Papers by Author: Vasile Danut Cojocaru

Abstract: Designing and processing of the alloplastic bone grafts represent one of the newest trends in bone tissue engineering, solving a lot of trauma problems of the patients simultaneously with technological and economical achievements. Recent developments in the field provide advantageous aspects concerning the internal architecture, mechanical properties and biocompatibility of the alloplastic bone grafts processed by the powder metallurgy (PM) technology. In this respect, the PM biocomposite materials based on hydroxyapatite powder particles reinforced by metallic or ceramic powders afford great benefits combining classic PM processes with different foaming techniques. The obtained biocomposites present special morphological and structural features matching the genuine bone tissue to be grafted, cortical respectively trabecular. This study focuses on the mechanical testing of the hydroxyapatite-based biocomposites reinforced by different foaming agents, specifically TiH2, CaCO3 and NH4HCO3 up to 25% mass. The overlapping of the obtained experimental results with those reported by the literature leads to the conclusion that the mechanical response of the PM biocomposites studied in this paper may play as a selection criteria to depict their application in hard tissue engineering.

Abstract: In the present days titanium and titanium alloys are extensively studied and used as biomaterials due to their biological, mechanical and physical properties. During last year’s special attention was paid to β-Titanium alloys due to their low elastic modulus. The present study investigates the twinning deformation features occurred during multi-pass cold-rolling processing of a biocompatible Ti-Nb-Zr-Fe alloy. Twinning deformation features were investigated using EBDS analysis by means of Inverse Pole Figures (IPF’s), Pole Figures (IP’s) and computed Schmid factor (SF’s) for the possible {332}<113> twinning variants.

Abstract: Equal-Channel Angular Pressing (ECAP) is a very interesting method for modifying microstructure in producing ultra-fine grained materials (UFG) and nanomaterials (NM). In the ECAP processing a sample is pressed through a die in which two channels of equal cross-section intersect at an angle of ϕ and an additional angle of ψ define the arc of curvature at the outer point of intersection of the two channels. ECAP is unique because significant cold work can be accomplished without reduction in the cross sectional area of the deformed workpiece. The key feature of ECAP processing is almost as a general rule the die design and manufacture as there is limited knowledge about ECAP and especially about ECAP die design, present paper offering an optimum solution to this problem, applied to multi-pass ECAP processing of an aluminum alloy. In this study a number of three ECAP dies with vertical configuration were considered, thereby combining the ease of execution with a relatively high maneuverability and posing minimum difficulties when extracting samples from the output channel. An optimum solution is also given by the present paper for designing and manufacturing the punches used for pressing the aluminum alloy through the three ECAP dies, minimizing this way the force required for plastic deformation and the power consumption and improving the ECAP process efficiency and stability.

Abstract: Severe plastic deformation (SPD) has received enormous interest over the last two decades as a method capable of producing fully dense and bulk ultra-fine grained (UFG) and nanocrystalline (NC) materials. Significant grain refinement obtained by SPD leads to improvement of mechanical, microstructural and physical properties. Compared to classical deformation processes, the big advantage of SPD manufacturing techniques, represented in particular by equal channel angular pressing (ECAP) is the lack of shape-change deformation and the consequent possibility to impart extremely large strain. In ECAP processing, the workpiece is pressed through a die in which two channels of equal cross-section intersect at an angle of ϕ and an additional angle of ψ define the arc of curvature at the outer point of intersection of the two channels. As a result of pressing, the sample theoretically deforms by simple shear and retains the same cross-sectional area to allow repeated pressings for several cycles. A commercial AlMgSi alloy was investigated in our study. The specimens were processed at room temperature for multiple passes, using three different ECAP dies. All samples (ECAP processed and as-received) were subjected to metallographic analysis and mechanical testing. Several correlations between the main processing parameters and the resulting microstructural aspect and mechanical features for the processed material were established. It was shown that severe plastic deformation by means of ECAP processing can be used in aluminum alloys microstructural design as an advanced tool for grain refinement in order to attain the desired microstructure and mechanical properties.

Abstract: In recent years a significant increase in new Ti-based biocompatible alloys (such as TiTaZr or TiTa-Nb-Zr) development was reported. Titanium and its alloys have been widely used in medicine since the 1960s because of their known biocompatibility, superior mechanical properties, low density and remarkable chemical stability. The present study investigates the microstructures and the mechanical properties of a Ti-29Nb-9Ta-10Zr (wt.%) alloys in order to investigate structural changes occurred during recrystallization treatment of 90% cold rolled Ti-29Nb-9Ta-10Zr (wt.%) alloy. The investigated alloy was fabricated by vacuum arc induction melting in levitation, using a FIVES CELES MP 25 furnace, starting from elemental components. Structural changes occurred during recrystallization treatment were investigated using X-ray diffraction, using a Philip PW 3710 diffractometer, in Bragg-Brentano θ-2θ geometry, with negligible instrumental broadening. Data concerning alloys component phases, average coherent crystallite size and internal average micro-strain was obtained.

Abstract: During the last decade the titanium alloys were extensively used in a variety of applications due to their good mechanical properties, high biocompatibility and corrosion resistance. β-type Ti alloys composed of Nb, Ta and Zr elements have received much attention, because they feature high specific strength, bio-corrosion resistance, no allergic problems and biocompatibility. A Ti-29Nb-9Ta-10Zr (wt.%) alloy was subjected to thermo-mechanical processing and testing. Two states were investigated: recrystallized and 80% cold-rolled. Data concerning phase structure and developed texture, expressed by Inverse Pole Figures (IPFs) and Orientation Distribution Functions (ODFs), was obtained and analyzed.

Abstract: A Ti-25Ta-25Nb β-type titanium alloy was subjected to thermo-mechanical processing and testing with the aim to observe the twinning deformation mechanism. Data concerning the evolution of twinning versus stress was obtained by SEM and micromechanical testing. Mechanical properties of the investigated alloy were also evaluated

Abstract: The design of novel Ti-based alloys for biomedical load-bearing implant application, such as hip or knee prostheses, aims at providing structural materials which are characterized by good corrosion stability in the human body, high fatigue resistance, high strength-to-weight ratio, good ductility, low elastic modulus, excellent wear resistance, low cytotoxicity and a negligible tendency to provoke allergic reactions. Low elastic modulus is required to be close to that of a human bone, in order to transfer the adequate mechanical stress to the surrounding bone. The Young’s modulus of biomaterials is desired to be equal to that of cortical bone because if the Young’s modulus of biomaterials is much greater than that of cortical bone, bone resorption occurs. Investigated samples were subjected to mechanical tests, having provided these mechanical properties: ultimate tensile strength (σ_UTS), yield strength (σ_YS), elongation to fracture (ε_f) and elastic modulus (E).