Revisiting fundamental welding concepts to improve additive manufacturing: From theory to practice

Highlights

• Vast knowledge on welding can be used in additive manufacturing.

• Criteria to optimize parameters in additive manufacturing is proposed.

• Proposed criteria provides an upper bound limit for key process parameters.

• A dimensionless parameter is proposed to compute the energy density.

• The dimensionless parameter allows to compare works from different research groups.

Abstract

Additive manufacturing technologies based on melting and solidification have considerable similarities with fusion-based welding technologies, either by electric arc or high-power beams. However, several concepts are being introduced in additive manufacturing which have been extensively used in multipass arc welding with filler material. Therefore, clarification of fundamental definitions is important to establish a common background between welding and additive manufacturing research communities. This paper aims to review these concepts, highlighting the distinctive characteristics of fusion welding that can be embraced by additive manufacturing, namely the nature of rapid thermal cycles associated to small size and localized heat sources, the non-equilibrium nature of rapid solidification and its effects on: internal defects formation, phase transformations, residual stresses and distortions. Concerning process optimization, distinct criteria are proposed based on geometric, energetic and thermal considerations, allowing to determine an upper bound limit for the optimum hatch distance during additive manufacturing. Finally, a unified equation to compute the energy density is proposed. This equation enables to compare works performed with distinct equipment and experimental conditions, covering the major process parameters: power, travel speed, heat source dimension, hatch distance, deposited layer thickness and material grain size.

Introduction

Additive manufacturing allows structures fabrication in a layer-by-layer deposition process and is revolutionizing the manufacturing industry due to its ability to obtain near-net shape products, in a short time span, with almost no material waste.

An upmost key feature of additive manufacturing based on melting and solidification, is the use of fundamental knowledge generated by decades of research on welding metallurgy and technologies, based on electric arc and high-energy density beams (laser and electron beam). This experience in welding can be used to better understand the additive manufacturing process and the implications on the microstructural features of the produced parts, including the origin of internal defects (type, morphology, location and quantity).

Most prevailing industrial applications of additive manufacturing use laser beam systems which, under appropriate process control, produce parts with good dimensional precision and surface finish [1], [2], [3]. However, electron beam-based systems tend to deliver sound parts with better mechanical properties, since the process is developed in a vacuum chamber at a uniform high temperature, which decreases residual stresses that build-up during layer deposition. Both electron and laser systems require high investment costs. Thus, additive manufacturing with electric arc and plasma are dedicated to large parts manufacturing.

Alternatively to high-power beams, electric arc heat sources can be used in additive manufacturing where the production rate is a major requirement [4]. Nevertheless, laser or electron beam sources are preferred for parts with fine features or requiring high dimensional tolerances, due to their small size heat source.

Additive manufacturing techniques based on melting and solidification induce complex thermal profiles throughout the material [5], [6]. Whether, the added material is powder or wire, the first layer is melted and solidified onto a substrate. When a second layer is deposited on the previous, the introduced heat must completely melt this second layer and partially re-melt the one underneath. Moreover, a heat affected zone is created in the first deposited layer, which, depending on the material, may promote additional solid-state transformations. This process is repeated over and over until the part is complete. Therefore, the heat introduced to deposit each layer generates successive heat affected zones on previously deposited layers, in a process similar to multipass fusion welding [7], [8], [9], [10]. As in welding, additive manufactured parts experience non-equilibrium solidification [11], [12], [13], due to fast cooling rates observed. The effect of multiple thermal cycles over these non-equilibrium structures may promote the formation of new phases and precipitates if the permanence time at a specific temperature for the solid-state transformation to occur is reached.

As additive manufacturing technologies based on electric arc or high-power beams are still being matured, well-established knowledge from welding metallurgy can be transferred to better understand and improve additive manufacturing techniques.

Currently, the processing parameters used by the additive manufacturing community lack comparability and, thus, a comprehensive effort to encompass the vast generated knowledge over the past years is necessary to foster the development of additive manufacturing techniques into new applications.

Recently, a very comprehensive revision on additive manufacturing was published by DebRoy et al. [1]. The present paper aims to revise fusion welding concepts applicable to additive manufacturing of metals, highlighting the similarities between both technologies and the procedures used in welding that can be directly applicable in additive manufacturing. Specifically, this paper focus on strategies to control the microstructure of the fusion zone in welds in terms of heat source manipulation, optimization of operating procedures and correction of microstructure, to improve mechanical performance and avoid internal defects in additively manufactured parts. In fact, most of existing literature on additive manufacturing is built on welding technology and welding metallurgy. Therefore, this paper intends to evidence how the welding and additive manufacturing communities can establish a synergetic relationship. Concerning additive manufacturing process optimization, distinct criteria (geometric, energetic and thermal) are proposed to determine an upper bound limit for the hatch distance. Finally, an equation to compare energy densities used with different additive manufacturing equipment and varying the most relevant process parameters (power, travel speed, hatch distance, layer thickness, grain size and diameter of the heat source) is proposed.