Within the past ten years, spark plasma sintering (SPS) has become an increasingly popular process for Mg manufacturing. In the SPS process, interparticle diffusion of compressed particles is rapidly achieved due to the concept of Joule heating. Compared to traditional and additive manufacturing (AM) techniques, SPS gives unique control of the structural and microstructural features of Mg components. By doing so, their mechanical, tribological, and corrosion properties can be tailored. Although great advancements in this field have been made, these pieces of knowledge are scattered and have not been contextualized into a single work. The motivation of this work is to address this scientific gap and to provide a groundwork for understanding the basics of SPS manufacturing for Mg. To do so, the existing body of SPS Mg literature was first surveyed, with a focus on their structural formation and degradation mechanisms. It was found that successful Mg SPS fabrication highly depended on the processing temperature, particle size, and particle crystallinity. The addition of metal and ceramic composites also affected their microstructural features due to the Zener pinning effect. In degradative environments, their performance depends on their structural features and whether they have secondary phased composites. In industrial applications, SPS'd Mg was found to have great potential in biomedical, hydrogen storage, battery, automotive, and recycling sectors. The prospects to advance the field include using Mg as a doping agent for crystallite size refinement and using bulk metallic Mg-based glass powders for amorphous SPS components. Despite these findings, the interactions of multi-composites on the processing-structure-property relationships of SPS Mg is not well understood. In total, this work will provide a useful direction in the SPS field and serve as a milestone for future Mg-based SPS manufacturing.
A hybrid, non-dimensional parameter descriptive of the plowing nature of surfaces is proposed for the case of sliding between a soft and a relatively hard metallic pair which are nominally flat, typically as one encounters in the case of hot forming. From a set of potential parameters which can be descriptive of the phenomenon, dimensionless groups are formulated and the influence of each one of them analyzed. Inter- dependence between certain groups are established which lead to retention of those which are independent. A non-dimensional parameter involving the root-mean square deviation (Rq) and the centroidal frequency (Fmean) deducted from the power-spectrum is found to have a high degree of correlation (as high as 0.93) with the coefficient of friction obtained in sliding experiments under lubricated condition. Variation of the coefficient of friction in dry condition (which brings in the effect of adhesion, in addition to plowing) as a function of the proposed parameter is also presented.
When machining miniaturized components, the contact conditions between the tool and the workpiece exhibit very small contact areas that are on the order of 10−5 mm2. Under these conditions, extremely high contact stresses are generated, and it is not clear whether macroscopic theories for the chip formation, cutting forces, and friction mechanisms are applicable. For this reason, the present investigation has focused on creating a basic understanding of the frictional behavior in very small scale machining processes so that evaluations of standard macroscale models could be performed. Specialized machining experiments were conducted on 70/30 brass materials using high-speed steel tools over a range of speeds, feeds, depths of cut, and tool rake angles. At each operating condition studied, the friction coefficient and the shear factor τk were obtained. Based on the experimental results, it was determined that the standard macroscopic theory for analyzing detailed friction mechanisms was insufficient in very small scale machining processes. An approach that utilized the shear factor, in contrast, was found to be better for decoupling the physical phenomena involved. Utilizing the shear factor as an analysis parameter, the parameters that significantly influence the friction in microscale machining processes were ascertained and discussed.
Understanding friction between material pairs is very crucial when utilizing the materials in various applications, including mechanical and biological systems. In the present investigation, tribological properties, such as coefficient of friction (COF) and transfer film formation, were investigated by varying surface texture and roughness features during sliding of Ultra High Molecular Weight Polyethylene (UHMWPE) pins against steel plates using an inclined pin-on-plate sliding tester. The roughness of the textures was quantified using optical profilometer. Scanning Electron Microscope was utilized to characterize the fracture features on the pin surface and transfer film formation on the plate surface. Results showed that the COF is significantly depend on surface texture when compared to surface roughness (Ra) of the harder steel material. The transfer film formation is found to depend on the COF. Detailed analysis of friction components, namely adhesion and hysteresis, demonstrated that the effect of surface texture on the COF and transfer film formation was attributed to the variation of the hysteresis friction component. The variations in hysteresis friction with surface texture is attributed to the nature of constraints imposed by the surface textures at the asperity level during sliding conditions.
In the current study, pure aluminum (Al) powders were cryomilled with and without 3 wt.% pure magnesium (Mg) dopant for varying durations followed by spark plasma sintering (SPS) of powders to prepare bulk components with superior mechanical properties. The crystallite sizes were determined for powders and the bulk components by analyzing the X-ray diffraction (XRD) spectrum. The calculations indicated a reduction in crystallite size with the increase in the cryomilling duration. The results also showed a more significant decrease in the crystallite sizes for Al-3Mg samples than that of pure Al. The changes in the surface morphology of powders were characterized using scanning electron microscopy (SEM). The elemental mapping analysis at nanoscale was carried out using Energy-dispersive X-ray spectroscopy (EDX) in Scanning transmission electron microscopy (STEM). The mechanical properties of the bulk components were assessed using a Vickers Microhardness tester. The test results demonstrated an improvement in the hardness of Mg-doped components. Higher hardness values were also reported with an increase in the cryomilling duration. This article discusses the mechanisms for the reduction in crystallite size for pure Al and Al-3Mg and its subsequent impact on improving mechanical properties.
