Elsevier

Journal of Cleaner Production

Volume 208, 20 January 2019, Pages 77-85
Journal of Cleaner Production

Electrical energy consumption and mechanical properties of selective-laser-melting-produced 316L stainless steel samples using various processing parameters

https://doi.org/10.1016/j.jclepro.2018.10.109Get rights and content

Highlights

  • Electrical energy and mechanical properties of SLMed parts were jointly studied.

  • Growth rate comparison is used to jointly analyze the results.

  • Acceptable density can be achieved without greatly increasing electrical energy.

  • Achieving higher mechanical strength requires more electrical energy.

  • The electrical energy can be effectively consumed for hardness and wear resistance.

Abstract

Various processing parameters in selective laser melting (SLM) affect power profile and scanning time, which directly relates to electrical energy consumption. These processing parameters also control the microstructure of materials, which further influence the mechanical properties of the fabricated parts. In this paper, we investigate the correlation between electrical energy consumption and mechanical properties, and study whether electrical energy can be effectively reduced without significantly compromising mechanical properties by optimizing processing parameters. 316 L stainless steel was used as powder materials. Two key parameters, laser power and exposure time, were selected, and several mechanical properties, including density, hardness, wear resistance, tensile strength, flexural strength, and torsional strength, were tested. The results of electrical energy consumption and mechanical properties were jointly analyzed using growth rate comparison. It was found that the improvement of various mechanical properties with increased electrical energy consumption differs greatly. Density can be effectively increased without significantly increasing the electrical energy, but the electrical energy needs to be greatly increased in order to achieve a high flexural strength. Growth rate three-dimensional maps of mechanical properties and electrical energy consumption are presented as a reference for processing parameter optimization.

Introduction

Presently, additive manufacturing (AM) is in the spotlight since it overturns conventional subtractive manufacturing, making it possible to fabricate complex structures additively. It has been successfully applied in the fabrication of aerospace components, medical apparatus, and molds. Selective laser melting (SLM) is one type of additive manufacturing technology that can produce quality metal parts in a layer-to-layer manner. A fiber laser melts powders selectively according to the pre-sliced computer-aided design (CAD) model on a layer of pre-spread powders (Uriondo et al., 2015). Compared to laser melting deposition, which is another type of metal AM technology, SLM can produce parts with relatively higher accuracy and better surface roughness, but smaller in size.

In SLM, laser is focused on a tiny spot with a diameter of usually less than 100 μm, which results in an extremely large thermal gradient. The temperature inside the spot is extremely high (can be over 2000 °C), while it is only several hundred degrees nearby (El Kadiri et al., 2008). Moreover, the laser scanning speed is also high, from tens to thousands of mm/s. Therefore, the materials undergo sharp and repeatedly heating/cooling cycles. Using such a unique process, SLM parts have very different microstructures and randomly dispersed defects, such as grain size, texture, defects types and size. Consequently, mechanical performance of the fabricated parts is affected. Moreover, there are many SLM processing parameters that control the laser energy absorbed by powders, which exacerbates the complexity of material microstructures (Yadollahi and Shamsaei, 2017). These processing parameters mainly include laser power, exposure time, point distance, hatch space, layer thickness, build orientation, and scanning strategy.

To understand the relationship between the process, structure, property, and performance of additive manufactured parts is of great importance (Daniewicz and Shamsaei, 2017). Kempen et al. (2012) studied the mechanical properties of SLMed AlSi10 Mg, including tensile strength, Young's modulus, impact toughness, and hardness. They found that SLMed samples show some anisotropy and the presence of pores reduces strength and becomes crack initiations, which is caused by the processing parameters. Gu et al. (2012) studied the influence of processing parameters on the material structures and wear properties of commercially pure titanium and found that inappropriate processing parameters increase pores which reduces density and wear resistance. Vrancken et al. (2014) studied the microstructure evolution of SLMed β titanium and found that heat treatment can great improve the mechanical properties. Zhang et al. (2013) investigated the laser parameters and environmental variables on the SLMed 316 L stainless steel. They found that almost fully densified samples are obtained using a high preheating temperature which also exhibit a high tensile strength and a stable Young's modulus that are similar to casted SS 316 L parts. Lu et al. (2015) studied micro-structure, mechanical property, and residual stress of SLMed Inconel 718. They reported that the relative density increased and pores decreased with enlarging the scanning island size. However, mechanical properties do not show much difference. Spiering et al. (Spierings et al., 2013) tested the SLMed 15-5 PH and 316 L. They found that both SLMed steels show a similar tensile and fatigue behaviors compared to conventionally processed steels but a lower fatigue life. Similar research can also be found in (Brandl et al., 2012, Thijs et al., 2013, Cherry et al., 2014, Sun et al., 2013, Sun et al., 2016, Zhu et al., 2016, Zheng et al., 2009, Attar et al., 2015, Gu et al., 2011, Li et al., 2014). It is generally accepted that the presence of defects, including pores, holes, and cracks, greatly reduce the mechanical properties regardless of materials. A suitable combination of processing parameters generates high performance of one aspect of mechanical properties. Too high or too low values of such processing parameters result in rather poor performance. More importantly, the different requirements of mechanical properties should be determined based on specific application scenarios.

On the other hand, as an immature technology, the consequences of applying AM to practical use on sustainability are not well understood. Ford and Despeisse (2016) reviewed the available publications and gave a discussion on the implication of AM on sustainability in terms of many aspects. Some researchers developed models for predicting energy consumptions of AM process from many different aspects. Le Bourhis et al. (Le Bourhis et al., 2014, Bourhis et al., 2013) considered all of the energy flows, including electricity, materials, and fluids, during the AM process, and developed a methodology to predict such consumption. Mognol et al. (2006) and Baumers et al. (2011) studied the electrical energy consumption by comparisons among various AM systems, orientations and positions, and process rate. Meteyer et al. (2014) performed a similar study to develop a model to predict consumption (energy and materials) based on life-cycle inventory. Paul and Anand (2012) investigated the sintering energy by considering slice thickness, part orientations, part geometry, and further process parameters.

As a newly developed technology, AM technology is often compared to conventional manufacturing process. Watson and Taminger (2018) developed a model for determine whether AM or subtractive manufacturing is more energy efficient for producing a given metallic part. Ingorao et al. (Ingarao et al., 2018) provides a comprehensive environmental manufacturing approaches comparison among subtractive, mass conserving, and selective laser sintering for components made of aluminum alloys. Morrow et al. (2007) compared the energy consumption of direct laser deposition with conventional tool and die manufacturing. They found that laser-based remanufacturing could greatly reduce the energy and environmental impacts.

Since AM parts often have relative rough surface finishes which do not meet the requirements of some industrial applications, researchers started to look at the hybrid manufacturing process (additive + subtractive manufacturing). Le et al. (2017) presented an investigation of the environmental impact of the combination of additive and subtractive manufacturing technologies, which helps designers to selective a the most suitable strategy. Piarone and Ingarao (Priarone and Ingarao, 2017) compared the primary energy and CO2 emissions of a machining and an integrated. Almost all of these studies focused on energy reduction (including materials, electricity, and fluids). However, the reduction of energy consumption becomes more meaningful only when the part quality meets the requirements. Paul and Anand (2015) reported that they optimized the AM processes for minimizing total energy and form errors of AM parts while maximizing part strength. However, their method was based on a calculation model and only yield strength was considered.

In the work reported in this paper, the influence of two main SLM processing parameters, i.e., laser power and exposure time, on the process electrical energy consumption and mechanical properties (density, hardness, tensile strength, flexural strength, torsional strength, and wear performance) were studied. The process energy consumption and mechanical properties were analyzed jointly in order to determine whether the electrical energy consumption can be effectively reduced without significantly compromising the mechanical properties.

SLM fabrication was performed using a Renishaw (UK) AM250 laser melting unit comprised of an SPI redPOWER 200 W ytterbium fiber laser, an automatic powder layering system, an argon gas protection system, and a process control system. The laser operated with a 70 μm focused beam diameter at a 1071 nm wavelength. A schematic of the SLM system is shown in Fig. 1.

At the onset of experiments, air must be pumped out of the sealed building chamber, and then filled with high-purity argon. Therefore, the oxygen content was reduced to below 100 ppm, preventing the powders from being oxidized. The working process (as shown in Fig. 2) starts with delivery of a layer of powders by the layering system. The spread layer of powders is scanned by the laser beam according to the pre-sliced CAD model. The building platform descends to deliver a new layer of powders. These steps continue until the entire part is built.

Fig. 3 shows the consumed power profile for an entire SLM process measured by a wattmeter. The entire SLM process can be divided into three stages: a pre-processing stage that includes pumping out air and filling with inert gas; a processing stage that includes powder layering, laser scanning parts and supports, and air pumping and aerating; and a post-processing stage that starts after the entire part is built. Energy consumption differs in different stages. The electrical energy consumed in the pre- and post-processing stages is determined by the SLM system and is relatively stable for each SLM process. In the processing stage, the energy consumed depends on many factors, such as part volume, build direction, support design, number of layers, etc. Since we aim to study general correlations between electric energy and mechanical properties, the design methodology of a specific part is not discussed in this work. Therefore, only the electrical energy consumed in the part processing is considered.

Section snippets

Materials and SLM processing parameters

The 316 L stainless steel powders were spherical in shape and ranged in size from 15 to 45 μm with a mean diameter of 30 μm. 316 L stainless steel was used in the experiments due to its wide application. Table 1 shows the chemical composition of the material.

The processing parameters studied here include laser power and exposure time. Exposure time is the time the laser is focused on one spot during scanning, while laser power refers to the power of the laser.

316 L stainless steel parts were

Energy consumption test results

Fig. 8 shows a segment of the collected electrical power data in order to illustrate the influence of laser power and exposure time on the measured power. When samples were made with the same exposure time, the electrical power consumed follows the ranking of laser power, as expected. However, the ratio of the consumed electrical power among samples does not exactly follow that of the nominal laser power. This is due to the fact that there are many other components that also consume electrical

Discussion

From the above results, it can be found that almost all mechanical properties generally increase with increased exposure times and laser power which coincides with the results in literature (Kruth et al., 2004, Dewidar et al., 2003, Olakanmi et al., 2015). However, the increasing rates are very different. The presence of pores caused by the balling effect, residual oxygen, and imperfections in the powders (Zou et al., 2017) is the main reason for the reduced mechanical properties, which has

Conclusions

In this paper, we studied whether electrical energy can be saved without significantly sacrificing the mechanical properties of SLMed parts and to provide a reference for optimizing SLM processing parameters. In this study, the mechanical properties and electrical energy of 316 L stainless steel parts fabricated by SLM were measured and compared using growth rate comparison. Various combinations of processing parameters, including laser power and exposure time, were used to produce parts. The

Funding

This work was supported by the National Natural Science Foundation of China [grant numbers 51775486, 51505423]; Fundamental Research Funds for the Central Universities [grant number 2017FZA4001, 2016QNA4002]; the National Basic Research Program of China [grant number. 2015CB058100].

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      Paul and Anand developed an interesting strategy for reducing energy consumption in AM processes by optimizing layer thickness, hatch overlap and part orientation as process parameters [16]. Zhu et al. [17] investigated the existing correlation between energy consumption involved in manufacturing 316 L stainless steel samples by SLM and mechanical properties obtained such as hardness, torsional strength, density. Peng et Chen [18] investigated the influence of energy density as energy consumption index on porosity of 316 L stainless steel SLM parts.

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