Study On Selective Laser Melting Of 24CrNiMo Alloy Steel

The parts manufactured by selective laser melting(SLM)can be directly used in many fields such as military and aerospace,only after simple post-processing due to their high density,good mechanical properties and high dimensional accuracy.Aiming at the problem of brake part of high-speed train,the SLM process of 24 CrNiMo alloy steel powder was systematically studied in this dissertation.With deeply consideration of the charateristics of SLM,the key process parameters including laser point distance,exposure time,and hatching distance,etc.,and the forming features including surface morphology,pool width,single track width,etc.of 24 CrNiMo alloy steel were investigated with single track and multi-track basic experiments.In order to obtain better process window for the formability of 24 CrNiMo,the properties and defects of the formed block were analyzed.Acoording to above mentioned research work,the following conclusions are drawn from the SLM process of 24 CrNiMo alloy steel.Firstly,for a single-track single-layer,the linear energy density decreases with the increase of point distance.With decreasing of the total height of the molten pool,the width of the molten pool decreases,and the wetting angle decreases.Also with decreasing of the melt track decreases,the surface becomes more and more uneven,and even breakpoints appear.With increasing the exposure time,more precisely,the linear energy density,the worsening surface morphology and bad forming quality gradually evolve into uniform surface morphology and good forming quality.With increasing the exposure time,the width of the molten track becomes wider,the total height of the molten track becomes larger,the molten pool becomes larger,and the wetting angle becomes larger.Also the breakpoint gradually disappear.Secondly,for a multi-track single-layer,the surface morphology is different if the hatching distance is different.The overlap rate of molten track is also different.When the overlap rate is about 0.4,the surface morphology of the single layer is an acceptable value.Simultaneously,the density of printed parts is very high.Thirdly,the influence of key process parameters on the density of 24 CrNiMo printed parts are discribled as follows.When 24 CrNiMo alloy steel is melted by SLM,the size of point distance has an obvious effect on the forming quality.Pore defects occur when point distance is too large,which reduces the density of printed parts.Point distance is too small to make the molten pool unstable and reduce the density.Hatching distance also affects the forming quality of 24 CrNiMo.It is mainly due to the overlap rate between the melt track and the melt track itself,and whether the forming quality of the melt track is good or not.When different exposure time parameters are used,the size of molten pool will also be affected due to the different laser residence time at each point,so too much or too little exposure time will cause the decrease of sample density.Forthly,the causes of pore defects in 24 CrNiMo alloy steel were analysed.When long holes appear,insufficient laser energy cannot penetrate the powder.If the hatch distance is too large,long holes appear.When a regular circular hole appears,it implies that the energy is too large under this parameter,and the metal powder is melted or even gasified,so that the thickness of the layer can not reach the designed thickness.There maybe exist three reasons for this phenomenon.The laser remelts the metal of the adjacent melting track.Or,the vaporization of the molten pool and the formed plasma bring back pressure to the molten pool,causing the protective gas to enter the molten pool and cause voids.Or,if the energy is too large,the temperature of the molten pool is high,and the high temperature metal reacts with a small amount of oxygen.The impurities produced by the reaction are insoluble in the metal liquid and cause splashing.Fifthly,the Vickers hardness of the upper surface edge,the hardness of the bottom side and the Vickers hardness of the upper part were significantly higher than the Vickers hardness of the middle part of the upper surface and the Vickers hardness of the middle part of the upper surface.The main reason is that the middle portion of the upper surface of the sample and the central portion of the side surface are subjected to repeated thermal cycling and a slow cooling rate,resulting in a low Vickers hardness.So the hardness changing is mainly caused by the difference in cooling rate,energy density,and heat treatment time.Sixthly,the the forming microstructures are composed of eutectoid ferrite and granular bainite and a small amount of martensite.The eutectoid ferrite is white slats,and the granular bainite is gray-black granular.Seventhly,the effect of oxygen content on the forming quality of 24 CrNiMo was studied.When the oxygen content is high,the 24 CrNiMo alloy steel powder easily reacts with oxygen to form an oxide impurity to form a pore defect to affect the density.The EDS image of pore defects illustrates the reason for the accumulation of carbides.Finally,the laser spot diameter has an important effect on the forming process.If the diameter is too large,the powder will gradually become non-melting along with the laser center.Since the laser density is a Gaussian distribution,the powders near the spot center is already overheated,resulting in too much liquid,making the molten pool liquid unstable.And the powders far from the center may be incompletely melted,causing splashes inside the molten pool.