Microstructure and Properties of Plasma Arc Powder Additive Manufactured 316L Stainless Steel with Different Cooling Conditions

Compared with laser additive manufacturing process, plasma arc powder additive manufacturing has a great deal of advantages, including higher energy efficiency, higher building efficiency, higher density and lower cost of device. However, because of the heat accumulation effect of multiple deposition layers, the shape and mechanical properties of additive manufactured part by plasma arc powder additive manufacturing were hard to control. The proper method to avoid excessive heat accumulation was to increase the inter-layer delay time for natural cooling of the building part or add forced cooling method to decrease the temperature of already formed part. Under the condition of forced cooling conditions, the microstructure and mechanical properties of plasma arc additive manufactured parts could be improved. Moreover, the relationship between microstructure and cooling condition of conventional forming process had been established in detail. In this paper, the thin-wall specimens were fabricated by plasma arc powder additive manufactured device under the condition of gas forced cooling and natural cooling, respectively. 316L powders was used as the deposition materials, which has been proved to be suitable for additive manufacturing large-scale part. After plasma arc powder additive manufacturing process, the macromorphology, microstructure, dendrite size, microhardness and room temperature tensile properties of the formal building specimens were analyzed by optical microscope (OM), scanning electron microscope (SEM), microhardness tester and universal material testing machine, to obtain the influence mechanism of different cooling conditions on the microstructure and properties of the samples fabricated by plasma arc powder additive manufacturing. The results show that: under the condition of gas forced cooling by compress air for 30 s between every adjacent deposition layer, the growth direction of the over deposited layer was more obvious, and the ratio of equiaxed crystal to columnar crystal in the deposited layer was obviously less than that in the natural cooling condition, especially for the bottom layer. The microstructure of the deposition specimens was in the morphology of dendrite under both cooling conditions, while the secondary dendrite arm spacing was smaller under the condition of gas forced cooling, and the difference was more obvious with the increase of deposition layer. This could be explicated by the solicitation theory, the secondary dendrite arm spacing (λ2) was proportional to the product of temperature gradient (G) and solidification rate (R), under the condition of gas forced cooling the value of G during solidification in the molten pool was increased, as a result, the secondary dendrite arm spacing (λ2) was smaller. The finer microstructure in uniform distribution lead to more uniform distribution of the overall hardness under the condition of gas forced cooling. The microhardness of natural cooled specimens was obviously decreased from HV 166.4 to HV 123, while the microhardness distribution of forced cooling specimens was more uniform, the amplitude variation was restricted to 10. According to the stress-strain curve and fracture morphology, it could be concluded that the tensile strength and yield strength of the sample under the condition of gas forced cooling were increased, which corresponds to the refinement of dendrite size in the deposited layer, and the tensile strength increased from 489 to 523 MPa. On the basis of the microstructure and mechanical test result, it was clear that the interlayer cooling process could improve the homogeneity of microstructrue and mechanical properties. The heat accumulation effect of the plasma arc powder additive manufacturing could be avoid by reasonable cooling method, especially for the thin-wall structure. However, the interlayer cooling condition on the microstructure of the other alloy might be more complex because of the solid-solid phase transformation, like martensitic stainless steel. And the different cooling method might also change the direction of heat flow, which might influence the microstructure orientation and morphology, and would be needed for the additive manufacturing and remanufacturing of directional solidified alloy. In future work, the influence between more complex cooling condition to the microstructure of nickel based alloy, martensite stainless steel will be discussed. © Editorial Office of Chinese Journal of Rare Metals. All right reserved.