Lisa Anne Deibler

Process modeling and experiments for forging and welding.

We are developing the capability to track material changes through numerous possible steps of the manufacturing process, such as forging, machining, and welding. In this work, experimental and modeling results are presented for a multiple-step process in which an ingot of stainless steel 304L is forged at high temperature, then machined into a thin slice, and finally subjected to an autogenous GTA weld. The predictions of temperature, yield stress, and recrystallized volume fraction are compared to experimental results.

Experiments and modeling to characterize microstructure and hardness in 304L

Drawn 304L stainless steel tubing was subjected to 42 different annealing heat treatments with the goal of initializing a microstructural model to select a heat treatment to soften the tubing from a hardness of 305 Knoop to 225–275 Knoop. The amount of recrystallization and grain size caused by 18 heat treatments were analyzed via optical microscopy and image analysis, revealing the full range of recrystallization from 0 to 100%. The formation of carbides during the longer duration and higher-temperature heat treatments was monitored via transmission electron microscope evaluation. The experimental results informed a model which includes recovery, recrystallization, and grain growth to predict microstructure and hardness. After initialization of the model, it was able to predict hardness with a R2 value of 0.95 and recrystallization with an R2 value of 0.99. The model was then utilized in the design and testing of a heat treatment to soften the tubing.

Effect of Hot Isostatic Pressing and Powder Feedstock on Porosity, Microstructure, and Mechanical Properties of Selective Laser Melted AlSi10Mg

AbstractAlSi10Mg tensile bars were additively manufactured using the powder-bed selective laser melting process. Samples were subjected to stress relief annealing and hot isostatic pressing. Tensile samples built using fresh, stored, and reused powder feedstock were characterized for microstructure, porosity, and mechanical properties. Fresh powder exhibited the best mechanical properties and lowest porosity while stored and reused powder exhibited inferior mechanical properties and higher porosity. The microstructure of stress relieved samples was fine and exhibited (001) texture in the z-build direction. Microstructure for hot isostatic pressed samples was coarsened with fainter (001) texture. To investigate surface and interior defects, scanning electron microscopy, optical fractography, and laser scanning microscopy techniques were employed. Hot isostatic pressing eliminated internal pores and reduced the size of surface porosity associated with the selective laser melting process. Hot isostatic pressing tended to increase ductility at the expense of decreasing strength. However, scatter in ductility of hot isostatic pressed parts suggests that the presence of unclosed surface porosity facilitated fracture with crack propagation inward from the surface of the part.

Failure analysis in support of deformation modeling.

While many aspects of the mechanical behavior of structural metals can be accurately modeled, the prediction of fracture and failure has proved particularly difficult. The annual Sandia Fracture Challenge [1] was launched in 2012 to test the abilities of models to capture ductile deformation and fracture. Every year a unique geometry sample made from a common engineering alloy is specified, and teams are invited to model the deformation and fracture of the sample based on uniaxial tensile characteristics and geometry. The models are then compared to experimental results.

Sliding Component Failure Due to Vibration Damage

Components with surfaces which must be able to slide freely after a period of use were subjected to vibration testing. After vibration testing, some units locked up and were unable to slide. The component consists of two interlocking pins which ride in a housing, as illustrated in Figure 1. During normal operation, the upper pin is pulled up out of the housing where it is free to disconnect from the lower pin. In the post-vibration locked up condition, the upper pin cannot be pulled out of the sleeve.