The solidification conditions during welding strongly influence the weld metal microstructure and mechanical properties of a weld. In the first part of this study, the grain morphology of gas tungsten arc (GTA) bead-on-plate welds was investigated for the aluminum Alloys 1050A (Al 99.5), 6082 (Al Si1MgMn), and 5083 (Al Mg4.5Mn0.7). The experiments revealed that increasing welding speed and alloy content allow the growth of small, equiaxed grains, particularly in the weld center. Furthermore, increasing grain refiner additions led to a strong reduction of the weld metal mean grain size and hence facilitated the columnar to equiaxed transition (CET). In addition, wavelength dispersive X-ray spectroscopy (WDS) and transmission electron microscopy (TEM) analysis revealed in the weld metal TiB2 particles that were surrounded by Al3Ti. This suggests the duplex nucleation theory for nucleation of aluminum grains in GTA weld metal.

One important aspect of a fusion weld is the weld metal microstructure. On the one side, the microstructure has a significant influence on the mechanical properties of the weld. For instance, several studies have shown that small, equiaxed grains (instead of large, columnar grains) can improve weld properties such as strength, ductility, and toughness (Refs. 1–4). On the other side, weld metal grain refinement is an important possibility to reduce the susceptibility to solidification cracking in aluminum welds (Refs. 5–8). Small, equiaxed weld metal grains can be achieved through the addition of a commercial grain refiner to the filler metal (Refs. 3, 7, 9). Such master alloys usually consist of the systems Al-Ti, Al-Ti-B, Al-TiC, or Al-B (Ref. 10), where Al-Ti-B master alloys are considered to be more efficient than Al-Ti or Al-B alloys (Ref. 11). Al Ti5B1 is one of the most important grain refiners (Ref. 12). Titanium and boron form particles such as TiB2 (Ref. 13) and Al3Ti (Ref. 14) that act during the solidification of the weld pool as heterogeneous solidification nuclei for aluminum grains. TiB2 particles were found in castings at the center of Al grains (Refs. 15, 16) where they nucleate aluminum grains (Ref. 17). In contrast, it was argued that Al3Ti is a more potent nucleant than TiB2 (Refs. 18, 19) because of the low atomic lattice mismatch between Al3Ti and α-Al. Furthermore, Al3Ti has more atomic planes that can nucleate aluminum grains than TiB2 (Refs. 15, 20). Others argued that, regarding Ti/B additions, AlB2 is the most efficient nucleus for Al, even though it dissolves quickly in the melt (Ref. 21). One widely accepted approach to explaining the exact role of each particle is the duplex nucleation theory (Refs. 20, 22, 23) that was developed from the peritectic theory (Ref. 14). The duplex nucleation theory suggests that the insolvable TiB2 particles are covered in liquid aluminum by a thin Al3Ti layer. Afterward, the peritectic reaction Al3Ti + AlL → AlS takes place on these particles. Accordingly, the Al3Ti layer reacts with liquid aluminum (AlL) to form a further layer of solid aluminum (AlS). This reaction converts such particles into efficient solidification nuclei for aluminum grains (Refs. 14, 20, 23). Properties that make the above particles favorable for nucleation of aluminum grains are, for example, their size and size distribution (Refs. 24, 25), and shape and atomic lattice (Ref. 26). Consequently, additions of grain refiners such as Al Ti5B1 to the aluminum weld pool can provide an increased number of active solidification nuclei and thus a fine, equiaxed weld metal grain structure. An important influence on nucleation, subsequent grain growth, and hence the resulting weld microstructure is the chemical composition of the weld metal. During solidification of the weld pool, the alloying elements partition in the melt and provide constitutional undercooling (Ref. 27), which is needed to activate the abovementioned particles for nucleation of Al grains (Ref. 28). Titanium is supposed to provide the highest degree of constitutional undercooling of all elements (Ref. 29). This explains why excess solute Ti (which is not tied up in particles) plays an important role in aluminum grain refinement and why commercial Al grain refiners usually contain Ti (Ref. 19). Furthermore, the aboveSolidification of GTA Aluminum Weld Metal: Part I – Grain Morphology Dependent upon Alloy Composition and Grain Refiner Content

Turbine components from conventionally cast nickel-base alloy René 80 show different hot cracking susceptibilities depending on their heat treatment conditions leading to slightly different microstructures. Electron probe micro-analysis, focused ion beam technique and analytical transmission electron microscopy were applied to reveal and identify grain boundary precipitates and the γ–γ′-microstructure. The distribution of borides along grain boundaries was evaluated statistically by quantitative metallography. The following features could be correlated with an increase of cracking susceptibility: i) Increasing grain size, ii) increasing fraction of grain boundaries with densely spaced borides, iii) lack of secondary γ′-particles in matrix channels between the coarse cuboidal γ′-precipitates. The latter feature seems to be responsible for linking-up of cracked grain boundary precipitates which occurred as an additional cracking mechanism after one heat treatment, whereas decohesion at the boride-matrix-interface in the heat affected zone of laser-drilled holes was observed for both heat treatments.