During the past decade, significant attention has been drawn to additive manufacturing (AM), which is a unique manufacturing process that allows for parts of various complexity to be built layer-by-layer [1
]. Selective Laser Melting (SLM) is a conventional Laser-Powder Bed Fusion (L-PBF) process that involves melting of a thin layer of metallic powder on a substrate plate and then moving a focused laser in a specified pattern along the powder bed. The laser irradiated powder melts forming a micro-sized melt pool. The melt pool rapidly solidifies upon the removal of the laser and individual tracks of solid material forms. The sum of individual tracks within a plane forms a layer. A layer of powder is again applied on the solidified layer and the laser scans the powder to build the next layer. The process is repeated until the part is completed.
Some limitations with the SLM technology are the low rate of production and the limited part size due to the size of building chamber [1
]. In order to evade these limitations, parts can be joined by welding technologies such as gas tungsten arc welding and laser beam welding. In this way, SLM manufactured parts can be joined with other SLM parts as well as with conventionally manufactured semi-finished products (i.e. cast and wrought material forms).
In this work, SLM manufactured Alloy 718 is investigated. Alloy 718 is the most widely used iron-nickel based superalloy in aero engines. It is a precipitation hardening superalloy that is commonly used in cast and wrought forms in manufacturing of hot structural components of gas turbine engines. This alloy has a Nb content of ~5 wt.% and is strengthened mainly by γ″ (Ni3
Nb) and slightly by γ′ (Ni3
(Al, Ti)) phases [2
]. Interdendritic segregation of Nb is very common in conventionally cast [2
], welded [2
], and L-PBF manufactured Alloy 718 [3
], which in turn result in the formation of the brittle Laves phase [(Ni, Fe, Cr)2
(Nb, Mo, Ti)]. Other phases occurring in the alloy include δ-phase (Ni3
Nb), and various metal carbides and nitrides, such as NbC, TiC, Cr23
The weldability of Alloy 718 has been extensively studied since the alloy was developed. The main problem when welding this alloy is the hot cracking sensitivity. Many approaches have been used to determine the hot cracking sensitivity of Alloy 718, one of them is Variable-Restraint (Varestraint) weldability test. Varestraint testing is a method of determining susceptibility to solidification and liquation cracking of the materials. The Varestraint test uses a controlled, externally applied bending strain to produce cracking during actual welding of the alloy. By varying the amount of strain, a threshold strain, i.e. the strain at which cracks start to form and a saturation strain, i.e. the strain at which crack length do not increase any further can be determined. After testing, the total and maximum crack lengths can be measured, usually on the upper surface of the specimen. These crack lengths, and the threshold and saturation strains are essential in determining the weldability of an alloy [5
]. The sensitivity to hot cracking in the alloy can be aggravated due to constitutional liquation of carbides, liquation of Laves phase, or the segregation of trace elements like B, P, C or S in the grain boundaries [6
]. Grain boundary segregation of these trace elements influences the formation and stability of the intergranular liquid that may form during the welding cycle by reducing the melting point of the grain boundary [6
However, there are no or just a very limited amount of research publications that have investigated the influence of grain orientation on the susceptibility of hot cracking in Alloy 718. Investigations regarding the influence of grain orientation with regard to hot cracking susceptibility have been carried out in other types of alloys. Lippold et al. [10
] studied liquation cracking in the partial melted zone (PMZ) of 5083 aluminum alloy plates and found that PMZ cracking was more severe in welds made transverse to the rolling direction in those made parallel to the rolling direction. The authors suggested that in the latter case the elongated grains produced by the rolling process were parallel to the weld, hence, it was more difficult for cracks to propagate into the base metal. Sidhu et al. [11
] investigated the HAZ cracking susceptibility in directionally solidified (DS) IN738 alloy, laser welded in longitudinal and transverse directions (with regard to grain orientation). In this way they intersected different numbers of grain boundaries while welding. Their results showed a decrease in HAZ cracking with the reduction in number of high angle grain boundaries that intersected the weld bead. It is known from the literature that the grain boundary energy is inherently high for high angle grain boundaries and impurity atoms often preferentially segregate along these boundaries because of their higher energy state. Guo et al. [12
] studied, by Gleeble testing, the correlation between grain boundary characteristics and intergranular liquation in Alloy 718 and reported that liquation mostly occurred at high angle grain boundaries when compared to low angle or special boundaries such as twin boundaries. An issue regarding the SLM manufactured parts, which has been widely investigated by researchers over the past few years, is the relationship between the sample building direction and grain orientation to tensile properties of SLM manufactured parts. Research regarding the influence of building direction on mechanical properties of Alloy 718 disclosed that the strength perpendicular to the building direction was generally higher than parallel to the building direction, while the ductility showed the opposite behavior [3
]. In terms of welding of SLM manufactured Alloy 718, hot cracking is a concern, and have previously been investigated by the authors of the present study regarding different heat-treated conditions [4
]. In that case, the welding was conducted parallel to the specimen building direction, i.e. parallel to the elongated grain orientation. The results showed that the cracks in HAZ were, to a large extent, following the vertical grain boundaries i.e. parallel to the building direction in the as-built welded samples. In the present study, the influence of grain orientation on hot cracking sensitivity was investigated by welding the specimen in both, parallel and transverse to the elongated grain orientation. Varestraint weldability testing with gas tungsten arc welding was used for conducting the experiments. Microstructures of the welded specimens were studied to understand the sensitivity towards hot cracking.