The Department of Mechanical Engineering (https:// www.mccormick.northwestern.edu/mechanical/graduate) prepares graduates for careers in industry, research, and academia. Students specialize in Design/Manufacturing/Tribology, Dynamics, Control, Robotics, Neural Engineering, Simulation Driven Engineering, Solid Mechanics, Fluid Dynamics, Nanotechnology/MEMS, Bioengineering and Biomechanics, Energy and Sustainability, Engineering Design and Innovation, Product Development, Engineering Management and other disciplines.

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Hot compression tests of a FGH96 superalloy under the conditions of temperatures from 1020 to 1110 °C and strain rates from 10−3 to 1 s−1 were carried out on a Gleeble-3180D thermo simulator. The microstructure evolution and nucleation mechanisms of dynamic recrystallization (DRX) for the superalloy were analyzed by electron backscatter diffraction (EBSD) technique and transmission electron microscope (TEM). A DEFORM-3D Finite Element (FE) software was used to simulate the distributions of deformation and microstructures of the compressed specimens. The results show that the flow stress curves of FGH96 superalloy exhibit typical features of dynamic recrystallization (DRX). The constitutive equation is established based on the friction-corrected flow stress data. Increasing temperature or decreasing strain rate is found to result in the increases of both the DRX fraction and the grain size. The nucleation mechanisms of DRX for the superalloy mainly include dominant discontinuous dynamic recrystallization (DDRX) and assisting nucleation by twins. The prediction models of DRX, which are expressed as a function of Z parameter, are obtained on the basis of the compression data and quantitatively microstructural characterization. By comparing the simulated results with that of the experiment, the accuracy of the developed DRX models is validated.

Experimental study and numerical simulation of dynamic recrystallization for a FGH96 superalloy during isothermal compression

This study confirmed that Al2O3 particles were formed in IN718 alloys during the fabrication by both selective laser melting (SLM) and electron beam melting (EBM). Different heat pattern and atmospheres in SLM and EBM result in different distribution and volume fraction of Al2O3 particles. The Al2O3 oxides would act as nucleation sites for the precipitation of Nb/Ti carbides, leading to the formation of unique core-shell composites with Al2O3 in the center and Ti/Nb at the periphery. In order to investigate the oxygen content introduced during SLM and EBM, the volume fraction of Al2O3 formed in spark plasm sintering (SPS)-fabricated substrate, by consolidating the pre-oxidized IN718 raw powders at 800 °C, was utilized. The oxygen contents introduced to IN718 substrates during SLM and EBM fabrication were calculated to be 0.030 wt% and 0.099 wt%, respectively.

Study of Formed Oxides in IN718 Alloy during the Fabrication by Selective Laser Melting and Electron Beam Melting

The aim of this study was to gain a deep understanding of the microstructure-mechanical relationship between solid-state sintering and full-melting processes. The IN718 superalloy was fabricated by hot isostatic pressing (HIP) and selective laser melting (SLM). Continuous precipitates were clearly localized along the prior particle boundary (PPB) in the HIP materials, while SLM materials showed a microstructure free of PPB. The mechanical properties of specimens that underwent SLM + solution treatment and aging were comparable to those of conventional wrought specimens both at room temperature and 650 °C. However, a drop was observed in the ductility of HIP material at 650 °C. The brittle particles along the PPB were found to affect the HIP materials’ creep life and ductility during solid-state sintering.

Influence of Powder Surface Contamination in the Ni-Based Superalloy Alloy718 Fabricated by Selective Laser Melting and Hot Isostatic Pressing

Nowadays, an increasing number of components are manufactured using Hot Isostatic Pressing (HIP), which is considered a near net or net shape forming technology, depending on the overstock thickness. One of the most challenging tasks with regard to improving HIP is limiting the interaction between the capsule and the consolidating powders, aiming to reduce material waste and costs. The capsule is typically made of mild carbon steel and it is used to correctly transfer the forming pressure to the loose powders at the HIP temperature. The great inhomogeneity between the two materials at such a high pressure and temperature lead to a strong elemental diffusion between the powders and the capsule, which leads to the formation of an interface. This study investigates in depth, the effects of this diffusion during the consolidation of Astroloy powders.

Net shape HIPping of Ni-superalloy: Study of the interface between the capsule and the alloy

Powder metallurgy (PM) processing of nickel-based superalloys has been used for a wide range of near net-shape fine grained products. In this paper a novel forming process, i.e. direct forging of unconsolidated powder superalloys is proposed. In this process, encapsulated and vacuumed powder particles are heated up to a forming temperature and forged directly at high speed to the final shape, by using a high forming load. Experiments of direct powder forging have been conducted on an upsetting tool-set. Microstructure, relative density and hardness of the formed specimen have been investigated. A finite element model of the direct powder forging process has been established in DEFORM and validated by the comparisons of experimental with simulation results of load variation with stroke as well as relative density distribution. The stress state and the relative density variation have been obtained from FE simulation. The correlation between the stress and consolidation condition has been rationalised. The developed FE model can provide a guide to design the geometry and thickness of preform for direct powder forging.

https://research.birmingham.ac.uk/portal/files/18357043/Dean_Study_of_Direct_Forging_Process_Materials_Science_Engineering_A_2015.pdf

G. Tian

Papers

A study of direct forging process for powder superalloys