【摘要】：AISI 304不銹鋼是應(yīng)用最為廣泛的一種奧氏體不銹鋼,很多情況下它的應(yīng)用都需要利用到一種重要的加工方法——焊接。相對于傳統(tǒng)的焊接方法,激光焊接因其具有能量集中、熱影響區(qū)小、熱變形小及焊接速度快等優(yōu)點(diǎn),故能提高304不銹鋼焊接接頭的質(zhì)量。由于接頭質(zhì)量與宏觀形貌和組織性能密切相關(guān),所以很有必要對304不銹鋼激光焊接接頭形貌和組織性能進(jìn)行研究。本文以304不銹鋼激光焊接接頭為對象,旨在研究焊接工藝參數(shù)對接頭宏觀形貌、微觀組織和機(jī)械性能的影響規(guī)律,得到合適的焊接工藝參數(shù)。此外,建立適用于激光焊接的三維熱源模型和有限元分析模型,實(shí)現(xiàn)焊接溫度場的三維動態(tài)模擬,從宏觀形貌和微觀組織兩方面將模擬結(jié)果與試驗(yàn)結(jié)果聯(lián)系起來,指導(dǎo)工藝參數(shù)的設(shè)計(jì)和優(yōu)化。首先,采用板厚為0.7 mm的304不銹鋼板進(jìn)行焊接工藝試驗(yàn),分析了工藝參數(shù)對焊縫宏觀形貌的影響規(guī)律。當(dāng)工藝參數(shù)選擇得當(dāng)時(shí),所獲得的焊縫具有合適的焊縫尺寸且沒有宏觀焊接缺陷的產(chǎn)生。然后,將得到的焊接接頭制成金相分析試樣并進(jìn)行電解腐蝕,利用光學(xué)顯微鏡對焊縫的微觀組織進(jìn)行觀察,發(fā)現(xiàn)焊縫的微觀組織不同于母材,且工藝參數(shù)對焊縫的微觀組織有影響,主要體現(xiàn)在焊縫邊緣柱狀晶區(qū)域?qū)挾群秃缚p中心等軸晶尺寸的變化上。對接頭不同區(qū)域進(jìn)行顯微硬度測試,發(fā)現(xiàn)焊縫的顯微硬度值高于母材。此外,對不同工藝參數(shù)下的焊接接頭進(jìn)行拉伸試驗(yàn),并將接頭的拉伸強(qiáng)度與焊縫的宏觀形貌和微觀組織聯(lián)系了起來。當(dāng)焊縫擁有較好的宏觀形貌和較小的晶粒尺寸時(shí),能獲得拉伸性能最好的焊接接頭,且接頭在拉伸試驗(yàn)后于母材處斷裂。利用掃描電鏡對該拉伸斷口的形貌進(jìn)行拍攝,發(fā)現(xiàn)斷面上存在大量韌窩,斷裂模式為韌性斷裂。最后,根據(jù)激光焊接的焊縫成形特征,構(gòu)造了雙橢球體熱源和高斯面熱源疊加的組合熱源模型,利用有限元分析軟件ABAQUS和FORTRAN語言編寫的用戶子程序,實(shí)現(xiàn)了激光焊接溫度場的三維動態(tài)模擬。在宏觀形貌方面,模擬結(jié)果中的熔池邊界線和試驗(yàn)結(jié)果中的熔合線基本吻合,驗(yàn)證了模型的可靠性;在微觀組織方面,通過焊接溫度場預(yù)測焊縫液相中的溫度梯度和焊縫中心的冷卻速度,以此定性地推斷焊縫結(jié)晶形態(tài)的傾向和晶粒的相對大小,探究焊接溫度場與焊縫微觀組織的關(guān)系。
[Abstract]:AISI 304 stainless steel is one of the most widely used austenitic stainless steels. Compared with the traditional welding method, laser welding can improve the quality of 304 stainless steel joint because of its advantages of concentrated energy, small heat affected zone, small thermal deformation and fast welding speed. Because the joint quality is closely related to macroscopic morphology and microstructure and properties, it is necessary to study the microstructure and properties of 304 stainless steel laser welded joint. The purpose of this paper is to study the effect of welding process parameters on the macroscopic morphology, microstructure and mechanical properties of 304 stainless steel laser welded joints, and to obtain appropriate welding process parameters. In addition, a three-dimensional heat source model and a finite element analysis model for laser welding are established to realize the three-dimensional dynamic simulation of the welding temperature field. The simulation results are related to the experimental results in terms of macroscopic morphology and microstructure. Guide the design and optimization of process parameters. Firstly, the welding process of 304stainless steel plate with a thickness of 0.7 mm was carried out, and the influence of process parameters on the macroscopic morphology of the weld was analyzed. When the process parameters are selected at that time, the obtained weld has a suitable weld size and no macroscopic welding defects. Then, the welded joints were made into metallographic analysis samples and electrolytic corrosion. The microstructure of the weld was observed by optical microscope, and it was found that the microstructure of the weld was different from that of the base metal. The effect of process parameters on the microstructure of the weld is mainly reflected in the variation of the width of the columnar crystal zone and the equiaxed grain size of the weld center at the edge of the weld. The microhardness of weld is higher than that of base metal. In addition, tensile tests were carried out on the welded joints with different process parameters, and the tensile strength of the joints was related to the macroscopic morphology and microstructure of the welds. When the weld has better macroscopic appearance and smaller grain size, the welded joint with the best tensile properties can be obtained, and the joint breaks at the base metal after tensile test. Scanning electron microscopy (SEM) was used to photograph the morphology of the tensile fracture. It was found that there were a large number of dimples on the fracture section and the fracture mode was ductile fracture. Finally, according to the weld forming characteristics of laser welding, the combined heat source model of double ellipsoid heat source and Gao Si surface heat source superposition is constructed, and the user subprogram written by finite element analysis software ABAQUS and FORTRAN language is used. The three-dimensional dynamic simulation of laser welding temperature field is realized. In terms of macroscopic morphology, the boundary line of the molten pool in the simulation results is basically consistent with the fusion line in the experimental results, which verifies the reliability of the model. The temperature gradient in the liquid phase of the weld and the cooling rate of the weld center are predicted by the welding temperature field. The tendency of the crystal morphology of the weld and the relative size of the grain are deduced qualitatively, and the relationship between the welding temperature field and the microstructure of the weld is explored.
【學(xué)位授予單位】：武漢理工大學(xué)
【學(xué)位級別】：碩士
【學(xué)位授予年份】：2015
【分類號】：TG456.7

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