Porous metals, typically produced through powder metallurgy, represent a class of relatively new materials with wide industrial applications, lately extending into the microscale domain. Although produced in near-net shapes, most components fabricated from these materials still require some form of secondary machining. Despite the progress made in the field, relatively little is known either on the inherent cutting mechanism or on the behaviour of these materials under micromachining conditions. The present study reviews the main cutting theories proposed in macroscale machining, along with one of the primary parameters used to describe its machinability performances, namely cutting forces. Then, the feasibility of macroscale concepts is discussed in the context of micromachining technology that is characterized by comparable tool and pore sizes. The microslot cutting experiment performed in a porous titanium sample outlined the relative interplay between the magnitude of the cutting force and porosity of the material. Based on this, it was concluded that the impact of structural porosity on cutting forces experienced during micromachining is significant and therefore further in-depth investigations will be required.
Porous titanium foam is now a standard material for various dental and orthopedic applications due to its light weight, high strength, and full biocompatibility properties. In practical biomedical applications, outer surface geometry and porosity topology significantly influence the adherence between implant and neighboring bone. New microfabrication technologies, such as micromilling and laser micromachining opened new technological possibilities for shape generation of this class of products. Besides typical geometric alterations, these manufacturing techniques enable a better control of the surface roughness that in turn affects to a large extent the friction between implant and surrounding bone tissue. This paper proposes an image analysis approach for optical investigation of the porosity that is tailored to the specifics of micromilling process, with emphasis on cutting force monitoring. According to this method, the area of porous material removed during micromilling operation is estimated from optical images of the micromachined surface, and then the percentage of solid material cut is calculated for each tool revolution. The employment of the aforementioned methodology in micromilling of the porous titanium foams revealed reasonable statistical correlations between porosity and cutting forces, especially when they were characterized by low-frequency variations. The developed procedure unlocks new opportunities in optimization of the implant surface micro-geometry, to be characterized by an increased roughness with minimal porosity closures in an attempt to maximize implant fixation through an appropriate level of bone ingrowth.
Porous titanium, characterized by interconnected and large open-cell structures, constitutes one of the most promising bone substitutes that are currently available for surgical orthopedic and dental implantation procedures. Since little is known about the behavior of this highly porous material during material removal operations, the main objective of this study was to develop a framework capable of evaluating the effect of cutting speed, cutting depth, and feed rate on the interplay between porosity and cutting force signatures, as experienced during microslot cutting experiments. The comparisons performed between optically determined porosity and cutting force profiles by means of standard random data analysis metrics (correlation coefficient, power spectral density, and coherence) revealed that the presence of a material discontinuity has a prevalent effect on cutting force variation in the case of micromilling processes characterized by (1) less intensive machining regimes and (2) larger cutter/workpiece engagement zones. The proposed methodology is useful in selection of the investigative approach to be taken in assessment of the micromachining-related behavior of highly porous foams subjected to micromilling operations.
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