Experimental investigation and statistical optimisation of the selective laser melting process of a maraging steel

doi:10.1016/j.optlastec.2014.07.021

Optics & Laser Technology

Volume 65, January 2015, Pages 151-158

https://doi.org/10.1016/j.optlastec.2014.07.021 Get rights and content

Highlights

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The analysis of microstructure has been corrected.
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The build pattern was explained.
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A paragraph on the hardness was added.

Abstract

Selective Laser Melting (SLM) is an Additive Manufacturing process (AM) that built parts from powder using a layer-by-layer deposition technique. The control of the parameters that influence the melting and the amount of energy density involved in the process is paramount in order to get valuable parts. The objective of this paper is to perform an experimental investigation and a successive statistical optimization of the parameters of the selective laser melting process of the 18Ni300 maraging steel. The experimental investigation involved the study of the microstructure, the mechanical and surface properties of the laser maraging powder. The outcomes of experimental study demonstrated that the hardness, the mechanical strength and the surface roughness correlated positively to the part density. Parts with relative density higher than 99% had a very low porosity that presented closed and regular shaped pores. The statistical optimization determined that the best part properties were produced with the laser power bigger than 90 W and the velocity smaller than 220 mm/s.

Introduction

Selective laser melting (SLM) is probably the most rapidly growing technique in rapid prototyping (RP) and rapid manufacturing technologies. It uses 3D computer aided design data as a digital information source and energy in the form of a high power laser beam to create three-dimensional metal parts by fusing together a fine metallic powder [1]. SLM represents an evolution of the Selective Laser Sintering (SLS) process that was developed and patented by Carl Deckard and Joe Beaman at the University of Texas at Austin in the mid-1980s for producing plastic prototypes. SLM started at the Fraunhofer Institute for Laser Technology ILT in Aachen, Germany, in 1995 with a German research project, resulting in the ILT SLM basic patent DE 19649865 [2], [3]. The SLM process is the same as SLS except for the much higher laser energy density required. The amount of energy density causes the all powder to melt in SLM and to melt in SLS only superficially. SLM can produce nearly full density parts without the need of post-processing treatment. In recent years, it has also been possible to produce parts for production purpose [4], [5].

Nowadays the main objective in this field is to produce parts with mechanical properties comparable with those of components produced with traditional processes. These properties depend on composition and size of the powder as well as the process parameters and manufacturing strategy [6], [7].

The SLM process has proved to be suitable for manufacturing hard tool steels [8], [9], stainless steels [10], [11], composites of Al–Si–Mg/SiC, stainless steel/hydroxyapatite, 663 copper alloy, Fe–Ni–Cr and TiC/Ti5Si3 [12], [13], [14], [15], [16], Ni based alloys [17], [18], [19] and recently Ti [20], [21], [22] and Mg alloys [23].

The goals of this study have been the investigation of the Nd:YAG SLM process of 18 Ni Maraging 300 steel and its statistical optimization.

Maraging steels are iron–nickel alloys with absence of carbon and have metals such as molybdenum, cobalt, titanium and aluminum. The alloys have ultra-high strength, superior toughness characteristics and weldability. Typically, these alloys are used in aerospace applications, machinery and tooling and ordnance components and fasteners.

The quality of the laser-molten part was assessed by the analysis of the microstructure, density, mechanical properties and surface roughness of built parts. The analysis of variance (ANOVA) permitted to test the main and first order interaction effects, i.e. laser power and scan speed, on the investigated output. The correlation between the relative density (ρ_r%) of built parts and hardness, ultimate tensile strength (UTS), elongation (%) and surface roughness (SR) was studied. The optimization analysis determined the more suitable process parameters in the investigated range.

Section snippets

Experimental procedure

For this investigation all the experimental tests were carried out on a laser machine equipped with a Nd:YAG laser source characterized by a 1.064 µm wavelength, 200 µm spot diameter and a maximum output power of 100 W. The laser light moved over the powder surface by means of scanning mirrors and drew selectively every layer of the powder.

The powder deposition system consisted of a working platform onto a coater deposited successive powder layers in one direction. Nitrogen gas filled the powder

Relative density

The relation between the Energy Density (E_d) and the Relative Density (RD) was plotted in Fig. 2. Values of E_d ranges from 1.29 to 2.78 J/mm². It is evident that density of parts increase with E_d, reaching average values higher than 99%. The true density of the material is 8.01 g/cm³; this clearly indicates that ρ% of built parts changes between 90.9 and 99.9%, obtaining nearly the full density. The average maximum value can be obtained for E_d=2.78 J/mm².

The metallographic examination of the

Statistical analysis and optimization

In order to investigate the influence of the considered factors on the quality of the parts, the Analysis of Variance (ANOVA) with a general linear model was used. The investigated factors were the relative density, the ultimate tensile strength, the hardness and surface roughness.

The data set contains the same number of observations for each combination of the factor levels. Factors for this model are discrete variables, therefore the ANOVA examines whether the variance of the factor is zero.

Conclusions

This paper describes the experimental investigation and the statistical optimization of the 18 Ni maraging 300 steel parts built by the selective laser melting process.

The experiments showed that parts with a relative density as high as 99% can be built and that ρ_r% increases with the energy density. Parts with the relative density higher than 99% had a very low porosity that presented closed and regular shaped pores. The pores diameter were smaller than 30 μm. High level of the relative density

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Experimental investigation and statistical optimisation of the selective laser melting process of a maraging steel

Highlights

Abstract

Introduction

Section snippets

Experimental procedure

Relative density

Statistical analysis and optimization

Conclusions

J Mater Process Technol

Cirp J Manuf Sci Technol

Int J Mach Tools Manuf

Phys Procedia

Appl Surf Sci

Mater Des

J Mater Process Technol

J Mater Process Technol

Trans Nonferrous Met Soc China

Surf Coat Technol

Mater Sci Eng: A

Acta Mater

Appl Surf Sci

Acta Mater

Mater Sci Eng A

Mater Sci Eng

J Mater Process Technol