Abstract
Electron beam melting (EBM) was used to produce Alloy 718 specimens with different microstructures (columnar, equiaxed and a combination thereof) by varying the process parameters. The present study aimed at assessing the response of such varying as-built microstructures to identical thermal post-treatments, which included hot isostatic pressing (HIPing) followed by heat treatment involving solution treatment and aging. The effect of these treatments on defect content, grain structure, hardness and phase constitution in the specimens was specifically analysed. Despite differences in defect content of as-built specimens with distinct microstructures, HIPing was effective in closing defects leading to samples exhibiting similar density. After HIPing, grains with equiaxed morphology or columnar grains with lower aspect ratio showed higher tendency for grain growth in comparison to the columnar grains with higher aspect ratio. The various factors affecting the stability of grains during HIPing of builds with distinct microstructures were investigated. These factors include texture, grain size, and secondary phase particles. The carbide sizes in the different as-built samples varied but were found to be largely unaffected by the post-treatments. Solution treatment following HIPing led to greater precipitation of grain boundary δ phase in regions with coarser grains than the smaller ones. After HIPing and heat treatment, all specimens exhibited similar precipitation of γ″ phase regardless of their grain morphology in the as-built condition.
Highlights
Powder bed fusion additive manufacturing (AM) techniques such as electron beam melting (EBM) enable production of near net shaped highly complex geometries in layer by layer fashion using difficult to machine materials like Alloy 718 [1]
The type of as-built microstructure that can be achieved has been shown to be governed by scan strategy and by manipulating EBM process parameters
It is worth mentioning that the range of G/R ratios that result in columnar and equiaxed morphologies is dependent on the material as elaborated by Kurz et al [12]
Summary
Powder bed fusion additive manufacturing (AM) techniques such as electron beam melting (EBM) enable production of near net shaped highly complex geometries in layer by layer fashion using difficult to machine materials like Alloy 718 [1]. The microstructure of EBM built Alloy 718 is commonly composed of columnar grains elongated along the build direction with preferred 〈001〉 orientation [2]. Due to this crystallographic texture the material typically exhibits anisotropic mechanical behaviour [6]. An equiaxed grain structure exhibits isotropic mechanical behaviour which could be beneficial for applications demanding such properties [2,6]. The type of as-built microstructure (from columnar to equiaxed) that can be achieved has been shown to be governed by scan strategy and by manipulating EBM process parameters. Helmer et al [10] studied equiaxed grain formation by altering the scanning rotation between layers.
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