Exploratory study of directed energy deposition conducted at low temperature −20 °C

Abstract

This experimental work is the first to investigate the directed energy deposition (DED) process at low temperature. DED is a metal additive manufacturing process commonly used to print or repair metal parts. Currently, DED can only be performed under laboratory conditions with the temperatures of 68°F to 77°F (20–25 °C). The manufacturing characteristics of DED make it have great potential in onsite additive manufacturing. However, the onsite DED needs to conducted at the temperature of the field environment, which cannot be changed to the optimal laboratory temperature. When onsite DED is carried out in a low-temperature environment, such as winter or the northern hemisphere, the ambient temperature is usually very cold, often below freezing temperature. The ambient temperature has a great influence on the DED process, such as the temperature gradient and cooling rate. So far, low-temperature DED has not been studied specifically for potential applications in on-site maintenance or remanufacturing. This experimental study fills this gap. In this work, the temperature of DED ambient (including substrate) was −20 °C, in order to simulate the DED process carried out in the field with low temperatures. The authors printed three thin-wall samples with stainless steel 316L powder at the low temperature condition. As comparative test, another thin-wall sample with same dimension and material was printed with same DED processing parameters at room temperature (20 °C). Then, the molten pool morphology, the sample geometry, the sample hardness and tensile mechanical properties were investigated experimentally. The results show that DED process can print SS316L powder smoothly at low temperature. Two interesting findings are that the sample height and molten pool size are larger at −20 °C temperature than the sample printed at 20 °C. In addition, the hardness values of thin-wall specimens printed at −20 °C are larger than those printed at room temperature and the tensile tests of low-temperature DED samples show the better mechanical properties. Comparing to the hardness value (107.5) of the sample printed at regular ambient temperature 20 °C, the hardness values of the low-temp DED printed sample had a trend of obvious increase to more than 120. In addition, the modulus acquired from tensile test was improved from 13,888 MPa to about 20,000 MPa. The Yield strength increased from 366 MPa to 400 MPa, and UTS increased from 602 MPa to about 630 MPa.