Bonding of 304L Stainless Steel to Cast Iron by Selective Laser Melting

While cast iron is widely used in industry, a major limitation is the weldability of a dissimilar material onto cast iron due to hot cracking as a result of lack of ductility from graphite flakes. Consequently, a significant amount of preheat is often employed to reduce the cooling rate in the fusion zone, which, however, may lead to distortion of the welded parts. A potential remedy could be the Selective Laser Melting (SLM) process, where only small melt pools are created and thus the overall energy input is reduced. The present paper describes an investigation of the SLM process to join 304L stainless steel with cast iron. In this study, 304L stainless steel particles ranging from 15-45 μm in size were melted on a grey cast iron substrate by the SLM process. Multiple sets of parameter values were chosen to test different energy densities on the tensile strength of the bond created. Subsequent characterization of the bonded area included energy dispersive spectroscopy (EDS) mapping for obtaining insight into the elemental diffusion, and metallography for visualization of the microstructure. A range of energy densities was identified for purposes of eliminating bond delamination and maximizing mechanical strength

Selective Laser Sintering and Freeze Extrusion Fabrication of Scaffolds for Bone Repair using 13-93 Bioactive Glass: A Comparison

13-93 glass is a third-generation bioactive material which accelerates the bone’s natural ability to heal by itself through bonding with surrounding tissues. It is an important requirement for synthetic scaffolds to maintain their bioactivity and mechanical strength with a porous internal architecture comparable to that of a human bone. Additive manufacturing technologies provide a better control over design and fabrication of porous structures than conventional methods. In this paper, we discuss and compare some of the common aspects in the scaffold fabrication using two such processes, viz. selective laser sintering (SLS) and freeze extrusion fabrication (FEF). Scaffolds fabricated using each process were structurally characterized and microstructure analysis was performed to study process differences. Compressive strength higher than that of human trabecular bone was achieved using SLS process and strength almost comparable to that of human cortical bone was achieved using FEF process

Freeze Extrusion Fabrication of 13-93 Bioactive Glass Scaffolds for Bone Repair

There is an increasing demand for synthetic scaffolds with the requisite biocompatibility, internal architecture, and mechanical properties for the bone repair and regeneration. In this work, scaffolds of a silicate bioactive glass (13-93) were prepared by a freeze extrusion fabrication (FEF) method and evaluated in vitro for potential applications in bone repair and regeneration. The process parameters for FEF production of scaffolds with the requisite microstructural characteristics, as well as the mechanical and cell culture response of the scaffolds were evaluated. After binder burnout and sintering (60 min at 700°C), the scaffolds consisted of a dense glass network with interpenetrating pores (porosity ≈ 50%; pore width = 100-500 μm). These scaffolds had a compressive strength of 140 ± 70 MPa, which is comparable to the strength of human cortical bone and far higher than the strengths of bioactive glass and ceramic scaffolds prepared by more conventional methods. The scaffolds also supported the proliferation of osteogenic MLO-A5 cells, indicating their biocompatibility. Potential application of these scaffolds in the repair and regeneration of load-bearing bones, such as segmental defects in long bones, is discussed

Selective Laser Sintering and Freeze Extrusion Fabrication of Bioglass Bone Scaffolds [abstract]

Biomedical Tissue Engineering, Biomaterials, and Medical Devices Poster SessionBioactive glasses are promising materials for bone scaffolds due to their ability to assist in tissue regeneration. When implanted in vivo, bioactive glasses can convert to hydroxyapatite, the main mineral constituent of human bone and form a strong bond with the surrounding tissues, providing an advantage over polymer scaffold materials. Bone scaffold fabrication using additive manufacturing (solid freeform fabrication) methods provides control over design and fabrication of pores in the scaffold. 13-93 bioglass (manufactured by Mo-Sci Corporation), a third-generation bioactive and resorbable material designed to accelerate the body's natural ability to heal itself, was used in the research described herein to fabricate bone scaffolds using two different additive manufacturing methods - Selective Laser Sintering and Freeze Extrusion Fabrication. Selective Laser Sintering (SLS) is a process where a laser light is controlled to selectively sinter the particles in a powder bed layer-by-layer to fabricate a 3D part based on a CAD model. The SLS machine used in this research was a DTM Sinterstation 2000. 13-93 bioglass mixed with stearic acid (as the polymer binder) by ball milling was used as the powder feedstock for the SLS machine. The fabricated green scaffolds underwent binder burnout to remove the stearic acid binder and then sintered at temperatures between 6500C and 7000C. After sintering, the scaffolds were mechanically tested, achieving a maximum compressive strength of 16 MPa for scaffolds with 60% apparent porosity. Bioactivity results showed the ability of the SLS scaffolds to support the growth of osteoblastic cells. Scanning electron microsocopy analysis and MTT formazan formation measurements provided evidence that the bioglass scaffolds fabricated by the SLS process offer a surface capable of supporting robust cell growth. Freeze Extrusion Fabrication (FEF) is a process where an aqueous-based glass paste is extruded and deposited layer-by-layer to fabricate a 3D part in a sub-freezing temperature environment. The FEF system, developed at Missouri S&T, consists of a 3-axis positioning system, a ram extruder for paste extrusion, and position and force sensors for measurement and control. Bioglass slurry was prepared by ball milling 13-93 bioglass particles along with a dispersant (surfynol) and a binder (aquazol). Further, a lubricant (PEG-400) was added to the paste to aid in extrusion. The bioglass slurry was then heated to obtain bioglass paste. Scaffolds with varying pore sizes from 300μm to 800μm were successfully fabricated using the FEF process. Post processing of green scaffolds, including binder burnout and sintering, is currently being performed. Scaffolds produced by the FEF process will be evaluated and compared with the scaffolds obtained using the SLS process

Freeze extrusion fabrication of 13-93 bioactive glass scaffolds for bone repair

We have investigated Freeze Extrusion Fabrication (FEF) of 13-93 bioactive glass to fabricate three dimensional scaffolds for bone tissue engineering. Bioactive glass paste was prepared for consistent extrusion through micro-nozzle and controlled porosity scaffolds were fabricated using additive manufacturing FEF. CAD models were prepared for different scaffold designs and computer controlled layer-by-layer deposition of pseudoplastic paste was carried out to in accordance. Liquid nitrogen assisted freezing of pastes was used for the consolidation of deposited layers. Post processing binder burnout and sintering schedules were designed for evaporation of organic binder and densification of glass. Sintering shrinkage was measured, and X-ray diffraction analysis was carried out to evaluate the nature of the sintered glass. SEM images of the sintered scaffolds were observed for pore interconnectivity required for tissue ingrowth. The sintered scaffolds demonstrated average compressive strength of 136 MPa. This is equivalent to human cortical bone compressive strength and it is the highest reported value with the additive manufacturing of bioactive glass --Abstract, page iv

Freeze Extrusion Fabrication of 13-93 Bioactive Glass Scaffolds for Bone Repair

A solid freeform fabrication technique, freeze extrusion fabrication (FEF), was investigated for the creation of three-dimensional bioactive glass (13-93) scaffolds with pre-designed porosity and pore architecture. An aqueous mixture of bioactive glass particles and polymeric additives with a paste-like consistency was extruded through a narrow nozzle, and deposited layer-by-layer in a cold environment according to a computer-aided design (CAD) file. Following sublimation of the ice in a freeze dryer, the construct was heated according to a controlled schedule to burn out the polymeric additives (below ~500°C), and to densify the glass phase at higher temperature (1 h at 700°C). The sintered scaffolds had a gridlike microstructure of interconnected pores, with a porosity of ~50%, pore width of ~300 µm, and dense glass filaments (struts) with a diameter or width of ~300 µm. The scaffolds showed an elastic response during mechanical testing in compression, with an average compressive strength of 140 MPa and an elastic modulus of 5-6 GPa, comparable to the values for human cortical bone. These bioactive glass scaffolds created by the FEF method could have potential application in the repair of load-bearing bones

Freeze Extrusion Fabrication of 13-93 Bioactive Glass Scaffolds for Repair and Regeneration of Load-bearing Bones

There is an increasing demand for synthetic scaffolds with the requisite biocompatibility, internal architecture, and mechanical properties for the bone repair and regeneration. In this work, scaffolds of a silicate bioactive glass (13-93) were prepared by a freeze extrusion fabrication (FEF) method and evaluated in vitro for potential applications in bone repair and regeneration. The process parameters for FEF production of scaffolds with the requisite microstructural characteristics, as well as the mechanical and cell culture response of the scaffolds were evaluated. After binder burnout and sintering (60 min at 700°C), the scaffolds consisted of a dense glass network with interpenetrating pores (porosity ≈ 50%; pore width = 100−500 μm). These scaffolds had a compressive strength of 140 ± 70 MPa, which is comparable to the strength of human cortical bone and far higher than the strengths of bioactive glass and ceramic scaffolds prepared by more conventional methods. The scaffolds also supported the proliferation of osteogenic MLO-A5 cells, indicating their biocompatibility. Potential application of these scaffolds in the repair and regeneration of load-bearing bones, such as segmental defects in long bones, is discussed