**1. Introduction**

Additive manufacturing (AM) is a new technology that has made its mark as an innovative and flexible manufacturing technology [1]. Continuously, there is a need for the development of this technique, with the main objective being to reach 100% component density [2,3]. The additive manufacturing of aluminum alloys in particular finds recent applications in the aerospace, rail and automotive industries for structural and non-structural parts, which are usually die casted. In particular, the powder bed fusion processing of aluminum alloys has gained interest in the aerospace, rail and automotive industries due to the versatile nature of the process. The shapes of the components are generally attained from rigidity-focused strategies. Usually low stress requirements are often the objective for these components relative to stressful loading circumstances where finite-life fatigue resistance should be considered as distinctively possible such as circumstances where load or vibrations counteracting on component could be extreme [4]. Fatigue resistance crack propagation relative to the direction of the load is inherently dependent to anisotropy of the process of manufacturing. It is

common knowledge that defects that are found on the surface are the most dangerous of them all since crack propagation is most likely to be initiated on the surface. In AM applications, majority of failures observed are those that generally instigated from the surface where there are defects [4].

Silicon based aluminum alloys such as AlSi10Mg, AlSi12, etc. are currently used for laser AM and these are characterized by good castability, low shrinkage and moderately low melting temperature and AlSi10Mg is one of the most common alloys characterized with a hypoeutectic composition [5,6]. Prashanth et al. [7] studied the heat treatment of hypereutectic Al–Si alloys, which are said to have a wide application in the automobile and aerospace sectors due to their high wear and corrosion resistance.

During laser AM processing, the rapid heating and cooling of the laser processing results in the residual stresses build up. These residual stresses affect the properties of the components such as the ultimate tensile strength and the fatigue life [8]. It is said to be common knowledge that AM material has comparatively lower fatigue resistance than traditionally manufactured materials in the as-built condition, the reason being that fatigue life is affected directly by impurities and the inhomogeneity of the microstructure [9]. In this case, post heat treatment is necessary in order to relieve the residual stresses while maintaining the desired mechanical properties.

For instance, Cabrini et al. [10] performed various heat treatment techniques on the AlSi10Mg samples with various direct metal laser sintering (DMLS). These were stress relieving, annealing at high temperature and water quenching. Their results determined that annealing resulted in intensification to the matrix of aluminum phase with precipitation of rounded silicon on the surface also. Fiocchi et al. [11] studied the low temperature annealing of Selective Laser Melting (SLM) produced AlSi10Mg. After stress relieving at 263 ◦C on the as built samples, minor microstructural differences were observed. However, for heat treatment at 294 ◦C the silicon network appeared to be disconnected. Other work by Fousova et al. [12], investigated the modifications in the microstructure and mechanical properties of additively manufactured AlSi10Mg alloy after exposure to temperatures of 120–180 ◦C. The current, heat treatment profiles, mainly die casting used for AM parts, are adapted from conventional profiles, which have been shown to affect the AM parts negatively in some instances [13].

Therefore in order to improve the fatigue performance of AlSi10Mg, T6 heat treatment, processes such as solution treatment at 520 ◦C, water quenching and artificial ageing at 160 ◦C, which are also known as peak-hardening have been used [14]. Microstructural coarsening and material softening during annealing of SLM produced AlSi12 were reported and revealed the same results as conventionally cast material [15].

This paper will focus on the effect of stress relieving on the microstructure, porosity, mechanical properties and fatigue life of SLM produced AlSi10Mg samples with a focus on the build direction effect. It is a continuation of the previously published work conducted by the authors investigating the effect of build direction on the as-built SLM samples using AlSi10Mg [16]. This is because although the effect of the build direction of powder bed fusion manufactured Ti6Al4V has been extensively studied, there is little information on the AlSi10Mg alloy.

## **2. Materials and Methods**

#### *2.1. Powder Bed Fusion Processing*

AlSi10Mg powder with a spherical morphology and particle size distribution of 30–65 μm was purchased from TLS Technik GmbH, Germany, and it was used as received (see reference [6]). The study was carried out on specimens produced by the SLM Solution M280 GmbH from Lubeck in Germany using a laser power of 150 W, 1000 mm/s scan speed, 50 μm hatch spacing and 50 μm powder layer thickness fixed processing parameters (the image of the samples on the base late is presented in previous work [16]). The samples were built in the XY, 45◦ and Z orientations. Tensile, fatigue and crack growth samples (three of each) were prepared according to the images in Scheme 1. Post build stress relieving on the specimens was carried out in a rotary furnace using a temperature of 300 ◦C and hold time of 2 h then furnace cooled to room temperature.

**Scheme 1.** An illustration of the Selective Laser Melting (SLM )produced AlSi10Mg mechanical samples; (**a**) tensile, (**b**) fracture toughness and (**c**) fatigue crack growth. Note: The length and width of (**b**) and (**c**) are the same.
