【摘要】：6系鋁合金的高強(qiáng)度、優(yōu)良焊接性能等特點(diǎn)使其廣泛應(yīng)用于軌道車輛的車體制造中,由于其導(dǎo)熱性好,焊接時需要大的熱輸入,這就導(dǎo)致焊接接頭軟化現(xiàn)象嚴(yán)重,力學(xué)性能明顯降低,采用小的熱輸入容易導(dǎo)致未熔合等缺陷,因此如何在保證接頭熔合良好前提下改善軟化問題對6系鋁合金的廣泛應(yīng)用有重要意義。本文對8 mm厚6系鋁合金板材進(jìn)行脈沖MIG焊接,焊接過程中焊縫底部墊板內(nèi)部通過循環(huán)水進(jìn)行冷卻(水冷),將水冷接頭與相同參數(shù)、自然冷卻狀態(tài)下焊接試板進(jìn)行對比,通過微觀組織和力學(xué)性能試驗結(jié)果分析快速冷卻對焊接接頭組織性能的影響。與自然冷卻相比,焊接過程中水冷明顯降低了焊接峰值溫度,提高了冷卻速度,減小了高溫停留時間;焊縫兩側(cè)的縱向殘余應(yīng)力有所降低;金相組織結(jié)果顯示焊縫中心等軸晶尺寸有所減小,焊縫邊緣處柱狀晶區(qū)域變長,填充和蓋面焊道部分熔化區(qū)晶粒重熔區(qū)域分別減小約40μm和80μm,熱影響區(qū)晶粒長大程度明顯減小;TEM試驗觀察到水冷接頭析出相尺寸明顯較小;顯微硬度結(jié)果顯示水冷接頭軟化區(qū)縮小了約6 mm,打底和蓋面焊道焊縫處硬度提高約4 HV,軟化區(qū)硬度值整體有所提高;拉伸結(jié)果表明斷裂位置發(fā)生在軟化區(qū),水冷焊接接頭抗拉強(qiáng)度提高約12 MPa,名義屈服強(qiáng)度提高約7 MPa,延伸率和斷面收縮率與自然冷卻條件接頭無明顯變化;斷口掃描結(jié)果觀察到大量等軸韌窩,說明斷裂型式為塑性斷裂,韌窩形態(tài)與自然冷卻條件下接頭斷口無明顯差別,說明兩者塑性性能相差不大。試驗結(jié)果表明焊接過程中快速冷卻有利于提高軟化區(qū)的強(qiáng)度,改善焊接接頭的組織,提高焊接接頭整體的力學(xué)性能。
[Abstract]:Due to its high strength and excellent welding properties, the 6 series aluminum alloy is widely used in the car body manufacture of rail vehicles. Due to its good thermal conductivity and large heat input during welding, the softening phenomenon of welded joints is serious. The mechanical properties are obviously reduced and the small heat input is easy to lead to defects such as non-fusion. So how to improve the softening problem on the premise of good fusion of the joints is of great significance to the wide application of 6-series aluminum alloys. In this paper, pulse MIG welding of 8 mm thick 6 series aluminum alloy sheet is carried out. In the process of welding, the bottom gasket of the weld is cooled (water-cooled) through circulating water, and the water-cooled joint is compared with the welding test plate under the same parameters and natural cooling condition. The effect of rapid cooling on the microstructure and properties of welded joints was analyzed by the results of microstructure and mechanical properties tests. Compared with natural cooling, water cooling decreases welding peak temperature, increases cooling rate and decreases residence time at high temperature, and the longitudinal residual stress on both sides of weld decreases. The results of metallographic structure show that the size of equiaxed grain in the center of the weld decreases, and the columnar region at the edge of the weld becomes longer. The grain remelting area in the partially melted zone of the filler and the cover welding pass was reduced by 40 渭 m and 80 渭 m, respectively, and the grain length of the heat-affected zone was significantly reduced by TEM test. The size of precipitated phase in the water-cooled joint was obviously smaller than that in the heat-affected zone. The results of microhardness show that the softening zone of water-cooled joint decreases about 6 mm, and the hardness of weld seam increases about 4 HV, and the tensile results show that the fracture occurs in the softening zone. Tensile strength of water-cooled welded joints increased about 12 MPa, nominal yield strength increased about 7 MPa, elongation and cross section shrinkage did not change obviously with natural cooling conditions and a large number of equiaxed dimples were observed by fracture scanning. It shows that the fracture type is plastic fracture and the dimple morphology has no obvious difference with the fracture surface under natural cooling condition, which indicates that the plastic properties of the two joints are not different from each other. The experimental results show that rapid cooling during welding process is beneficial to increase the strength of softening zone, improve the microstructure of welded joints and improve the mechanical properties of welded joints as a whole.
【學(xué)位授予單位】：河北科技大學(xué)
【學(xué)位級別】：碩士
【學(xué)位授予年份】：2016
【分類號】：TG407

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