Performance of discontinuity-free components produced by additive turning computer aided manufacturing strategy

Abstract

Computer aided manufacturing (CAM) techniques for directed energy deposition (DED) affect the material properties of the manufactured component based on the scan strategy used. This research investigates the material characteristics of turning-style toolpath strategies to generate axisymmetric components with additive manufacturing (AM), referred to in this research as additive turning. This novel approach leverages existing CAM technology for turning, where the component rotates around a stationary cutting tool, to generate toolpath trajectories for DED with varying wall-thicknesses and controlled deposition angles. This strategy allows for entire components to be deposited in one continuous deposition, resulting in reduced cycle-time and improved material usage efficiency compared to conventional AM strategies where the beam is switched off at the end of every layer. Results from this study show that the use of additive turning can produce over 99 % dense components with less variation and anisotropy in texture and hardness, as well as a lower variation in elongation to failure when compared to conventional strategies. This research highlights that various CAM strategies could be deployed for AM to improve process efficiency or enable localized control over part performance.

Introduction

It’s well known that scan strategies in additive manufacturing (AM) impact the material properties of the resulting component. 316 L stainless steel has been widely studied by the AM community. Various AM techniques such as powder bed fusion, blown powder DED, wire arc, and binder jet are being utilized in current work by Dryepondt et al. (2021), Lecis et al. (2021), Lee (2020), and Kim et al. (2019), respectively. The majority of reported literature on 316 L stainless steel manufactured by AM indicate the microstructure of 316 L stainless steels to be fully austenitic with a columnar grain structure with solidification cells. However, considering the rapid cooling rates of AM and associated solute segregation, researchers such as Yadollahi et al. (2015), Ziętala et al. (2016), Bajaj et al. (2020), and Lecis et al. (2021) have reported δ ferrite getting retained at room temperature in 316 L stainless steel fabricated by DED. Irrespective of the retention of δ ferrite, considering the absence of any solid-state phase transformations post solidification, 316 L stainless steel enables a better understanding on the effect of the toolpath on microstructure.

Honeycutt et al. (2021), Praniewicz et al. (2019), and Feldhausen et al. (2020) have indicated that generating directed energy deposition (DED) toolpaths for large axisymmetric components with complex features is difficult and time consuming. Applications such as rocket nozzles, nose cones, and pipes used in the oil and gas industries have been explored by Gradl et al. (2018), Panchagnula and Simhambhatla (2018), and Yamazaki (2016), respectively. Common computer aided manufacturing (CAM) strategies for these components offer the use of one-dimensional spiraling toolpaths for thin-walled components where only one bead thickness is needed. For applications where a wall thickness greater than one bead is needed, a strategy that employs the use of concentric circles in a piecewise layer fashion (similar to the strategy shown in Fig. 2a) is often used. With this strategy, the manufacturing time and material waste is increased at each start/stop point, known as a process discontinuity. To the best of the authors’ knowledge, there are currently no studies that explore spiral toolpath strategies that produce components with more than a one-bead wall-thickness.

The toolpath explored in this study, termed additive turning, utilizes a three-dimensional spiral toolpath, allowing axisymmetric components greater than one bead thick to be manufactured with a continuous deposition strategy. The constant spiraling of the toolpath results in the weld-tracks to be constantly crossing, analogous to layer rotations in raster-styled slicing strategies. This ensures that any defects are homogeneously distributed throughout the part. This continuous deposition toolpath strategy allows for components to be made faster with less wasted material. However, the material properties of such a strategy are unknown and is the focus of this study.