Effect of the Processing Parameters on the Porosity and Mechanical Behavior of Titanium Samples with Bimodal Microstructure Produced via Hot Pressing

Abstract

Commercially pure (c.p.) titanium grade IV with a bimodal microstructure is a promising material for biomedical implants. The influence of the processing parameters on the physical, microstructural, and mechanical properties was investigated. The bimodal microstructure was achieved from the blends of powder particles with different sizes, while the porous structure was obtained using the space-holder technique (50 vol.% of ammonium bicarbonate). Mechanically milled powders (10 and 20 h) were mixed in 50 wt.% or 75 wt.% with c.p. titanium. Four different mixtures of powders were precompacted via uniaxial cold pressing at 400 MPa. Then, the specimens were sintered at 750 °C via hot pressing in an argon gas atmosphere. The presence of a bimodal microstructure, comprised of small-grain regions separated by coarse-grain ones, was confirmed by optical and scanning electron microscopies. The samples with a bimodal microstructure exhibited an increase in the porosity compared with the commercially available pure Ti. In addition, the hardness was increased while the Young’s modulus was decreased in the specimens with 75 wt.% of the milled powders (20 h).

In-situ particle analysis with heterogeneous background: a machine learning approach

“…The characteristics of suspensions, emulsions, and mixtures often used in powder-based manufacturing depend largely on powder materials size, shape and distribution of powder materials. These factors control the porosity and mechanical properties of sintered 13 and additively manufactured samples 14 , improves flowability and wettability of yield stress fluids 15 , and influence thermal and electrical conductivity 16 . Therefore, characterizing the particle size distribution before and during manufacturing becomes important at advanced production and scientific facilities 17 – 19 .…”

Understanding the Synergetic Effects of Mechanical Milling and Hot Pressing on Bimodal Microstructure and Tribo-Mechanical Behavior in Porous Ti Structures

“…Previous research has shown that porosity percentages greater than 45 % are necessary to attain an elastic modulus comparable to that of human bone [11]. These porosity levels lead to a drastic reduction in the mechanical resistance, compromising implant reliability [12]. Nonetheless, there exist three main reinforcement strategies that can counteract the decrease in mechanical strength stemming from porosity: (i) composition design, (ii) processing technique, and (iii) thermal treatment [13].…”

“…A wide range of materials can be employed as space-holder particles, from ceramic particles, soluble salts, polymers, to metallic spheres [20]. For biomedical applications, the commonly materials for space-holders are NH 4 HCO 3 [5,12,[21][22][23], NaCl [20,21,[23][24][25], starch [26,27], Mg [1,28,29], PMMA [30,31], saccharose crystals [27,32], PVA [33], and carbamide [19,34,35], due to its easy removal ability by decomposition at relatively low temperatures or by means of dissolution process, generally in water [9,12]. According to the literature, the use of NaCl as a spacer has advantages, such as low cost, rapid dissolution in water, reduced metal etching during dissolution, as well as much lower toxicity due to residual content [36,37].…”

“…In addition to various other factors, extending the milling duration leads to a decrease in particle size. In order to avoid excessive grain growth and retain the fine grains which comprise the bimodal structures, hot consolidation techniques such as hot pressing sintering (HP) [12,42], hot isostatic pressing (HIP) [43], and field assisted sintering technique/spark plasma sintering (FAST/SPS) [40,44,45] are used due to their characteristics of rapid heat/cooling rates, low sintering temperatures, and short sintering times, compared to conventional sintering processes [12,46].…”

“…The application of pressure during pressure-assisted sintering processes, such as HP, has been shown to be effective in reducing matrix microporosity in the manufacture of Ti [47,48]. In prior research [12], an approach to producing porous structures that exhibit bimodal microstructure was carried out combining hot-pressing and the space-holder technique using NH 4 HCO 3 as the pore former. In that study, the final porosities were different depending on the amount of space-holder particles added (50 vol%), and Young's moduli were estimated from the measured values of porosity neglecting the effect of the bimodal structure.…”