本文選題：超聲輔助激光焊接 + 熔池流動��； 參考：《哈爾濱工業(yè)大學》2017年碩士論文

【摘要】：鎂合金是近年來在工程領(lǐng)域應用廣泛也是最輕的金屬材料,廣泛地應用于各個領(lǐng)域。因為鎂合金自身的一系列特殊的物理性質(zhì),導致在激光焊接過程中極易出現(xiàn)氣孔、晶粒粗大等問題。超聲輔助加工技術(shù)在金屬凝固、表面改進以及焊接領(lǐng)域得到了廣泛的研究,實現(xiàn)了細化晶粒,消除氣孔和裂紋,從而改善材料綜合性能的效果。因此,本課題提出超聲輔助激光焊接工藝,利用超聲波在焊接熔池中產(chǎn)生的空化效應、聲流效應以及機械與熱效應,消除鎂合金在焊接時出現(xiàn)的一系列問題,并研究其機理,為鎂合金的高質(zhì)量焊接打下理論基礎(chǔ)。與常規(guī)的鎂合金激光焊相比,施加超聲后,焊縫表面平整美觀,但超聲輔助無法解決焊縫的下塌缺陷;增大激光功率或降低焊接速度,焊縫熔寬和下塌量均有所增加;在其他工藝參數(shù)相同的情況下,只有負離焦能夠得到穩(wěn)定的焊接過程,正離焦和零離焦均會出現(xiàn)匙孔塌陷而導致焊接過程不連續(xù);偏移量和變幅桿壓力主要影響傳播到鎂合金熔池的超聲能量。隨著偏移量的增大,損失的超聲能量越多,超聲對焊縫成形的影響越小;變幅桿壓力大小主要影響超聲從變幅桿端部傳到工件表面處的能量損失,當壓力很小或很大時,超聲能量損失較大,引起焊縫成形不良。通過掃描電鏡以及EDS能譜分析表明鎂合金焊接時主要產(chǎn)生氫氣孔以及匙孔不穩(wěn)定產(chǎn)生的“小孔型”氣孔。在焊接速度較低的情況下,向熔池中施加超聲波可以減小焊縫中氣孔缺陷的產(chǎn)生;采用示蹤元素法對熔池流動進行分析研究,發(fā)現(xiàn)施加超聲后,焊接熔池的流動性增強。但隨著焊接速度的提高,這種現(xiàn)象變得越來越不明顯;結(jié)合超聲對熔池流動特性以及超聲波產(chǎn)生的效應,闡述了超聲抑制焊接氣孔機理:施加超聲波輔助后,液態(tài)熔池中產(chǎn)生空化效應、聲流效應以及機械攪拌效應,促進氣泡的形核、長大及逸出過程,從而抑制氣孔缺陷的產(chǎn)生。對焊縫微觀組織及力學性能進行分析,AZ31B鎂合金激光焊焊縫相組成主要由α-Mg基體和第二相β-Mg17Al12構(gòu)成,β相彌散分布于晶界和晶粒內(nèi)部,焊縫熔合線處的柱狀晶區(qū)較大。施加超聲輔助后,熔池中產(chǎn)生空化效應、聲流效應以及機械攪拌作用,能夠改善液態(tài)金屬的潤濕性,有利于熔池金屬的形核,同時超聲波效應也會促進熔池的流動,使已結(jié)晶的晶粒更容易被流動的金屬卷到熔池中,促進等軸晶的形成;且由于超聲波的加入,焊接接頭的力學性能有所提高。
[Abstract]:Magnesium alloy is widely used in engineering field and is the lightest metal material in recent years. It is widely used in various fields. Due to a series of special physical properties of magnesium alloy, it is easy to appear porosity and coarse grain in laser welding process. Ultrasonic assisted machining technology has been widely studied in the fields of metal solidification, surface improvement and welding. It can refine grains, eliminate pores and cracks, and improve the comprehensive properties of materials. Therefore, the ultrasonic assisted laser welding process is proposed in this paper. The cavitation effect, acoustic flow effect and mechanical and thermal effect of ultrasonic wave in welding pool are used to eliminate a series of problems in welding of magnesium alloy, and its mechanism is studied. It lays a theoretical foundation for high quality welding of magnesium alloy. Compared with the conventional laser welding of magnesium alloy, the weld surface is flat and beautiful after ultrasonic application, but ultrasonic assistance can not solve the collapse defect of weld, and the weld width and collapse amount increase with increasing laser power or decreasing welding speed. When other process parameters are the same, only negative defocusing can obtain a stable welding process, both positive defocusing and zero defocusing will cause keyhole collapse, which leads to discontinuity of welding process. The ultrasonic energy propagating to the molten pool of magnesium alloy is mainly affected by the offset and the pressure of the variable amplitude bar. With the increase of deviation, the more ultrasonic energy is lost, the smaller the influence of ultrasonic on weld formation, and the pressure of the horn affects the energy loss from the end of the horn to the surface of the workpiece, when the pressure is very small or large, The loss of ultrasonic energy is large, which leads to bad weld formation. Scanning electron microscopy and EDS energy dispersive spectrum analysis showed that hydrogen pores and keyhole instability were mainly produced in magnesium alloy welding. In the case of low welding speed, ultrasonic wave applied to the weld pool can reduce the formation of porosity, and the fluidity of the weld pool is enhanced by the tracer element method. However, with the increase of welding speed, this phenomenon becomes less and less obvious. Combined with the ultrasonic effect on the flow characteristics of molten pool and ultrasonic wave, the mechanism of ultrasonic suppression of welding porosity is expounded: after the application of ultrasonic aid, Cavitation effect, acoustic flow effect and mechanical stirring effect are produced in liquid molten pool, which can promote the nucleation, growth and escape of bubbles, thus inhibiting the formation of pore defects. The microstructure and mechanical properties of the weld are analyzed. The weld phase composition of the laser welding of AZ31B magnesium alloy is mainly composed of 偽 -Mg matrix and the second phase 尾 -Mg17Al12. The 尾 phase is dispersed in grain boundary and inside the grain, and the columnar crystal zone is larger at the weld fusion line. With the aid of ultrasonic, cavitation, acoustic flow and mechanical stirring can improve the wettability of liquid metal, which is beneficial to the nucleation of molten pool metal, and the ultrasonic effect can also promote the flow of molten pool. The crystallized grains are more easily rolled into the molten pool by the flowing metal to promote the formation of equiaxed crystals, and the mechanical properties of welded joints are improved due to the addition of ultrasonic wave.
【學位授予單位】：哈爾濱工業(yè)大學
【學位級別】：碩士
【學位授予年份】：2017
【分類號】：TG457.19

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