Study On Defects Of Laser Direct Deposition Of 24CrNiMoY Alloy Steel

DLD as a research hotspot of the creative industry today,is in the stage of rapid development.Laser additive manufacturing technology is a multi-physics coupling process.Due to the influence of temperature,process parameters,scanning methods and other factors during the forming process,it will cause inclusion defects,crack defects,and porosity defects in the deposited layer samples.The existence of defects will affect the mechanical properties of the formed parts.The formed parts prepared in industrial production must avoid the existence of these defects,so the behavior of the defects and the cause of formation need to be studied.In order to study the behavior of defects,a fiber laser was used to directly deposit24CrNiMoY alloy steel powder on the Q235 steel substrate.DLD was used to prepare three sets of 24CrNiMoY alloy steel samples under different parameters.The first set changes the scanning speed(5 mm/s,6 mm/s,7 mm/s,8 mm/s)to optimize process parameters;the second set changes the number of deposited layers(3 layers,5layers,7 layers);The third set changes the scanning mode(the included angles of the scanning directions between adjacent layers are 0~o,45~o,and 90~orespectively).The deposited samples were characterized by metallographic,scanning,hardness,tensile,densification and other experimental methods.The defect behavior and mechanical properties of the sediment samples under three different parameters were studied.The findings are as follows:In order to study the behavior of defects,two sets of 24CrNiMoY alloy steel samples were prepared by DLD.The first set changes the scanning speed(5 mm/s?6mm/s?7 mm/s,?8 mm/s)to optimize the process parameters;the second set changes the scanning method(the angle between the scanning directions between adjacent layers is 0~o?45~o?90~o respectively).Metallographic,scanning,hardness,tensile,friction and abrasion experiments were used to study the defect behavior and mechanical properties of the deposited samples.The findings are as follows:When the scanning speed was changed,inclusion defects and crack defects were found in the sample of the deposited layer.Under the experimental conditions,the inclusion defects are mainly composed of carbides and oxides.As the scanning speed increases,the content of inclusion defects in the sample increases.The cracks crack along the grain boundaries and belong to thermal cracks.Stress is the direct cause of cracks.There are cracks starting from the lowest layer in the sample,and the cracks that propagate along the top have the greatest stress during the first layer scanning.At the same time,the existence of crack defects was also found around the inclusion defects.As the source of the cracks,the inclusion defects would cause stress concentration,thereby forming crack defects.When the number of deposited layers changes,only the inclusion defects exist in the sample,and the area ratio of the inclusion defects decreases with the superposition of the number of deposited layers.With the increase of the number of deposited layers,the temperature around the sample increased,resulting in remelting.When the scanning method is changed,it is found that the mechanical properties of the deposited layer sample are the best in the parallel scanning mode.At this time,the hardness value distribution is uniform,and the average hardness,density,and tensile properties are the best.When the scanning mode is 45~o,porosity defects are generated,and crack defects are found around the porosity defects.The porosity defects serve as the crack source and cause the formation of crack defects.When the scanning method is vertical,the inclusion defect does not fall off in the sample.This study provides a certain reference for defect elimination of DLD alloy steel.