Alloy design and adaptation for additive manufacture

Abstract

In this review the authors provide a comprehensive insight into additive manufacturing process mechanics which explore how thermodynamics give rise to characteristic microstructure and property. Discussion is then given over to how solidification microstructure is modified using processing conditions and utilisation of traditional casting methods which give rise to grain refinement and some cases ‘texture by design’.

The review then makes observation and provides analysis on the typical material types reported in powder bed fusion and directed energy deposition. These are organised to efficiently highlight the key observations in terms of alloy creation and adaption with a specific focus on the relevant additive manufacturing technique. The emergent alloy families are also explored alongside the rapidly expanding topic of functionally graded alloys produced by the breadth of additive manufacturing techniques.

The review concludes with a detailed critique of the state-of-the-art and an examination of where opportunities exist to advance the interdependency between process and emergent alloys exhibiting superior properties. The authors propose several areas worthy of consideration and attempt to provide inspiration for our community in the pursuit of new alloys and accompanying process designs for additive manufacturing.

Introduction

The triple point that occurs when we consider process, microstructure and property relationships is nowhere else more poignant than in the field of Additive Manufacturing (AM). Since the inception of the technologies which bring together energy beams and material delivery, focus has been given to the geometrical freedoms which became available to the designer. However, recent advances in the field allow us to adjust process and composition in unison to advance the properties of components built using AM technologies. We can now conceive of engineered artefacts which exhibit remarkably distinct properties as compared to their conventionally produced counterparts. This review will explore the role of adapted alloys in AM and link these to process characteristics with the purpose of providing a diverse research community with inspiration to exploit these opportunities further.

Comprehensive consideration of materials in additive manufacture has been provided by Bourell et al. previously (Bourell et al., 2017). The Bourell review provides a general introduction to all types of materials (ceramics, polymers and metals) processed by AM. Consideration is also given over here to the role of emergent systems including functional grading and the development of AM specific composite materials. It is apparent from this work that the role of materials innovation in the wider AM space is dramatic. As such, the present review will focus on metallic systems and how these are adapted for AM processes.

The precise beginnings of metal AM are difficult to pinpoint with numerous innovations being made simultaneously in both powder bed fusion (PBF) and directed energy deposition (DED). Excellent reviews by Yap et al. (2015) and Dass et al. (Dass and Moridi, 2019) explore the origins and usage of PBF and DED respectively. In unison these works highlight the rapid development of process capability alongside our understanding of the underpinning process physics. As such, this review will explore processes to a limited degree only in favour of focussing on the interplay between composition and process thermodynamics which define the microstructure and properties of materials produced by both DED and PBF.

Characteristic of the first twenty years in AM materials development has been a preference for deploying conventional materials within this process family. The current economics of AM technologies obliges researchers to concentrate on high value components which are often comprised of advanced alloys for specific applications. As such there is much evidence in the literature for exploration of commonly used alloys which benefit from the use of AM. It is challenging to highlight these individually and as such the reader is referred to comprehensive reviews in the processing via AM of the following materials Titanium (Liu and Shin, 2019), Aluminium (Aboulkhair et al., 2019), Nickel (Sanchez et al., 2021a), Iron (Bajaj et al., 2020) and Magnesium (Karunakaran et al., 2020). An authoritative review of Copper processing by AM is yet to be published but the reader is referred to experimental studies via electron beam melting (EBM) (Ramirez et al., 2011), laser PBF (LPBF) (Silbernagel et al., 2019) and DED (Zhang et al., 2021a) which underline the growing interest in Copper as an AM compatible material.

In terms of aggregating prior contributions to the literature a simple consideration of the rate of publications with time is a useful approach. Fig. 1 shows the sharp increase in the rate of publication in relation to AM of specific materials of interest to this review.

The rate of increase is remarkable, and the reader will note the dominance of exploration relating to steel, aluminium and titanium-based systems. Anecdotally interest in these materials will be driven by applications in the aerospace, biomedical and tooling sectors. It is within these sectors that the most readily available opportunities for AM technology exploitation reside.

While this review focuses on the innovations made to materials the rapid development of the machine tool technology is also worthy of consideration in this introduction. Two primary approaches are reported in the literature for the creation of macro scale components in metals. Powder bed fabrication has two primary approaches which use laser beam (King et al., 2015) or electron beam (Körner, 2016) to allow melting and resulting in localised consolidation. Similarly, DED may draw upon laser or electron beam-based approaches, but the current research activity is dominated by laser-based approaches. Hybridization of these technologies with subtractive methods has also developed prevalence in the literature with machining of DED (Dávila et al., 2020) and PBF (Pragana et al., 2021) within process now reported. This is accompanied by marked efforts to enhance process control through increased instrumentation (Everton et al., 2016). While the advancement to process technology is also central to the capability of AM it falls outside of the scope of this project in favour of concentrating on materials innovation.

With regards to materials innovation in AM, this is very much a rapidly moving feast. New materials are being processed successfully for the first time on a regular basis and these are being reported with increasing frequency. A comprehensive pallet of ‘processible’ materials can be found in Table 1.

It is apparent that there are a number of materials which have been processed by AM where additional work is required to verify process-material combination. Further, some widely used materials are yet to be explored in a meaningful way. It is from this basis that the present review seeks to elucidate the latest and most meaningful adaptations to alloy compositions to enhance processability.

Having identified the gaps in the literature for a review in materials technology for AM the authors propose the present review which is built around technologies to solve a simplistic problem to define to but a complex problem to address, Fig. 2.

The structure of this review is intended to provide systematic observations on the mechanics of the techniques of DED and PBF and elucidate how the characteristics of heat and mass transfer influence the processability and the resulting microstructure which are characteristic of the processes of interest. Process thermodynamics in both process types are driven by the means of energy delivery (energy beam) and mode of material delivery (powder bed, powder/wire dynamic delivery). In all processes considered in this review complete melting of the feed material takes places. When these transition into the liquid phase and form a melt-pool the specifics of the work material dictate firstly molten behaviour, cooling, recrystallisation and solidification. The literature is speckled with reports of characteristic defects associated with AM techniques (Kyogoku and Ikeshoji, 2020) and means by which to identify them (du Plessis et al., 2020) identifying the current process-material shortfalls. The authors argue here that these serve to limit the uptake of AM for industrial application and hence must be overcome. Indeed, process innovation will contribute to this but the role of material innovation will be significant. It is seemingly inevitable that alloys will require modification and adaptation such that they become more compatible with the thermodynamics associated with AM. This is explored in the early stage of this review and general observations are related to specific processes.

Following a fundamental exploration of material-process interaction this review moves review the literature pertaining to key materials of interest. Fig. 1 highlights the primary materials reported for DED and PBF in the literature. As such this review explores the augmentation reported for these materials specifically to AM. A suite of methods have been explored in the literature to adapt alloys to enhance AM product. In general alloy adaptation is undertaken for three reasons i) enhance processability/broaden process windows ii) to enhance the performance characteristics of the product material and iii) reduce the requirement for post processing by any means. The methods by which alloy augmentation can be achieved are varied and draw upon insight to fundamental materials science and are often bespoke to the ‘parent’ metal.

This review brings these together for the first time with the intention of providing a succinct resource from which scholars developing new materials or requiring strategies to process troublesome materials may find inspiration. This review will be of interest to those exploring approaches to overcome the inherent materials characteristics (in Fe, Ni, Ti, Al and high entropy alloy systems) which make them currently suboptimal in combination with the process state-of-the-art.