Arc-based directed energy deposition of Inconel 718 with cold metal transfer

Abstract

Directed energy deposition via electric arc (DED-Arc) using wire as the feedstock, also known as wire-arc additive manufacturing (WAAM), is an important additive manufacturing technique capable of producing large-scale components of hard-to-machine materials such as Inconel 718 (IN718). The present work is aimed at investigating the resulting microstructural and mechanical performance obtained with DED-Arc deposition using a low heat input variant of gas metal arc welding (GMAW) process: cold metal transfer (CMT). Single-bead depositions were employed to determine the effect of decreasing the heat input by adjusting the wire feed speed from 6 m/min to 10 m/min and the travel speed from 0.2 m/min to 0.8 m/min using the CMT process. Microstructural observations demonstrate that the Laves length decreased from 2.60 to 1.68 micrometre and NbC diameter from 0.90 to 0.59 micrometre with increasing the travel speed. In addition, the scale of the solidification structure also decreased, which increased the hardness of the deposition from 212 to 232 HV. Furthermore, the process stability quantified by the regularity of the CMT droplet transfer also increased from 79.2% to 96.8%. Based on this, a single-bead-linear wall sample was produced with parameters selected to minimise the heat input, by using a high travel speed of 1.0 m/min and low inter-pass temperature of 80 degree Celsius. Microstructural analysis showed that a good metallurgical bond between the layers has been achieved despite the decrease of heat input to about 0.18 kJ/mm. The response to standard heat treatments applied in DED-Arc IN718 conforming to aerospace and oil and gas standards (AMS 5663 and API 6ACRA, respectively) were evaluated and compared against wrought IN718 under similar heat treatments. Digital image correlation (DIC) was validated against clip-on extensometer and subsequently employed during quasi-static tensile tests at room temperature with as-built and heat-treated DED-Arc IN718 coupons. Overall, the mechanical properties were lower than the wrought counterpart after heat treatment (e.g. 11 - 33% lower yield strength along the horizontal direction). However, the API heat treatment applied to DED-Arc IN718 promoted a higher degree of Laves dissolution and slightly less anisotropic tensile properties in comparison to AMS 5663 (e.g. 7% vs. 11% difference in yield strength along the build and travel direction) resulting in properties more close to the wrought counterpart. This was mainly due to the differences in the solution treatment temperatures (1040 vs. 980 degree Celsius). Nevertheless, the API heat treatment also led to localised low hardness (~ 50 HV below the average) and grain growth in the interlayer region, as identified by optical microscopy. It was also found via fractography, microstructural analysis and DIC that solidification defects were linked to appreciable reduction and scatter in tensile strength and ductility (~ 0.05 standard deviation in tensile strain and 40 - 140 MPa in tensile strength). Systematic deposition experiments were then conducted to correlate the as-built tensile properties aimed at minimising the defect formation in DED-Arc IN718. This was achieved by decreasing the travel speed from 1.0 m/min to 0.4 m/min, changing the inter-pass temperature by using a short inter-pass time of 30 s instead of ~ 500 s and using a pulsed-spray droplet metal transfer (GMAW-P) to increase the arc power from 3025 W in the CMT mode to 3923 W. In addition, build interruptions were added mid-length of selected samples to understand how this would impact the observed properties. The temperature distribution and local cyclic thermal history were measured via calibrated infrared imaging to interpret the effect of heating cycles on the obtained mechanical performance. Overall, a higher ductility and tensile deformation capability were obtained by decreasing the travel speed from 1.0 m/min to 0.4 m/min with a controlled inter-pass temperature o

Date of Award	31 Dec 2023
Original language	English
Awarding Institution	The University of Manchester
Supervisor	John Francis (Supervisor) & Matthew Roy (Supervisor)