AA5754鋁合金電阻點焊接頭的微觀組織和力學(xué)性能

AA5754鋁合金電阻點焊接頭的微觀組織和力學(xué)性能Microstructure and mechanical property of AA5754 aluminum alloy resistance spot welds

以汽車車身用AA5754鋁合金板材制備的電阻點焊接頭為研究對象，通過金相觀察、顯微硬度矩陣測試和拉伸試驗對焊接接頭的微觀組織、工藝缺陷與力學(xué)性能進行表征與分析。結(jié)果表明：根據(jù)組織和性能的差異，接頭可分為3個典型區(qū)域：焊核區(qū)、熱機械影響區(qū)和母材區(qū)。接頭焊核心部為等軸枝晶組織，焊核邊部為柱狀枝晶組織，焊核周邊的熱機械影響區(qū)內(nèi)的材料發(fā)生部分熔化，產(chǎn)生了晶內(nèi)及晶界枝晶。接頭中主要缺陷為噴濺、焊核區(qū)氣孔、縮孔和沿晶裂紋、液化裂紋。母材區(qū)硬度沿厚度方向漸變，邊部呈現(xiàn)最大值，心部呈現(xiàn)最小值；焊核硬度與母材硬度相近；焊核兩側(cè)的熱機械影響區(qū)中，在接頭的壓痕邊緣和板材接觸面縫隙尖端處存在1~2 mm寬的硬化區(qū)。接頭的強度與焊點直徑呈正相關(guān)關(guān)系，拉剪試驗的失效模式為焊點熔核界面斷裂，剝離試驗的失效模式為紐扣剝離斷裂。

AA5754 aluminum alloy sheets used for automotive body were joined by resistance spot welding. The microstructure, discontinuities and mechanical properties of the welds were characterized and analyzed by optical microscope observation, Vickers microhardness grid measurement and tensile tests. The results show that, according to the differences of microstructure and properties, the weld joint can be divided into three regions, i.e. nugget, thermo-mechanically affected zone (TMAZ) and base metal (BM). An equiaxed dendritic structure exists in the nugget center while a columnar structure is observed at the nugget perimeter. Minor amounts of inner and inter-grain dendritic structures, resulting from partial melting, is found in TMAZ. Discontinuities in the welds are expulsion, gas bubbles, shrinkage cavity, shrinkage cracks and liquation cracks. The thru-thickness hardness of the BM sheet displays a distinct variation. A 1-2 mm wide hardened zone was observed around the indentation perimeter and crack tip on the faying surface in the TMAZ. The positive correlation is found between the weld strength and weld button diameter. The fracture mode for tensile-shear tests is interfacial fracture while button pull-out fracture for coach-peel tests.

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