Volume 57, Issue 4

Micrographs of metallographic sections show that additive manufacturing, with its local heat input and rapid cooling rates, brings about microstructures in metal components that differ from those generated in conventional manufacturing processes, such as casting or forging. The example of samples/components made from the material Alloy 718 and manufactured using a laser (Laser Powder Bed Fusion, abbr.: LPBF, designation according to DIN EN ISO ASTM 52900 [1]) shows a very low coarsely distributed porosity. The porosity is limited to the gas porosity already introduced during the atomization step of the powder manufacturing process. As opposed to the microstructure of conventionally produced Alloy 718, the precipitates formed are very small and can only be revealed in the light microscope when high magnifications are applied. Evenly distributed, cruciform γ” precipitates are formed. The grain structure is formed independent of the LPBF-typical melting zones. Isolated dendritic structures can be observed at the interfaces of the melting zones. In the samples manufactured by LPBF, grain structures in the longitudinal section differ from those in the cross section: It can be observed that the grains are elongated in the build direction, which explains the anisotropic behavior of the materials in the tensile test. The width of the melting zones and the laser track spacing can easily be measured in the cross section, while the depth of the melting zones is well recognizable in the longitudinal section. Mechanical properties such as those found in cast or forged Alloy 718 are already present in the stress relief annealed condition.

Microstructural evolution of Ge x Pb 1–x Te (x=0.5, 0.87, 0.9) alloys with respect to spinodal decomposition and continuous/discontinuous precipitation is investigated. Solution treatment at 630 °C for 10 days leads to a single-phase grain structure with homogeneous concentration distribution. In Ge 0.5 Pb 0.5 Te, after aging at 300, 400, and 450 °C for 3 days a lamellar spinodally decomposed microstructure was observed. For the alloys with higher Ge content, a diffusion controlled decomposition of the supersaturated solid solution occurs, during which two different microstructures form either simultaneously or separately. Inside the miscibility gap, but outside the spinodal line, a nucleation and growth mechanism leads to continuous and discontinuous precipitation at the grain boundaries, followed by continuous precipitation in the grain interior.

In this study, the microstructural features and mechanical properties of dissimilar joints between Hastelloy C276 and AISI 316L by gas tungsten arc welding were characterized. ERNiCrMo-4, a Ni-Cr-Mo based filler metal, was used and the welding was performed by multi-pass. Both macro and microstructural features of joints were examined using microscopic techniques. Macrostructural characterization revealed no defects like cracking, pores and undercuts within the joints. The microstructure of weld metal had several forms of solidified austenitic matrix and P phases embedded in dendritic structure. Partially melted zones were observed at the fusion line between the weld metal and AISI 316L and grain growth was determined within the fusion line between the weld metal and Hastelloy C276. In mechanical characterization, not only the variation of hardness but also mechanical properties under tensile and impact loadings were investigated as a function of microstructural features of the joint. The hardness profile throughout the dissimilar joint had a gradual increase from AISI 316L to Hastelloy C276, both having polygonal grains and hardness values of the weld metal strongly affected its solidified form. Due to the superior mechanical properties of Hastelloy C276, failure under tensile loading occurred within AISI 316L. Under impact loading, heat affected zone of Hastelloy C276 showed the highest impact strength.

With a novel class of heavy-duty gas turbine engines, turbine exhaust temperatures are expected to further increase. There are concerns that the propensity to cracking in service of the welded turbine exhaust casing (TEC) liners might also increase. Cold-formed “collars”, featuring large stress-reducing fillet radii, were designed to decrease thermal stresses in the welds, where the struts of the turbine bearing casing are aerodynamically clad by liner plates. Such collars were received from a supplier in the course of a product and process qualification (PPQ). The results of the metallurgical investigation ordered by an internal client are the subject of this contribution. It was found that deformation martensite was present in the subject cold-formed fillet pieces, despite a pre-heat of 150 °C and a post-fabrication 920 °C annealing process. However, the amount of deformation martensite found, at below 5 % volume fraction, was considered not to be detrimental for engine operation.