Effect of building direction on porosity and fatigue life of selective laser melted AlSi12Mg alloy

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Abstract

Gas porosity is one of the most common defects in aluminum alloy parts manufactured by solidification processing, and can have a strong influence on fatigue properties. This study shows that gas pores with a fraction of 0.2–1.6% and an average size of 20–55 µm are present in the Al-Si alloy parts manufactured by Selective Laser Melting (SLM). Failure after fatigue testing was found to initiate from surface or subsurface gas pores and fatigue life prediction equations were developed considering the influence of pores. The building direction did not have a statistically verifiable effect on the average gas porosity fraction, size and distribution, although the scatter in porosity fraction was greater in the vertically built specimens. At the same applied stress, the fatigue life of SLM manufactured specimens decreased with an increase in pore size, and specimens built horizontally exhibited a greater fatigue life than those built vertically. The cause is attributed to greater propensity of cracks to propagate along lower strength melt pool boundary layers in vertically built specimens.

Introduction

Hypoeutectic Al-Si alloys close to the eutectic composition (12.6 wt%Si) are important cast Al alloys due to their excellent castability and low propensity to solidification defects. Consequently, AlSi10Mg and AlSi12 alloys have been commonly used for selective laser melting (SLM) [1]. In fact, among current commercial Al alloys, Al-Si based alloys are the most extensively studied Al alloys in the context of SLM due to their robust welding characteristics [2].

Fatigue properties are important for parts in applications subject to dynamic loads. It has been well documented that the fatigue strength of Al alloy castings is determined by the metallurgical defects [3], [4], and porosity is more detrimental to fatigue properties than inclusions and oxide films [5], [6]. Ammar et al. [7] reported that the fatigue life of both hypereutectic and hypoeutectic Al-Si alloys decreases with an increase in pore area at the initiation sites, Wang et al. [8] suggested that fatigue properties are controlled by the largest pores (based on cross-sectional area normal to the applied loading). Gao et al. [9] found by experiment and finite element analysis that both the distribution and size of pores significantly influence fatigue crack initiation, a result confirmed by Wan et al. [10]. In addition, Nicoletto et al. [11] argued, using finite analysis, that the morphology of the pores also affects the fatigue properties.

In SLM manufactured parts, three typical types of porosity exist, namely, “lack of fusion”, gas and shrinkage porosity [1]. “Lack of fusion” porosity is a bonding defect and only occurs in parts where there was low imparted energy density [12]. It is less common in aluminum alloys than in titanium alloys as Al alloys are more easily melted. However, gas porosity is common in Al alloys due to the large difference in the solubility of H2 from liquid to solid. Shrinkage porosity is generally not a major concern because of the small melt pools. To date, experimental studies have been conducted to correlate the fatigue performance with “lack of fusion” porosity or oxides [13], [14]. However, to the authors’ best knowledge, no study in the literature has focused on the effect of gas porosity on fatigue performance of SLM Al alloys although it is the predominant defect, nor has there been a study on the effect of building direction on gas porosity and fatigue properties of SLM Al alloys.

Section snippets

Materials and methods

Spherical AlSi12Mg powder was used to build the test specimens using SLM. The powder has a size distribution between 32.7 µm (D10) and 71.9 µm (D90) with mean volume diameter around 49.5 µm (D50), as illustrated in literature [15]. Table 1 lists the chemical compositions of the powder and the as-built specimens determined using inductively coupled plasma-atomic emission spectroscopy (ICP-AES).

A SLM250HL was used to manufacture the samples, which has a build volume of 250 × 250 × 350 mm with a

Fatigue properties

The fatigue strength was found to be around 120–130 MPa for the SLM specimens at 107 cycles (Fig. 2), which is higher than that of conventional casting [19], i.e., 90 MPa, but comparable with data for SLM processed AlSi10Mg alloy [14]. However, there was no clear difference between the HO and VO specimens.

Significant variation was observed in fatigue life under the same applied stress regardless of the building orientation, especially for HO specimens tested at 160 MPa and VO specimens at

Discussion

For SLM parts, the difference in fatigue properties results from variations in porosity distributions, microstructure, building direction and residual stress [22]. The microstructures for specimens with the same processing parameters have no significant difference except porosity. It also can be seen from Fig. 4 that the specimens with different building directions have similar grain size and second phase in spite of the difference in texture. As the platform was preheated to 200 °C in our

Conclusions

An investigation into the influence of microstructural features on the fatigue properties of AlSi12Mg parts has been undertaken. The following specific conclusions can be made:

  • The dominant defect in the SLM produced AlSi12Mg specimens was observed to be gas pores with a fraction of 0.2–1.6% and an average size of 20–55 µm. The building orientation showed no statistically verifiable effect on the gas porosity fraction and size of SLM specimens although greater variations were observed in

Acknowledgement

The authors acknowledge the facilities, and the scientific and technical assistance, of the Advanced Manufacturing Precinct, the Australian Microscopy & Microanalysis Research Facility at RMIT University, and the financial support from China Scholarship Council (201407005051).

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