Application Of Nanosecond Pulse Laser In Dissimilar Metals Laser Microwelding Of Steel-Aluminum

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.