【作者】 肖华；

【导师】 闫培光；

【作者基本信息】 深圳大学 ， 光学工程（专业学位）， 2019， 硕士

【摘要】 随着科技的进步,在实际工程应用中,对材料的性能要求越来越高,在一些场合,单一的材料已经无法满足更严格的工程需求,而这些应用场合往往对加工精度要求特别高,这就为异种材料激光焊接创造了广泛的施展空间。纳秒脉冲激光因其独特的性能,在难焊、难熔、脆性材料的焊接应用具有明显优势,而钢、铝是应用最广泛的两种材料,但其之间的焊接性很差,因此纳秒脉冲激光微焊接工艺成为解决钢-铝异种材料焊接问题的有效手段。本文采用纳秒激光焊接系统以及0.2 mm厚的304不锈钢和0.4 mm厚的6063铝合金作为对象,并以钢上铝下搭接接头方式,利用金相显微镜、扫描电镜、能谱仪、硬度和拉力测试仪器等设备对焊接效果进行综合分析研究。首先研究了激光功率、焊接速度、工作频率、图形间距、波形等参数变化对焊接效果的影响,结果表明,随着激光功率的提高以及焊接速度、工作频率、图形间距、波形峰值功率50%处的脉宽的减小,焊接熔深和剪切强度逐渐增加,但当接头内部出现裂纹后,剪切强度开始急剧下降。论文考察了焊接参数对外观的影响,例如随着功率的降低、频率的增加,焊点氧化程度减弱,且表面更加光洁平滑,可以作为焊接外观要求较高时参数选择的依据,亦可对焊点进行重熔修复,但需注意额外输入热量的影响。本文提出一种通过功率、速度、频率、图形间距等参数初步评估裂纹是否产生的方式即裂纹影响因子,并通过试验验证了其准确性,为预判裂纹提供了一种思路。论文还通过0.2 mm厚的6063铝合金和0.4 mm厚的304不锈钢研究了钢下铝上的反置搭接方式进行了对比研究,发现但所需激光功率和速度增加,强度略微降低,但焊接缺陷明显增多,外观变差。通过对选定参数试样的微观组织分析发现,Fe-Al金属间化合物呈现规律性分布,在靠近Al基材的区域主要生成FeAl₃,在靠近Fe基材的区域主要生成Fe₃Al,并且由α-Al过渡到Fe₃Al的速度很快,FeAl₃生成物也较薄,焊点底部和侧面的岛状物主要是Fe₃Al,絮状物和层状物主要是FeAl₃,另外还研究裂纹产生的原因是主要剧烈的熔池反应、过高的热输入量导致脆性金属间化合物增多,在焊接应力作用下发生开裂。通过对选定参数试样的硬度测试发现,接头硬度基本分布在400⁵00 HV之间,与母材相比硬度出现了较大幅度的提升,尤其接头底部界面区产生脆硬相的某些区域硬度值高达780 HV。对试样进行剪切强度测试的结果为139.8 MPa,力学性能优异。对强度测试断口形貌的分析表明,断裂模式是以微孔聚集型剪切断裂为主,但局部微小区域伴有脆性解理断裂的混合断裂模式。对断口特征区域成分分析结果显示,焊接接头破坏主要发生在铝合金基材靠近焊接区域附近,而非Fe-Al金属间化合物生成区域,最后针对这种现象分析了几种可能原因。更多还原

【Abstract】 With the development of science and technology,material performance plays an increasingly significant role in practical engineering applications,however,for some cases,a single material has been unable to meet the more and more strict engineering requirements where processing accuracy is usually very high,which all pave a wide path for laser welding of dissimilar materials.Due to its unique properties,ultra-narrow pulsed laser shows obvious advantages in the welding of difficult to weld,refractory and brittle materials.Steel and aluminum alloys are ranked as the two of most extensively used materials,but they can’t be directly welded together by traditional welding ways.In recent years,ultra-narrow pulsed laser micro-welding technology has been recommended as an effective way to solve the welding problem between steel and aluminum alloys.In this paper,nanosecond laser welding system was used to weld 304 stainless steel with a thickness of 0.2 mm and 6063 aluminum alloy with a thickness of 0.4 mm,in which overlap welding was applied with the former metal on the top,and the latter on the bottom.Welding spots were inspected and analyzed by metallographic microscope,scanning electron microscope,energy spectrometer,hardness and tensile testing instruments etc.Firstly,the effect of welding parameters such as laser power,welding speed,frequency,line distance,and waveform on welding spot was studied,and it was found that with the increase of laser power and the decrease of welding speed,frequency,line distance,and the pulse width where peak power was at 50 %,weld penetration and shear strength rose gradually,but if cracks occurred inside the welded joint,shear strength began to fell sharply.The influence of welding parameters on spot appearance was investigated too,and the result shows that both low power and high frequency contribute to a smooth and polishing surface appearance,in the mean time,the degree of oxidation drops to a great extent,which can be served as a reference for parameter selection where appearance weighs a lot,or be applied in laser re-melting repair,but the consequence of additional heat input should be paid attention.A judge method was proposed to preliminarily evaluate whether the crack was generated or not by parameters such as power,speed,frequency and line distance,namely the crack influence factor,and its accuracy had been verified by experiments,providing an idea of predicting crack.In addition,inverted overlap welding,as a contrast,was also arranged,in which 0.2 mm thick 6063 aluminum alloy lay the upper layer,0.4 mm thick stainless steel located in the lower.It was observed that on the one hand,higher power and faster welding speed were needed,and on the other hand,welding defects were apparently increased along with bad appearance and slightly declined strength.By analyzing the spot microstructure of samples with selected parameters,it could be seen clearly that Fe-Al intermetallic compounds had a regular distribution.The area closed to Al substrate mainly generates Fe Al3 that is relatively thin,on the other side,the region near Fe substrate primarily forms Fe3 Al that can be quickly transformed from a-Al.Besides,on the bottom and lateral side of the spot,Fe3 Al morphology presents like island form,reversely lamellar and flocculent shape represent Fe Al3.What’s more,welding crack had been further researched and the reason why cracks emerged could be concluded as following: intensive reaction in melting pool and over-high heat input collectively lead to much more brittle intermetallic compound,and combined with welding stress,crack occurs.Hardness test of selected samples demonstrated that the value of the welded joint was basically distributed between 400 and 500 HV,which manifested a big improvement compared with that of base material,especially in some interface areas where brittle phase occurred at the bottom of the joint,the hardness could even reach up to 780 HV.Shear strength of the specimens is around 139.8 MPa,which indicates the spot owns excellent mechanical property.After strength test,the analysis of the fracture morphology had been conducted.The fracture belongs to mixed type which is mainly characterized by shear fracture in the form of micro-pore accumulation,and accompanied by brittle cleavage fracture in local micro-area.Component analysis of the fractured area shows that the failure of the welded joint primarily occurs in the welding area near aluminum alloy substrate,rather than the formation area of Fe-Al intermetallic compound.Finally,several possible reasons for this phenomenon are analyzed.更多还原