LOW-CYCLE FATIGUE STRENGTH OF THE HEAT-RESISTANT ALLOY SPECIMENS PRODUCED BY SELECTIVE LASER MELTING PROCESS 

Abstract

Interest in additive manufacturing has increased recently. The mechanical characteristics of the Inconel 718 superalloy conventionally applied in aerospace engineering were studied in detail. Insufficient attention was paid to the low-cycle fatigue (LCF) of materials produced by selective laser melting (SLM) at stresses and frequencies characteristic of gas-turbine engines. This paper describes comparative LCF studies and examines the microstructure and fracture of fatigue specimens produced by hot rolling and SLM from Inconel 718 alloy powders. The studies were conducted at room and elevated temperatures. The SLM specimens were subjected to hot isostatic pressing and heat treatment. For comparative studies, a part of the specimens was prepared by hot rolling from the same alloy. The rolled specimens were not subjected to hot isostatic pressing. The results indicate that heat treatment imparts polyhedric structure to the hot rolled materials and leads to clear layer-by-layer fusion areas approximately 100 mm in size. After heat treatment, the alloy is strengthened with an intermetallic g''-Ni3Nb phase, g' phase, and carbides. A lamellar d phase was also found in the microstructure. Upon the LCF tests, ‘strain–number of cycles to failure’ logarithmic curves were plotted to determine, with adequate reliability, the reasonable level of stresses to allow the predetermined number of cycles to failure. The analysis suggests that the precipitates present at grain boundaries of the basic g phase (MC carbides and lamellar d phase) and the amount of the intermetallic g'' phase are likely to influence the slope of the ‘strain–number of cycles to failure’ curves.