The positive effect of hot isostatic pressing on improving the anisotropies of bending and impact properties in selective laser melted Ti-6Al-4V alloy

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Abstract

Selective laser melting (SLM) is a versatile additive manufacturing process for fabricating solid or porous metallic materials with complicated three-dimensional shapes. SLM Ti alloys, particularly Ti-6Al-4V, and other alloys have been manufactured and analyzed in numerous studies. However, the high anisotropy of the microstructures and inconsistent mechanical properties of SLM materials have been extensively reported, and these disadvantages could prohibit its widespread use. To clarify how to alleviate the anisotropic behaviors of SLM materials, the main objective of this study was to evaluate the influences of hot isostatic pressing (HIP) on the microstructure, densification, bending strength, impact toughness, and fracture behavior of the as-built Ti-6Al-4V alloy.

The results showed that the vertical and horizontal building directions obviously affect the bending and impact properties of as-built alloys. The transverse rupture strength (TRS) and impact energy of the horizontally-built alloy were respectively found to be 48% and 100% higher than those of the vertically-built one. In the vertically-built alloy, disc-shaped building defects, identified by X-ray computed tomography (CT) and microscopy, obviously reduce the effective load-bearing cross-section and deteriorate the bending and impact performances. After HIP at 1000 °C/150 MPa, the α′-martensite structure in the as-built alloy is transformed into an α+β lamellar one, and the disc-shaped building defects are evidently eliminated. As a result, the impact energies of as-built vertical and horizontal specimens are improved by 28 J (560%) and 19 J (190%), respectively, and the TRS of the as-built vertical alloy is raised by 550 MPa (37%). Consequently, the discrepancies in TRS and impact energy between the HIPed vertical and horizontal specimens are merely 3% and 14%, respectively, and the anisotropic behaviors of the SLM Ti-6Al-4V alloy are thus substantially lessened.

Introduction

Selective laser melting (SLM) is a versatile and novel additive manufacturing (AM) technique for various metallic materials. SLM can be used to produce not only fully dense materials but also high-porosity ones [1], [2], [3], [4]. The shapes and porosities of the SLM metallic materials can be precisely controlled by adjusting the manufacturing parameters [3], [4], [5], [6], [7]. SLM Ti-6Al-4V and other Ti-based alloys have been widely investigated due to their excellent properties and wide-ranging applications [1], [2], [3], [4], [5], [6], [7], [8], [9], [10], [11]. In the search for high-density and high-performance Ti-6Al-4V alloys, the effects of SLM parameters, including laser power, scan speed, hatching distance, layer thickness, and scanning strategy, on SLM Ti-6Al-4V have been studied extensively [1], [2], [10], [11]. SLM Ti-6Al-4V with a relative density higher than 98% can be obtained by optimizing the SLM parameters [1], [2], [11], [12], [13].

In most cases, the microstructure of as-built SLM Ti-6Al-4V is extremely fine α′-martensite due to the high cooling rate after SLM [1], [12], [13], [14], [15], [16], [17], [18], [19]. The ultimate tensile strength (UTS) of as-built Ti-6Al-4V alloy ranges from 1100 to 1300 MPa and is generally higher than those of alloys produced by conventional processes [12], [13], [14], [15], [17]. Unfortunately, the ductility of as-built Ti-6Al-4V is less than 10% due to the low ductility of α′-martensite [12], [13], [14], [15], [16], [17]. Moreover, several studies have investigated the influences of building direction on the mechanical properties of as-built SLM Ti-6Al-4V alloys, particularly the tensile properties [12], [13], [14], [15], [17]. The anisotropies in the mechanical properties of as-built Ti-6Al-4V have been clearly demonstrated [12], [13], [14], [15], [16], [17], [19].

Vilaro et al. [12] investigated the effects of horizontal and vertical building directions on the tensile properties of SLM Ti-6Al-4V alloys and found that the UTSs of these two directions were comparable. The longest lengths of the samples built in the horizontal and vertical directions were parallel and perpendicular to the substrate, respectively. However, the ductilities of the horizontal and vertical directions were 7.6% and 1.7%, respectively, and the role of the building orientation in the ductility was clear. Rafi et al. [15] also found that horizontally-built specimens exhibited slightly better tensile properties than vertically-built ones. In contrast, Qiu et al. [13] indicated that the tensile elongation of a vertically-built sample was higher than that of a horizontally-built one. Simonelli et al. [14] studied the tensile properties of SLM Ti-6Al-4V specimens built with three different orientations. The vertical, edge, and flat specimens consisted of 2000, 200, and 60 layers, respectively. The results showed that the elastic modulus and UTS did not obviously vary with the build directions. However, the elongations at break of the edge, vertical, and flat specimens were 11.8%, 8.9%, and 7.6%, respectively.

To improve the ductility of as-built SLM Ti-6Al-4V, several researchers have tried heat treatment and hot isostatic pressing (HIP) to increase the ductility by phase transformation of the α′-martensite structure to α+β duplex structures [12], [13], [20], [21], [22], [23], [24]. However, these researchers focused on mainly the heat-treated tensile properties in one building direction, and thus the anisotropy in the tensile properties could not be fully understood. Vilaro et al. [12] demonstrated that, after heat treatment at 1050 °C, the ductilities of horizontal and vertical specimens were 8.9% and 7.9%, respectively, and the anisotropy in ductility was sufficiently alleviated. In addition, Qiu et al. [13] investigated the influence of HIP treatment (920 °C/103 MPa) on the tensile strength and elongation of as-built SLM Ti-6Al-4V and reported that HIP slightly decreased the strength but considerably improved the elongation of both vertical and horizontal samples. They also found that after HIP, the vertically-built specimen had a greater elongation than the horizontally-built one.

Besides the tensile performance, impact toughness is also an important mechanical property of structural materials because it represents the capacity for absorbing an unexpected loading before fracture. Unfortunately, the influence of building orientation on toughness has rarely been clarified. Yasa et al. [18] demonstrated that the building axis does not obviously affect the toughness of as-built SLM Ti-6Al-4V if there is no directional porosity between the successive layers. Recently, Wu et al. [19] investigated the influences of the vertical and horizontal building directions on the impact toughness of as-built SLM Ti-6Al-4V. They reported that the impact energy of a horizontal specimen is sufficiently higher than that of a vertical specimen due mainly to the morphologies of the building defects.

Based on previous studies, it is clear that the influences of building direction on the tensile and other mechanical properties are complicated and are not conclusively understood, particularly those after HIP or heat treatment. HIP treatment can improve the densification of porous materials and eliminate defects by combining high gas pressure and high temperature [25], [26], [27]. The tensile elongation of SLM Ti-6Al-4V can also be improved by HIP [13], [22], [23], [24]. However, information about the bending and impact properties of SLM Ti-6Al-4V is still insufficient, and the role of HIP in the bending and impact performances of SLM Ti-6Al-4V has not been identified yet. To further understand how to effectively alleviate the anisotropic behaviors in more depth, this study investigated the anisotropies in the bending and impact properties of as-built and HIPed SLM Ti-6Al-4V alloy. The positive effect of HIP on the anisotropic mechanical properties of SLM Ti-6Al-4V is demonstrated.

Section snippets

Experimental procedure

Fig. 1(a) shows the plasma-atomized Ti-6Al-4V powder used in this study. As can be seen in that figure, the raw powder was spherical. The particle size distribution of the raw powder was also examined using a laser light scattering particle analyzer (Mastersizer 3000E, Malvern Instruments LTD., Worcestershire, UK), as shown in Fig. 1(b). The results showed that the D10, D50, and D90 sizes were 21 µm, 34 µm, and 54 µm, respectively. The standard specimens for bending and impact tests were

As-built microstructure

The cross-section microstructures of the as-built V and H specimens were identical, so only the microstructures of V specimens are presented in Fig. 3. The cross-section microstructures on the xy plane consisted of nearly-equiaxed grains with an average size of about 100 µm. In contrast, the cross-section microstructures on the yz planes exhibited columnar grains that were as long as 1 mm. The diameters of the nearly-equiaxed grains on the xy plane were close to the widths of the columnar grains

Conclusions

This study investigated the microstructure, relative density, TRS, impact toughness, and fracture behavior of the as-built and HIPed SLM Ti-6Al-4V alloys to clarify the influences of building direction and HIP on the bending and impact properties. The findings are summarized as follows.

  • 1.

    The microstructure of as-built Ti-6Al-4V is fully α′-martensite, and the distributions of Ti, Al, and V atoms are homogeneous. The degree of preferred orientation in the as-built alloy is obviously lower than

Acknowledgment

The authors thank the Ministry of Science and Technology of the Republic of China for assistance under grant number MOST 103-2218-E-027-014.

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