1 Introduction
Laser-based powder bed fusion of metals (PBF-LB/M), also referred to as selective laser melting (SLM), is a premier metal additive manufacturing (AM) method for the fabrication of customized metal components. Along with stainless steels, titanium alloys, superalloys (nickel and cobalt-chrome based), and copper alloys, additively manufactured aluminum (Al) alloys are finding increasing applications [
1‐
3]. However, Al alloys present several challenges for AM. The sensitivity of Al to surface oxidation, the propensity to absorb hydrogen from moisture in the atmosphere, the possibility of powder blowing by gas flow in the PBF-LB/M chamber, and the poor flowability during layering make porosity a common issue for Al components [
4]. Furthermore, the tenacious oxide film on the surface of Al powder particles can obstruct melting of powder particles and provides nucleation sites for defect formation [
5‐
8]. Moreover, Al has high thermal conductivity and high reflectivity to laser wavelengths usually used in PBF-LB/M, which results in a relatively low melting efficiency. Although these deficiencies can be partially alleviated using a high laser power and an inert atmosphere, the use of a high laser power can have an undesirable effect of vaporizing critical strengthening elements with low boiling points (i.e., Mg and Zn).
Another challenge for PBF-LB/M of high strength or precipitation hardenable Al alloys (i.e., 2xxx, 6xxx, and 7xxx series) is their susceptibility to hot tear during the terminal stages of solidification when the alloys are in the mushy state, resulting in an impairment to the mechanical properties of the printed component [
9]. In general, the hot tearing mitigation strategies developed for conventional casting and welding of high strength Al alloys have been adopted or adapted to reduce hot tearing during PBF-LB/M, which include process control, parameter optimization, and grain refinement.
Build plate pre-heating is a commonly used process control technique for mitigating hot tearing, by which the elevated temperature of the build plate reduces the thermal gradient in the printed material. Several studies have investigated build plate temperatures between 200 and 400 °C for PBF-LB/M of 2xxx alloys, but the success was limited as cracks still occurred even with pre-heating in some cases [
4,
10,
11]. Process parameter optimization has largely focused on volumetric energy density (i.e., laser power, scan speed, hatch spacing, and layer thickness). For instance, it has been found that hot tearing in 2xxx alloys decreases with decreasing scan speed or increasing volumetric energy density [
2,
12‐
16]. The effect of other process parameters has received less attention, although the selection of hatch spacing appears to critically affect hot tearing [
14,
16]. However, there is a lack of consensus on the effect of process parameters on hot tearing in 2xxx alloys and how the hot tearing susceptibility is affected by process parameters remains to be explored.
A characteristic of the PBF-LB/M process which exacerbates hot tearing is the formation of coarse columnar grains, which often traverse multiple build layers due to epitaxial growth and large thermal gradients. Therefore, a promising route to mitigating hot tearing is through grain refinement, as fine equiaxed grains are known to reduce the propensity to hot tearing due to a lower coherency temperature, thinner liquid films, greater capillary pressure between grains, and the ability to rotate and deform to accommodate the thermally induced stress of parts [
17,
18]. The addition of scandium (Sc) is a popular grain refinement strategy used for AM of Al alloys, where primary Al
3Sc particles form in the melt pool to provide heterogeneous nucleation sites [
19‐
21], although it has not been commonly used for PBF-LB/M of 2xxx alloys. Moreover, the high cooling rates of the PBF-LB/M process allow for greater solid solubility of Sc in the Al matrix, providing enhanced precipitation hardening during subsequent heat treatment, which has been already demonstrated in electron beam melting of an Al-2Sc master alloy [
22].
Significant grain refinement and hot tearing reduction during PBF-LB/M of 2xxx alloys have been achieved using zirconium (Zr), which forms Al
3Zr particles with Al in the melt pool as heterogeneous nucleation sites [
12,
15]. The use of Zr has further proven to be effective in eliminating hot tearing through grain refinement during PBF-LB/M of the crack susceptible 7075 alloy [
23]. As reviewed recently [
24], there have been a number of successful approaches to utilizing grain refinement for suppression of hot tearing during PBF-LB/M of Al alloys by adding a relatively large amount of nano-sized particles [
25‐
28]. The use of commercially available AlTiB grain refiners has not been widely studied until recently, despite AlTiB appearing to have a stronger grain refining effect in rapidly solidified Al alloys than Sc [
29]. Wang et al. [
13] reported a reduction in the average grain size from 23 to 2.5 μm for PBF-LB/M of an Al-3.5Cu-1.5 Mg alloy with an addition of TiB
2 powder, which was, however, introduced as a reinforcing phase rather than as a master alloy. A similar reduction in grain size was reported by Jiang et al. [
30] during laser directed energy deposition of a 7075/TiB
2 composite. More recently, TiB
2 has been investigated as an intentional grain refiner in PBF-LB/M for 2xxx [
31] and 7xxx [
32] alloys, demonstrating effective grain refinement and hot tearing reduction in both cases.
The main objective of this study is to obtain crack-free Al2139 by PBF-LB/M, with mechanical properties matching those of the wrought or cast counterparts. Al2139 is an Al–Cu–Mg–Ag alloy with excellent mechanical properties and creep resistance at elevated temperature, due to the formation of the fine and uniformly distributed Al
2Cu precipitates on the 〈111〉
α planes [
33]. In this paper, we show the elimination of hot tearing in PBF-LB/M of Al2139 through parameter selection and grain refinement using a commercial Al5Ti1B grain refiner. Thermomechanical and thermodynamic simulations are used to assist the analysis of hot tearing susceptibility.