Comparative environmental impacts of additive and subtractive manufacturing technologies

Abstract

Additive manufacturing technologies are opening new opportunities in term of production paradigm and manufacturing possibilities. Nevertheless, in term of environmental impact analysis supplementary research works require to be made in order to compare and evaluate them with traditional manufacturing processes. In this article, we propose to use Life Cycle Assessment (LCA) method and to associate decision criteria to support the selection of manufacturing strategies for an aeronautic turbine. The dimensionless criteria allow to define environmental trade-offs between additive and subtractive methods. This study provides an approach generalizable to other parts and processes.

Introduction

The use of additive manufacturing (AM) technologies for industrial applications has increased substantially during the past years [1], [2]. Technological advances contributed to the deeper understanding of AM processes, such as selective laser sintering (SLS) and electron beam melting (EBM) [3]. Currently, these AM processes allow cost effective manufacturing of metal components for end-use applications, especially when production volumes are low and geometrical complexity is high [4]. In this scenario, AM technologies could compete with traditional manufacturing methods based on formative and subtractive processes [5]. Nevertheless, criteria to support the selection of different manufacturing methods have still to be developed to compare technologies and select easily the most appropriate manufacturing methods. The purpose of this article is to propose and present combined criteria taking into account not only the manufacturability but also the environmental impacts.

The principles of metal component manufacturing using AM technologies are based on building the geometry layer by layer in a sequential manufacturing process [6]. Typically, the EBM process selected in this study requires sintering and melting the base material which is in powder form. After the additive process, the final geometry of the part is close to nominal values. However, finishing operations are needed when technical requirements imply high geometrical and dimensional tolerances as well as good surface quality [7].

Some of the advantages of the additive process versus conventional subtractive manufacturing methods include that the raw material consumption is reduced. The volume of raw material used during the AM process is in practice close to the volume of the part before the finishing phase, and therefore the metal powder that has not been affected by the laser or electron beam during the AM process can potentially be recycled. The waste of the process, such as material or fluid, is decreased substantially as opposed to traditional subtractive manufacturing processes, in which the generated waste is usually higher [8].

Based on this initial presentation, it seems that AM is capable of reducing the impact of the industrial and manufacturing activity on the environment [9]. However, this assumption must be demonstrated. For instance, to obtain the powder material for the AM process, a considerable amount of energy is required, and this process intrinsically generates waste, which is released to the environment. Consequently, the trade-offs in emerging AM processes need to be studied further to be able to replace established conventional subtractive methods. This study proposed an approach to define this trade-off between additive and subtractive methods.

In the context of a sustainable manufacturing process, it is necessary to estimate and compare the environmental impact and energy efficiency of established and emerging manufacturing processes. To achieve this goal, cooperation initiatives, such as “CO2PE!” [10], have the aim to research in deep the environmental footprint of manufacturing industry. Also, more standardized methodologies for systematic analysis and improvement of manufacturing process life cycle inventory [11] need to be implemented, as presented by [12].

Although, Life Cycle Assessment (LCA) method is the most commonly used methodology by which environmentally conscious design is carried out, substantial improvements have to be made in order to develop simple criteria allowing engineers to select quickly between different manufacturing options for given objectives. The present article is proposing a combination of criteria for comparing additive and subtractive methods from the environmental impact.

The document is organized in the following manner. In Section 2, different eco-indicators developed in the literature are briefly summarized and key literature references are provided. In Section 3, the case study key characteristics are described. In Section 4, the different manufacturing strategies considered in the article are summarized, as well as the initial conditions and hypotheses of the study. This section is also introducing a new dimensionless indicator specifically proposed to compare additive and subtractive methods. Its usage and its interest to support selection decision between both processes are presented. Section 5 summarizes the key results of the study. Finally, Section 6 concludes the article and presents the future work.