Influencing Factors And Restraining Methods Of Crack Formation In Selection Laser Melting Forming Nickel-based Superalloy IN738

Selective laser melting technology has developed rapidly around the world because of its unique “layer by layer” additive manufacturing method,which has the characteristics of short processing cycle,high material utilization rate,and the ability to form complex structural parts that are difficult to achieve with traditional processes.IN738 alloy is better than most of the existing nickel-based high-temperature alloys due to its high-temperature creep properties and heat-resistant corrosion properties.It is widely used in hot-end components such as aircraft engines and gas turbines.Selective laser melting technology has unique advantages in directly forming such complex structural parts.However,high crack susceptibility and poor density of molded parts are the shortcomings of selective laser melting forming IN738 alloy,due to the high alloying degree and the high Al and Ti content in IN738.For this reason,IN738 alloy powder is used as the forming material.Using microstructure characterization and analysis,solidification model establishment,mechanical property testing and other means,the forming performance and crack generation and suppression of powder in the laser melting technology of the selected area were studied.Mainly in order to reveal the formation mechanism of cracks,establish the relationship between “powder raw materials-forming process parameters-part performance”,and then provide a reasonable reference for powder evaluation standards and the adjustment of the performance of formed parts.The study of single molten pool found that the shape of the molten pool changes continuously from irregular intermittent shape to regularity with the increase of laser power and the decrease of scanning speed.When the laser power is high and the scanning speed is low,the formed molten pool is in the form of regular continuous strips.When the laser power is less than 270 W and the speed is greater than or equal to 1750 mm/s,the energy absorbed by the powder is reduced,the spheroidization is serious,and the typical teardrop-shaped particles or even spherical particles are formed,which cannot form a continuous molten pool.As the scanning speed increases,the laser power decreases,the melt pool width decreases,and the melt penetration depth also decreases.The research results of two batches of IN738 alloy powder shaped parts show that: the increase of the Zr element content in the original powder increases the crack density of the XOY and XOZ surfaces of the formed part;the increase of the Zrelement content reduces the solidus temperature of the alloy and promotes the The formation of M(Ti,Nb,Mo,W,Zr)C carbides and ?+?' eutectic phase at the boundary;the internal cracks of the IN738 alloy formed by SLM are mainly solidification cracks,and they follow the <001> in the center of the molten pool The orientation of the columnar grain boundary distribution reduces the ultimate tensile strength of the part from 1113 MPa to 610 MPa,and the plasticity also decreases seriously.For the selective melting of IN738 alloy by laser melting,the powder chemical composition,especially crack sensitive elements,should be strictly controlled,for example,the Zr element content should be kept below 0.02 wt.%.After optimizing the process parameters,it was found that when the laser power was 270 W,the scanning speed was 2250 mm/s,the scanning interval was 0.07 mm,and the scanning angle was 67°,the internal crack density and size of the formed samples were the smallest,and the cracks were significantly suppressed The density reached 99.2%,and the mechanical properties increased by 55.2% to 947 MPa,and the plasticity increased significantly.For different batches of the same alloy,especially powders with different element contents,when forming with the same process parameters,the uniformity of the forming quality and the performance of the parts cannot be guaranteed;when the powder properties change,the SLM process parameters must be optimized before forming Different powders have matching optimal forming parameters,and this matching increases the cost and cycle in actual production.Optimal adjustment of process parameters can improve defects and performance of powder molded parts with deviations in composition,but the improvement effect is limited.