Freeform Extrusion of High Solids Loading Ceramic Slurries, Part II: Extrusion Process Control

Part I of this paper provided a detailed description of a novel fabrication machine for high solids loading ceramic slurry extrusion and presented an empirical model of the ceramic extrusion process, with ram velocity as the input and extrusion force as the output. A constant force is desirable in freeform extrusion processes as it correlates with a constant material deposition rate and, thus, good part quality. The experimental results in Part I demonstrated that a constant ram velocity will produce a transient extrusion force. In some instances the extrusion force increased until ram motor skipping occurred. Further, process disturbances, such as air bubble release and nozzle clogging that cause sudden changes in extrusion force, were often present. In this paper a feedback controller for the ceramic extrusion process is designed and experimentally implemented. The controller intelligently adjusts the ram motor velocity to maintain a constant extrusion force. Since there is tremendous variability in the extrusion process characteristics, an on-off controller is utilized in this paper. Comparisons are made between parts fabricated with and without the feedback control. It is demonstrated that the use of the feedback control reduces the effect of process disturbances (i.e., air bubble release and nozzle clogging) and dramatically improves part quality.Mechanical Engineerin

Reaction Sintered Zirconium Carbide/Tungsten Composite Bodies and a Method for Producing the Same

A method of sintering a composite body characterized by a transition metal carbide phase (such as a ZrC phase) substantially evenly distributed in a second, typically refractory, transition metal (such as W) matrix at ambient pressures, including blending a first predetermined amount of first transition metal oxide powder (such as ZrO2) with a second predetermined amount of second transition metal carbide powder (such as WC powder). Next the blended powders are mixed to yield a substantially homogeneous powder mixture and a portion of the substantially homogeneous powder mixture is formed into a green body. The body is fired to a first temperature, wherein the first transition metal oxide is substantially reduced and the simultaneously generated CO and gas are evolved from the body to substantially eliminate oxides from the green body, and the body is heated to a second temperature and sintered to yield a composite body of about 99 percent theoretical density and characterized by a first transition metal carbide phase distributed substantially evenly in a second transition metal matrix

Freeform Extrusion of High Solids Loading Ceramic Slurries, Part I: Extrusion Process Modeling

A novel solid freeform fabrication method has been developed for the manufacture of ceramic-based components in an environmentally friendly fashion. The method is based on the extrusion of ceramic slurries using water as the binding media. Aluminum oxide (Al2O3) is currently being used as the part material and solids loading as high as 60 vol. % has been achieved. This paper describes a manufacturing machine that has been developed for the extrusion of high solids loading ceramic slurries. A critical component of the machine is the deposition system, which consists of a syringe, a plunger, a ram actuated by a motor that forces the plunger down to extrude material, and a load cell to measure the extrusion force. An empirical, dynamic model of the ceramic extrusion process, where the input is the commanded ram velocity and the output is the extrusion force, is developed. Several experiments are conducted and empirical modeling techniques are utilized to construct the dynamic model. The results demonstrate that the ceramic extrusion process has a very slow dynamic response, as compared to other non-compressible fluids such as water. A substantial amount of variation exists in the ceramic extrusion process, most notably in the transient dynamics, and a constant ram velocity may either produce a relatively constant steady-state extrusion force or it may cause the extrusion force to steadily increase until the ram motor skips. The ceramic extrusion process is also subjected to significant disturbances such as air bubble release, which causes a dramatic decrease in the extrusion force, and nozzle clogging, which causes the extrusion force to slowly increase until the clog is released or the ram motor skips.Mechanical Engineerin

Method for Producing Pressurelessly Sintered Zirconium Diboride/Silicon Carbide Composite Bodies

A method of sintering a ZrB2-SiC composite body at ambient pressures, including blending a first predetermined amount of ZrB2 powder with a second predetermined amount of SiC powder, wherein both powders are characterized by the presence of surface oxide impurities. Next the blended powders are mixed to yield a substantially homogeneous powder mixture and a portion of the substantially homogeneous powder mixture is formed into a green body. The body is fired to a first temperature, wherein substantially all surface oxide impurities are reduced and/or volatilized to substantially eliminate oxides from the green body, and the body is heated to a second temperature and sintered to yield a composite body of at least about 99 percent theoretical density and characterized by SiC whisker-like inclusions distributed substantially evenly in a ZrB2 matrix

Optimization of Selective Laser Sintering Process for Fabrication of Zirconium Diboride Parts

Selective Laser Sintering (SLS) was investigated to fabricate Zirconium Diboride (ZrB2) parts for ultra-high temperature applications. Experiments were conducted to determine values of SLS process parameters (laser power, scan speed, line spacing, and layer thickness) that can be used to build ZrB2 parts with high integrity and sharp geometrical features. A sacrificial plate with a proper number of layers (determined from experimentation) separated from the main part was built in order to reduce thermal gradients when building the main part. The sacrificial plate was found to assist in eliminating cracks in the bottom of the main part. The fabricated green parts then went through post processing steps including binder burnout and sintering at proper temperature schedules, to remove the binder and sinter the ZrB2 particles. The test bars after sintering had an average relative density of 87% and an average flexural strength of 250 MPa.Mechanical Engineerin

Aqueous-Based Extrusion Fabrication of Ceramics on Demand

Aqueous-Based Extrusion Fabrication is an additive manufacturing technique that extrudes ceramic slurries of high solids loading layer by layer for part fabrication. The material reservoir in a previously developed system has been modified to allow for starting and stopping of the extrusion process on demand. Design pros and cons are examined and a comparison between two material reservoir designs is made. Tests are conducted to determine the optimal deposition parameters for starting and stopping the extrudate on demand. The collected test data is used for the development of a deposition strategy that improves material deposition consistency, including reduced material buildup at sharp corners. Example parts are fabricated using the deposition strategy and hardware design.Mechanical Engineerin

Mechanical properties of borothermally synthesized ZrB2

Mechanical properties of borothermally synthesized, highly pure ZrB2 were tested at room and elevated temperatures. Commercially available ZrB2 powder typically contains 1 to 4 wt % hafnium which has been shown to lower thermal properties of dense ZrB2 ceramics. Further, commercial grade ZrB2 contains other impurities (0.6 wt% O, 0.11 wt% N, 0.04 wt% Fe and others) which are also known to decrease its high-temperature mechanical strength. Purer grades of zirconia and boron powders, containing \u3c 75 ppm hafnium and \u3c0.5 wt% of other metal impurities, were reacted to produce ZrB2 for room and elevated temperature mechanical property studies. The zirconia and boron powders were reacted at 1000°C in a graphite vacuum furnace for two hours. The synthesized ZrB2 powder was then rinsed with methanol to remove boria from its surfaces, sieved with a #45 mesh, and hot pressed to near full density with 32 MPa applied pressure in a flowing argon atmosphere at 2100°C. The hot pressed billets were machined to ASTM standard test bars with the flexure surface polished to 1 um. Young’s modulus, Vickers Hardness, fracture toughness, and four-point bend strength were measured, and the results will be discussed

Freeform Extrusion Fabrication of Titanium Fiber Reinforced Bioactive Glass Scaffolds

Although implants made with bioactive glass have shown promising results for bone repair, their application in repairing load-bearing long bones is limited due to their low fracture toughness and fairly fast degradation response in vivo. In this paper, we describe our investigation of freeform extrusion fabrication of silicate based 13-93 bioactive glass scaffolds reinforced with titanium fibers. A composite paste was prepared with 13-93 bioactive glass filled with titanium fibers (~16 µm in diameter and aspect ratio of ~250) having a volume fraction of 0.4 vol. %. This paste was then extruded to fabricate scaffolds with an extrudate diameter of about ~0.8 mm. The sintered scaffolds, with and without titanium fibers, had measured pore sizes ranging from 400 to 800 µm and a porosity of ~50%. Scaffolds produced with 0.4 vol. % titanium fibers were measured to have a fracture toughness of ~0.8 MPa•m1/2 and a flexural strength of ~15 MPa. Bioactive glass scaffolds without titanium fibers had a toughness of ~ 0.5 MPa•m1/2 and strength of ~10 MPa. The addition of titanium fibers increased the fracture toughness of the scaffolds by ~70% and flexural strength by ~40%. The scaffolds’ biocompatibility and their degradation in mechanical properties, in vitro were assessed by immersing the scaffolds in a simulated body fluid over a period of one to four weeks.Mechanical Engineerin

Room-Temperature Mechanical Properties of a High-Entropy Diboride

The mechanical properties of a (Hf,Mo,Nb,Ta,W,Zr)B2 high-entropy ceramic were measured at room temperature. A two-step synthesis process was utilized to produce the (Hf,Mo,Nb,Ta,W,Zr)B2 ceramics. The process consisted of a boro/carbothermal reduction reaction followed by solid solution formation and densification through spark plasma sintering. Nominally, phase pure (Hf,Mo,Nb,Ta,W,Zr)B2 was sintered to near full density (8.98 g/cm3) at 2000°C. The mean grain size was 6 ± 2 µm with a maximum grain size of 17 µm. Flexural strength was 528 ± 53 MPa, Young\u27s modulus was 520 ± 12 GPa, fracture toughness was 3.9 ± 1.2 MPa·m1/2, and hardness (HV0.2) was 33.1 ± 1.1 GPa. A Griffith-type analysis determined the strength limiting flaw to be the largest grains in the microstructure. This is one of the first reports of a variety of mechanical properties of a six-component high-entropy diboride

Effect of Architecture and Porosity on Mechanical Properties of Borate Glass Scaffolds Made by Selective Laser Sintering

The porosity and architecture of bone scaffolds, intended for use in bone repair or replacement, are two of the most important parameters in the field of bone tissue engineering. The two parameters not only affect the mechanical properties of the scaffolds but also aid in determining the amount of bone regeneration after implantation. Scaffolds with five different architectures and four porosity levels were fabricated using borate bioactive glass (13-93B3) using the selective laser sintering (SLS) process. The pore size of the scaffolds varied from 400 to 1300 μm. The compressive strength of the scaffolds varied from 1.7 to 15.5 MPa for porosities ranging from 60 to 30%, respectively, for the different architectures. Scaffolds were soaked in a simulated body fluid (SBF) for one week to measure the variation in mechanical properties. The formation of the Hydroxyapatite and in-vitro results are provided and discussed