發(fā)布時(shí)間：2018-10-29 18:17

【摘要】：隨著列車輕量化和高速化,給鋁合金車體的焊接技術(shù)及其焊接結(jié)構(gòu)可靠性帶來更大的挑戰(zhàn)。攪拌摩擦焊(FSW)是鋁合金車體最具潛力的材料連接技術(shù)。7075鋁合金由于比強(qiáng)度、比剛度較高、塑性較好,在載運(yùn)工具制造以及航空航天等領(lǐng)域應(yīng)用廣泛。本文利用攪拌摩擦焊(FSW)技術(shù),對厚度為6 mm的7075鋁合金進(jìn)行不同工藝平板對焊試驗(yàn),使用一系列微觀/納觀組織表征手段對接頭焊核區(qū)(WNZ),熱機(jī)影響區(qū)(TMAZ),熱影響區(qū)(HAZ)及母材(BM)中的沉淀相尺寸及形狀進(jìn)行定量表征,并借助透射電鏡(TEM)對不同形狀沉淀相的高分辨圖像進(jìn)行觀測,通過晶格條紋間距計(jì)算,實(shí)現(xiàn)對沉淀相種類的精確表征,以探求7075鋁合金FSW接頭不同微區(qū)強(qiáng)度失配機(jī)理。在此基礎(chǔ)上,對接頭進(jìn)行FSW焊后常規(guī)熱處理與分級熱處理試驗(yàn),并對熱處理后接頭微觀組織以及顯微硬度分布進(jìn)行表征,進(jìn)而優(yōu)化熱處理工藝參數(shù)。最后,借助拉伸卸載以及原位拉伸試驗(yàn)對7075鋁合金FSW接頭變形損傷機(jī)理進(jìn)行研究。研究結(jié)果表明:(1)7075鋁合金FSW接頭顯微硬度分布呈極其不均勻分布的“W”型,微區(qū)顯微硬度值與微區(qū)中沉淀相種類和尺寸、晶粒尺寸及加工硬化行為有關(guān);(2)接頭中沉淀相主要有棒狀MgZn2相、橢圓狀A(yù)lCuMg相以及膠囊狀A(yù)l2CuMg相三種;強(qiáng)化相種類不同,微區(qū)顯微硬度值不同,AlCuMg相和Al2CuMg相強(qiáng)化效果好于MgZn2相;WNZ中沉淀相主要是AlCuMg和Al2CuMg,加之細(xì)晶強(qiáng)化,顯微硬度較高,達(dá)到145 HV;相比WNZ,TMAZ中AlCuMg和Al2CuMg相對較少,MgZn2相對較多,顯微硬度降低;HAZ中MgZn2相對更多,加工硬化和細(xì)晶強(qiáng)化效果也較弱,顯微硬度進(jìn)一步下降,HAZ與TMAZ交界處顯微硬度達(dá)到整個(gè)接頭的最低值,約為125 HV;(3)FSW接頭中WNZ、TMAZ、HAZ及BM均出現(xiàn)無析出區(qū)(PFZ),PFZ的存在使接頭在一定程度上發(fā)生軟化;(4)7075鋁合金最佳FSW工藝為攪拌頭旋轉(zhuǎn)速度800 r/min,焊接速度300mm/min,FSW接頭抗拉強(qiáng)度為514 MPa,達(dá)到BM的91%。相比于原始接頭,常規(guī)熱處理與分級熱處理后,接頭中沉淀相經(jīng)歷了固溶處理+人工時(shí)效、沉淀相發(fā)生顯著細(xì)化,顯微硬度明顯提高并趨于均勻分布,常規(guī)熱處理與分級熱處理后FSW接頭平均顯微硬度分別達(dá)到165 HV、160 HV;最佳熱處理工藝為固溶溫度470℃、時(shí)效時(shí)間36 h,最佳分級熱處理工藝為一級時(shí)效時(shí)間12 h、二級時(shí)效時(shí)間12 h,常規(guī)熱處理效果好于分級熱處理。(5)7075鋁合金FSW接頭在拉伸過程中,微裂紋首先在WNZ底部形核,接著又在前進(jìn)側(cè)HAZ與TMAZ交界處形核,最終前進(jìn)側(cè)HAZ與TMAZ交界處的裂紋發(fā)展成主裂紋,主裂紋沿前進(jìn)側(cè)HAZ與TMAZ的交界處向前擴(kuò)展導(dǎo)致接頭斷裂。前進(jìn)側(cè)HAZ與TMAZ的交界處是整個(gè)7075鋁合金FSW接頭的最薄弱環(huán)節(jié)。
[Abstract]:With the lightening and high speed of the train, the welding technology of aluminum alloy body and the reliability of welding structure are more challenging. Friction stir welding (FSW) is the most potential material joining technology for aluminum alloy body. 7075 aluminum alloy is widely used in the manufacture of carrier tools and aerospace due to its high specific strength, high specific stiffness and good plasticity. In this paper, friction stir welding (FSW) technique is used to test the plate butt welding of 7075 aluminum alloy with a thickness of 6 mm. A series of microscopic / nano microstructure characterization methods are used to characterize the (WNZ), thermal impact zone (TMAZ),) in the nuke zone of the joint. The size and shape of precipitate phase in heat-affected zone (HAZ) and base metal (BM) were quantitatively characterized. The high-resolution images of precipitated phases with different shapes were observed by transmission electron microscopy (TEM), and the lattice fringe spacing was calculated. The types of precipitated phases were accurately characterized to find out the mismatch mechanism of different microstructures in 7075 aluminum alloy FSW joints. On this basis, the FSW welding routine heat treatment and graded heat treatment tests were carried out, and the microstructure and microhardness distribution of the joints after heat treatment were characterized, and the heat treatment process parameters were optimized. Finally, the deformation damage mechanism of 7075 aluminum alloy FSW joint was studied by tensile unloading and in situ tensile test. The results show that: (1) the microhardness distribution of 7075 aluminum alloy FSW joints is "W" type with extremely uneven distribution, and the microhardness value is related to the type and size of precipitate phase, grain size and work hardening behavior; (2) there are three kinds of precipitated phases in the joint: rod-like MgZn2 phase, ellipsoid AlCuMg phase and capsule Al2CuMg phase, the strengthening effect of AlCuMg phase and Al2CuMg phase is better than that of MgZn2 phase. The precipitation phase in WNZ is mainly composed of AlCuMg and Al2CuMg, and fine grain strengthening. The microhardness of the precipitated phase is higher than that of WNZ,TMAZ. Compared with that of WNZ,TMAZ, there are fewer AlCuMg and Al2CuMg, more MgZn2 and lower microhardness. There are more MgZn2 in HAZ, and the effect of work hardening and fine grain strengthening is also weaker. The microhardness of HAZ / TMAZ joint decreases further. The microhardness at the junction of HAZ and TMAZ reaches the lowest value of the whole joint, about 125 HV;. (3) the absence of (PFZ), PFZ in WNZ,TMAZ,HAZ and BM in FSW joints makes the joints soften to some extent; (4) the optimum FSW process for 7075 aluminum alloy is that the rotating speed of the stirring head is 800rmin, the welding speed is 300mm / min, the tensile strength of the joint is 514 MPa, and the tensile strength of the joint is 91cm of BM. Compared with the original joint, after conventional heat treatment and graded heat treatment, the precipitation phase in the joint experienced the artificial aging of solution treatment, the precipitation phase was refined significantly, and the microhardness increased obviously and tended to uniform distribution. The average microhardness of FSW joints after conventional heat treatment and graded heat treatment is 165 HV,160 HV;, respectively. The optimum heat treatment process is as follows: solution temperature 470 鈩，

本文編號：2298435