Yusheng Shi

Porous niobium coatings fabricated with selective laser melting on titanium substrates: Preparation, characterization, and cell behavior

Nb, an expensive and refractory element with good wear resistance and biocompatibility, is gaining more attention as a new metallic biomaterial. However, the high price of the raw material, as well as the high manufacturing costs because of Nbs strong oxygen affinity and high melting point have limited the widespread use of Nb and its compounds. To overcome these disadvantages, porous Nb coatings of various thicknesses were fabricated on Ti substrate via selective laser melting (SLM), which is a 3D printing technique that uses computer-controlled high-power laser to melt the metal. The morphology and microstructure of the porous Nb coatings, which had pores ranging from 15 to 50 μm in size, were characterized with scanning electron microscopy (SEM). The average hardness of the coating, which was measured with the linear intercept method, was 392±37 HV. In vitro tests of the porous Nb coating which was monitored with SEM, immunofluorescence, and CCK-8 counts of cells, exhibited excellent cell morphology, attachment, and growth. The simulated body fluid test also proved the bioactivity of the Nb coating. Therefore, these new porous Nb coatings could potentially be used for enhanced early biological fixation to bone tissue. In addition, this study has shown that SLM technique could be used to fabricate coatings with individually tailored shapes and/or porosities from group IVB and VB biomedical metals and their alloys on stainless steel, Co-Cr, and other traditional biomedical materials without wasting raw materials.

An Integrated Forming Process for Manufacturing Ti6Al4V Impeller with a Functional Coating Layer Using Hot Isostatic Pressing

The complex aircraft components are difficult to manufacture because of their complicated shapes and high requirements of mechanical properties. Besides, those components usually need a functional coating layer on the surface to improve the reliability, which adds an extra production cycle after manufactured. In this study, an integrated forming process was proposed to manufacture a complex-shaped impeller together with the surface coating layer using hot isostatic pressing (HIP). In this new process, an HIP mold with the graphite core pre-deposited a Ni coating layer was designed and conducted with the HIP experiment. The results show that the manufactured impeller had a dimensional precision of 0.15 mm. The scanning electron microscope/X-ray diffraction (SEM/XRD) analyses show that the internal microstructure was compact with α + β phases and the surface had a uniform Ti2Ni/TiNi coating layer. The mechanical experiments show that the HIPped specimens had a 70 MPa higher yield strength and 95 MPa higher tensile strength than the same-size forgings. Besides, a Ti2Ni/TiNi coating layer was formed on the surface of the impeller and the hardness and wear resistance had an obvious improvement. All the results show that the new process was successfully applied to near net shaping of an integrated Ti6Al4V impeller with a functional coating layer.

Metal-ceramic bond mechanism of the Co-Cr alloy denture with original rough surface produced by selective laser melting

The porcelain fracture caused by low metal-ceramic bond strength is a critical issue in porcelain fused to metal(PFM) restorations. Surface roughening methods, such as sand blasting, acid etching and alkaline degreasing for the metal matrix are used to increase bond strength. However, the metal matrix of PFM processed by selective laser melting(SLM) has natural rough surface. To explore the effect of the original roughness on metal-ceramic bond strength, two groups of specimen are fabricated by SLM. One group of specimen surface is polished smooth while another group remains the original rough surface. The dental porcelain is fused to the specimens’ surfaces according to the ISO 9693:1999 standard. To gain the bond strength, a three-point bending test is carried out and X ray energy spectrum analysis(EDS), scanning electron microscope(SEM) are used to show fracture mode. The results show that the mean bond strength is 116.5 ± 16 MPa of the group with rough surface(Ra=17.2), and the fracture mode is cohesive. However, when the surface is smooth (Ra=3.8), the mean bond strength is 74.5 MPa ± 5 MPa and the fracture mode is mixed. The original surface with prominent structures formed by the partly melted powder particles, not only increases surface roughness but also significantly improves the bond strength by forming strong mechanical lock effect. Statistical analysis (Student’s t-test) demonstrates a significant difference (p<0.05) of the mean value of bond strength between the two groups. The experiments indicate the natural rough surface can enhance the metal-ceramic bond strength to over four times the minimum value (25 MPa) of the ISO 9693:1999 standard. It is found that the natural rough surface of SLM-made PFM can eliminate the porcelain collapse defect produced by traditional casting method in PFM restorations.

Thermal elasto‐viscoplastic analysis based on Perzyna model for hot isostatic pressing of stainless steel powder

Abstract The present study has investigated the hot isostatic pressing (HIP) process of a stainless steel powder using finite element method. Thermal elasto‐viscoplastic constitutive equations are deduced from the Perzyna viscoplastic rheological model. A three‐dimensional simulation was performed within MSC.Marc software. Temperature distribution and evolution of relative density were discussed. The results of the simulation indicate that noticeable temperature gradients would cause the nonuniform relative density distribution during the HIP process; it is also found that there is a slight decrease in the relative density of the compact at the early stage of the HIP cycle. The finite element results were compared with experimental data.