由外部向內(nèi)部變化呈現(xiàn)先增大后減小最后趨于穩(wěn)定的趨勢(shì)；疲勞裂紋擴(kuò)展速率數(shù)據(jù)分布趨勢(shì)表明，氣孔對(duì)長(zhǎng)裂紋擴(kuò)展過(guò)程的影響較小，可忽略不計(jì)，但對(duì)裂紋前緣形貌有較大影響。

With the rapid development of Chinese high-speed railway system, the urgent demand for lighter weight is increasing, and aluminum alloys are increasingly applied into manufacturing the railway train and critical safety components. As a medium strength aluminum alloy, the 7020 aluminum alloy shows a great potential. Hybrid laser welding has currently become one of the most important welding techniques for medium and high strength aluminum alloys. Nevertheless, intrinsic defects such as pores and shrinkages physically determine the fatigue resistance of the welded joint. Based on in situ synchrotron radiation X-ray computed microtomography (SR-μCT), the number and spatial distribution of gas pores within AA7020 hybrid welded joints are identified and counted. The critical size of gas pores, affecting the fatigue properties of welded joints, is acquired by combining the statistical results of the pores and the average grain size of the hybrid weld. Meanwhile, the qualitative relationship between pore size, effective stress and fatigue life is discussed under in situ fatigue data via SR-μCT and fracture shape. By utilizing the finite element analysis, detailed work has been performed on the stress state near the pores of different positions inside the joint. Through the simulation analysis, the stress concentration coefficient around the pores firstly increases, then decreases, and finally tends to a stable trend as the location of the pore-like defect is transferred from the outside to the inside. Besides, the influence of porosity on fatigue crack initiation, fatigue crack growth and breaking process is also analyzed using fatigue crack growth experiment. In conclusion, the results show that the critical pore size of hybrid laser welding joint can be qualitatively considered as 30 μm, and the SR-μCT and fracture analysis show that larger surface and sub-surface pores are more likely to initiate fatigue cracks, the experiment of fatigue crack propagation rate shows that the porosity has little effect on the long crack growth process but significant influence on the crack front.

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