Investigation On Laser Cleaning Mechanism Of Oxide Layer,Process Parameters And The Influence On Laser Welding Of Aluminum Alloy

Laser cleaning,as a new cleaning method,has a combination of advantages including high efficiency,environment friendliness,easy operation and high precision,etc.Therefore,laser cleaning has a great potential application in manufactural industries.In this thesis,5754 Al alloy was chosen as the experimental materials.The experiment was carried out using a laser cleaning equipment and fiber laser welding system.The scanning electron microscopy(SEM),energy disperse spectroscopy(EDS)and high-speed camera(HSC)was used to test samples.The cleaning mechanism of oxide layer on Al alloy and the relationship between process parameters and laser cleaning effect was studied in detail.The influence on laser welding quality after laser cleaning was also explored.The model of materials thermodynamics and mathematic relationship among different laser cleaning parameters was used to illustrate and analyze experimental results in detail.In terms of laser mechanism,due to different physical and chemical properties between surface oxide layer and Al alloy substrate,the differences of laser energy absorption and states change after absorbing laser energy were significant.Based on the model of materials thermodynamics,the theorical energy density of the oxide layer removal is 16.4 J/cm~2.At low energy density,phase explosion caused by laser ablation was the main cleaning mechanism.At high energy density,besides phase explosion,the separation of the oxide layer and substrate induced by thermal stress coupling effect after absorbing laser energy,and impact effect induced by evaporation pressure caused the splattering and removal of oxide layer.The cross-section area,brightness and existent time of plasma have positive correlation with energy density during laser cleaning.In terms of process parameters,experiment and relationship among process parameters showed that energy density was the key factor to affect laser cleaning effect and surface roughness.Process parameters were mainly through affecting energy density and distribution of pulse crater on the sample surface to achieve good laser cleaning effect.Energy density has positive relationship with laser cleaning effect and surface roughness,it changed surface roughness mainly through changing the morphology of the pulse crater.Overlap ratio changed laser cleaning effect and surface roughness through changing the number of pulse crater at same zone.The maximum roughness was in the overlap ratio at 50-67%.The results showed the initial and complete cleaning thresholds of 2?m oxide layer on Al alloy is 12.7 J/cm~2 and 25.5 J/cm~2,respectively.In terms of the influence on laser welding,compared with uncleaned sample,the weld of laser cleaning was brighter.The penetration depth and across-section aera of weld showed tendency of increasing,which was more obvious at low heat input.Based on the differences of surface oxide layer and morphology between cleaned and uncleaned samples,qualitative analysis was given for weld size changing.The sizes of sample weld were similar with different cleaning methods,and at low heat input,compared with laser cleaning,the welds after chemical and mechanical cleaning become narrower and deeper.