Deborah C. Blaine

Application of Work-of-sintering concepts in powder metals

One of the current challenges facing the particulate materials industry is developing simple, accurate models to predict sintered properties. Work-of-sintering concepts, where time-temperature integrals are used in such models, offer a solution to this problem. In this study, the master sintering curve concept is applied to several powder metal systems: 17 to 4PH stainless steel, 316L stainless steel, nickel, niobium, tungsten heavy alloys with two different compositions, and molybdenum. A detailed explanation of the construction of these curves is given, including methods used to calculate the apparent activation energy for sintering and to curve-fit experimental data to a sigmoid function describing the master sintering curve. Discussion of the results shows that the master sintering curve can be applied to powder metal systems, even those that use liquid phase during sintering.

Application of finite element analysis to the design of tissue leaflets for a percutaneous aortic valve

Percutaneous Aortic Valve (PAV) replacement is an attractive alternative to open heart surgery, especially for patients considered to be poor surgical candidates. Despite this, PAV replacement still has its limitations and associated risks. Bioprosthetic heart valves still have poor long-term durability due to calcification and mechanical failure. In addition, the implantation procedure often presents novel challenges, including damage to the expandable stents and bioprosthetic leaflets. In this study, a simplified version of Fungs elastic constitutive model for skin, developed by Sun and Sacks, was implemented using finite element analysis (FEA) and applied to the modelling of bovine and kangaroo pericardium. The FEA implementation was validated by simulating biaxial tests and by comparing the results with experimental data. Concepts for different PAV geometries were developed by incorporating valve design and performance parameters, along with stent constraints. The influence of effects such as different leaflet material, material orientation and abnormal valve dilation on the valve function was investigated. The stress distribution across the valve leaflet was also examined to determine the appropriate fibre direction for the leaflet. The simulated attachment forces were compared with suture tearing tests performed on the pericardium to evaluate suture density. It is concluded that kangaroo pericardium is suitable for PAV applications, and superior to bovine pericardium, due to its lower thickness and greater extensibility.

Effects of residual carbon content on sintering shrinkage, microstructure and mechanical properties of injection molded 17-4 PH stainless steel

Carbon contamination from the thermoplastic binder is an inherent problem with the metal powder injection molding process. Residual carbon in the compacts after debinding has a strong impact on the sintering process, microstructure, and mechanical properties. In this study, injection molded 17-4 PH stainless steel was debound to two levels of residual carbon, 0.203 ± 0.014 wt% and 0.113 ± 0.008 wt%, by elevating the debinding temperature from 450°C to 600°C. Dilatometry in H2 atmosphere shows that the 600°C-debound compacts shrink much faster than those debound at 450°C when the sintering temperature rises to over 1200°C. Density measurements for tensile bars sintered between 1260°C and 1380°C confirm the beneficial effect of low residual carbon content on sintering shrinkage. Quantitative metallography reveals that more δ-ferrite forms along austenite grain boundaries during sintering of the 600°C-debound compacts. In both samples, density gradients across the compact section are correlated with the residual carbon content and corresponding δ-ferrite formation. Finally, tensile tests show that the 600°C-debound compacts have lower tensile strength but higher ductility than those debound at 450°C. The relevant mechanisms are discussed with a focus on the effects of residual carbon content, δ-ferrite amount, and porosity.

Critical use of video-imaging to rationalize computer sintering simulation models

Sintering, a thermally activated diffusion process, is used to densify particulate materials. During the densification process, there is concomitant volumetric shrinkage of the powder compact to the final desired part shape. Additionally, shear deformation can occur in response to deviatoric stresses in the body, such as gravity, thermal stresses, and stress gradients due to internal density differences. Finite element modeling of sintering deformation can greatly aid in reverse engineering green part shapes, thus improving dimensional precision of final parts. However, the constitutive equations that govern the rheological response of a porous body during sintering are notoriously difficult to characterize. In situ video-imaging of a simply supported beam, deforming under its own weight during sintering, has been effectively employed in determining the apparent viscosity of the densifying material. This paper describes this experimental technique and the implementation of the results in a finite element analysis. It demonstrates the efficacy of the characterized constitutive equations by a finite element simulation example using 316L stainless steel with 0.2% boron addition.

INFLUENCE OF HEAT TREATMENTS ON THE MICROSTRUCTURE AND TENSILE BEHAVIOUR OF SELECTIVE LASER MELTING-PRODUCED TI-6AL-4V PARTS

In industry, post-process heat treatments of Ti-6Al-4V are performed with the aim of improving its tensile behaviour. While heat treatments of wrought Ti6Al4V have been standardised (e.g., Aerospace Material Specification H-81200), heat treatments of selective laser melting (SLM)-produced Ti-6Al-4V lacks research and understanding. Significant concern exists about SLM Ti6-Al-4V’s achievable ductility attributed to its martensitic (α’) phase. In this research, heat treatments at a range of temperatures are applied to SLM-produced Ti-6Al-4V tensile samples. Microstructural analysis (both optically and through electron backscatter diffraction) was used to identify links between heat treatments and microstructure. Subsequently, uniaxial tensile tests were performed to determine the respective tensile properties of all samples. Correlations in the data show a significant loss in strength with respect to an increase in annealing temperature due to grain growth, while no noticeable trend was observed for fracture strain with regard to annealing temperatures.

Numerical comparison of lattice unit cell designs for medical implants by additive manufacturing

ABSTRACT The aim of this study was to compare traditional strut-based lattices with minimal surface designs using morphological analysis and image-based simulations of design files. While the two types have been studied widely, no direct comparison has ever been done. Surprisingly, there are no major differences in performance between the two types generally, but minimal surface designs do outperform slightly on angular load simulation. However, minimal surface designs in this density range are shown to have very thin walls, potentially making their accurate production more challenging, or more suitable for applications where larger pore sizes and sheet thicknesses may be applicable. Interesting results such as dual pore size distributions and variations in tortuosity of pore networks are demonstrated, with differences between various designs. The results show that all the tested designs are suitable for bone implants, but the best design might be selected based on its specialised performance requirements.

Distortion and Densification Control during Liquid Phase Sintering of High‐Performance Materials

Liquid phase sintering is used for net‐shape fabrication of high performance materials. This work reviews microstructure evolution models needed to simulate the densification and distortion events observed with the semisolid powder‐liquid mixtures during sintering. Critical insight in required from the capillarity, pore collapse, solid‐liquid morphology, solid phase connectivity, grain growth, and system rheology. These complications are handled using a modified time‐dependent viscous flow model that requires data on grain size, density, and boundary conditions (such as substrate friction) versus time. The rheological response model coupled with finite element analysis has proven most effective in predicting final size and shape of engineering components. A simplified material parameter extraction scheme is used to determine many the material constants, providing hope for a generic model in the future.