Materials Science Forum Vols. 783-786

Abstract: The physical basis and a mathematical formulation of a softening model nicknamed Alsoft, accounting for the combined effect of recovery and recrystallization during annealing of heavily deformed aluminium alloys have been presented. The prediction power of the model is tested against experiments in terms of softening kinetics and final grain structure of selected Al-Mn-Fe-Si-model alloys with different as homogenized microchemistries in terms solid solution levels of Mn (potential of concurrent precipitation) and different constituent particle and dispersoid structures. It is demonstrated that good model predictions may be obtained for alloys and conditions which are not or too a limited extent influenced by particle drag effects and concurrent precipitation while conditions strongly affected by these effects are increasingly difficult to model and in the most extreme cases impossible with reasonable input model parameters.

Abstract: This paper describes the fracture assessment standards developed in Japan, where the Weibull stress is implemented to correct the CTOD toughness for constraint loss in structural components. ISO 27306 and WES 2808 (Japan Welding Engineering Society Standard) are presented. Discussion is given on the effects of strength mismatch and residual stress in welds.

Abstract: Bone microstructure is dominantly composed of anisotropic extracellular matrix (ECM) in which collagen fibers and epitaxially-oriented biological apatite (BAp) crystals are preferentially aligned depending on the bone anatomical position, resulting in exerting appropriate mechanical function. The regenerative bone in bony defects is however produced without the preferential alignment of collagen fibers and the c-axis of BAp crystals, and subsequently reproduced to recover toward intact alignment. Thus, it is necessary to produce the anisotropic bone-mimetic tissue for the quick recovery of original bone tissue and the related mechanical ability in the early stage of bone regeneration. Our group is focusing on the methodology for regulating the arrangement of bone cells, the following secretion of collagen and the self-assembled mineralization by oriented BAp crystallites. Cyclic stretching in vitro to bone cells, principal-stress loading in vivo on scaffolds, step formation by slip traces on Ti single crystal, surface modification by laser induced periodic surface structure (LIPSS), anisotropic collagen substrate with the different degree of orientation, etc. can dominate bone cell arrangement and lead to the construction of the oriented ECM similar to the bone tissue architecture. This suggests that stress/strain loading, surface topography and chemical anisotropy are useful to produce bone-like microstructure in order to promote the regeneration of anisotropic bone tissue and to understand the controlling parameters for anisotropic osteogenesis induction.

Abstract: Recently, low-modulus β-type titanium alloys have been the focus of considerable attention because of their high biocompatibility and their low moduli that make them effective for inhibiting bone atrophy and for enhancing bone remodeling. However, the biofunctinalities of titanium alloys, such as bone conductivity, blood compatibility, and soft tissue compatibility, are poor. Therefore, surface modification techniques such as bioactive ceramic surface modification and blood-and soft-tissue-compatible polymer surface modification are applied to titanium alloys. Hydroxyapatite (HAp) surface modification via metal organic chemical vapor deposition (MOCVD) and segmented polyurethane (SPU) surface modification via silane coupling treatment are effective techniques to add biofunctionalites to titanium alloys. HAp surface modification via MOCVD and SPU surface modification using three kinds of silane coupling agents on a low modulus beta-type titanium alloy, namely, TNTZ, are discussed. Moreover, the bonding strengths of HAp and SPU on the surface of TNTZ, which are important parameters, are also discussed.

Abstract: Low carbon bainitic steels are important in applications such as linepipe, and the details of the bainite microstructure control strength and toughness. The transformation of austenite to bainitic ferrite has been widely researched over the years, although recent use of electron backscatter diffraction techniques has provided opportunity to advance the characterization of various crystallographic aspects. In recent work, microstructures were characterized in a base steel containing 0.04 C and 1.7 Mn (wt. pct.) and two additional steels having modest carbon and manganese variations to influence the transformation behavior, with an interest in the MA (martensite-austenite) constituent and characteristics of the bainite developed at different transformation temperatures. Effects of austenite conditioning were also examined, as these steels contained an addition of 0.04 wt. pct. Nb. Microstructural details including crystallographic characteristics assessed using EBSD are presented, along with comments related to the implications of the results.

Abstract: Grain size determines to a large degree the mechanical properties of the friction stir processed (FSP) material. Developed in this work is a numerical (FEM) based-model for predicting values of the Zener-Hollomon parameter (Z-parameter) as function of input process parameters during friction stir processing of AZ31B. Prediction of Z values is desirable given that direct relations exist between the Z-parameter and the average grain size in the dynamically recrystallized zone (DRX). For this purpose, utilized in this work is a robust finite element model with a suitable constitutive equation and boundary conditions the results of which have been previously validated against published experimental data. A virtual test matrix constituting of 16 cases (4 spindle speed, N, x 4 feed, f) was run. Based on resulting state variables of strain rates and temperatures at a representative point within the stir zone, a statistically-validated power equation model was developed that relates Z-parameter values to input parameters of speed and feed. The results of the numerically developed power equation were validated against experimental results. This model can be readily used in future control frameworks to FSP produce AZ31B sheets of a predefined target grain size.

Abstract: Aluminum and its alloys are characterized by low density, high electrical and thermal conductivity, and good resistance to corrosion in certain media such as air. The mechanical strength alloy is achieved. The objective of the present research consist on studying the type of structure (columnar, equiaxed or with columnar to equiaxed transition, CET) using parameters of the solidification process and electrochemical parameters in Al, Cu and Al-Cu alloys with different concentrations. In order to obtain columnar, equiaxed and CET structures, the alloys were directionally solidified upwards in an experimental set up with a set of thermocouples in the samples which permit to determine the time dependent profiles during the process. The electrochemical studies of the samples were realized by using an electrochemical impedance spectroscopy (EIS) technique and potentio-dynamic polarization curves immersed in 3% NaCl solution at room temperature. In general, we observed that the susceptibility to corrosion of the different structures depends on the size of the secondary dendritic spacing and the proportion of Al2Cu phase and Al-rich phase.

Abstract: The main aim of this paper is to investigate the effects of different processing conditions on the behavior of a P/M (Powder Metallurgy) aluminum alloy with respect to the microstructure, fracture and mechanical properties. Moreover, the evolution of porosity as a consequence of pressing, sintering and ECAPing processes was investigated. A commercial Al-Mg-Si-Cu-Fe powder was used as material to be investigated. Different compacting pressures (400, 500, 600, 700 MPa) were applied. Specimens were dewaxed in a ventilated furnace at 400 °C for 60 min before sintering. Sintering was carried out in a vacuum furnace at 610 °C for 30 min. The specimens were ECAPed for 1 pass. The 2-dimensional quantitative image analysis was carried out by means of SEM and OM for the evaluation of the dimensional and morphological porosity characteristics. The detailed microstructure revealed the main features of sintering processes as well as secondary pores at the prior alloying particle sites. The tensile fracture surfaces in both studied processing condition (as-sintered and ECAP) show limited ductility, with fracture occurring on a plane normal to the tensile stress axis. Examination at higher magnifications revealed predominantly transparticle ductile features. In terms of mechanical properties, ECAP is almost doubling the tensile strength of the as-sintered materials

Abstract: The production of aluminum alloy components through sheet forming processes conducted at elevated temperatures is gaining more and more interest as it gives raise to the possibility of a significant enhancement of the metal formability characteristics, compared to room temperature forming. However, conventional forming processes at elevated temperatures on aluminum alloy sheets are usually carried out under superplastic forming regime conditions, which are too slow to be applicable to mass production typical of the automotive industry. The aim of the present study is to investigate the formability characteristics of AA6016 aluminum alloy sheets when deformed at elevated temperature, but in a range of strain rates higher than those usually applicable in superplastic forming. To this aim, uni-axial tensile tests were carried out to evaluate both the material ductility in terms of true strain at fracture as a function of the temperature and strain rate, and the alloy post-forming characteristics after testing. In such a way, the optimal forming conditions in terms of temperature, strain rate and microstructural features were identified.

Abstract: High thermal conductivity aluminum has special advantages for electronic packaging and thermal management applications because of the combination of excellent thermal conductivity and relatively low density. Recent development of new press-and-sinter aluminum materials with low levels of alloying that sinters to a high density yielding a high thermal conductivity approaching the theoretical value for pure aluminum. The sintered materials possess thermal conductivity (TC) exceeding 200 w/m-oK (typically 215 – 230 w/m-oK), which makes it unique, since cast and wrought aluminum materials typically fall below 175 w/m-oK. This allows the benefits of powdered metal for low cost manufacturing at high volumes of parts to be realized. This unique combination of low cost and high TC makes these materials an attractive alternative to higher TC materials such as copper. In addition, a metal matrix composites (MMCs) press and sinter approach to tailoring the coefficient of thermal expansion (CTE) can also be used.