Materials Science Forum Vol. 879

Abstract: The chemical heterogeneities of alloying elements were evaluated in the hot top plus the top of a 40-ton ingot of as-cast high strength low alloy steel. The chemical compositions of small samples, taken from a slice cut along the longitudinal axis of the ingot, were obtained using mass spectroscopy. The chemical results were used to construct the chemical heterogeneity maps of C, Mn, Ni, Cr and Mo in the entire slice. The analyses of the different maps indicate the existence of positive segregation for all segregated elements except Ni where no segregation was observed. The most important macrosegregation was revealed in the centerline of the ingot. Carbon presents the highest degree of segregation whereas Mo presents the lowest one. In term of homogeneity degrees, Mn, Ni, Cr and Mo present better homogeneity than C whether in the top of the ingot or in the hot top.

Abstract: As-cast low-carbon low-niobium steels fabricated by direct strip casting (DSC) were treated by simulated coiling in the lab. Coiling temperatures were carefully selected: (1) 900 ̊C (in the austenite); (2) 700 ̊C (during the austenite-to-ferrite transformation); (3) 650 ̊C (in the ferrite). Optical microscopy and transmission electron microscopy were used to examine the microstructure constituents and the precipitates. Mechanical properties were evaluated by Vickers macrohardness measurements. The results show that coiling treatment has a strong influence on the final microstructure and mechanical properties, thus highlighting the necessity to carefully design the coiling treatment. In addition, the differences in hardness for the three coiling temperatures derive from a complex combination of different strengthening mechanisms.

Abstract: Metal matrix nanocomposites (MMNCs) have excited great interest in recent years, due to their very good properties. In this work, an efficient process by combining high-energy ball milling (HBM) with ultrasonic vibration (UV) was employed to prepare MMNCs. The composite granules containing nanoSiC_P were produced by milling the nanoSiC and Al powders, and then were remelted in the matrix melt and treated by UV to prepare MMNCs. The MMNCs were finally formed by squeeze casting. The results indicate that globular nanoSiC_P/Al compound granules with diameters between 1.5-2mm are obtained by dry HBM, and the nanoSiC particles are uniformly distributed in the granules. After remelting, nanoSiC particles in compound granules release in the matrix melt and are uniformly dispersed by UV within 2min. In MMNCs, nanoSiC particles concentrate mainly around eutectic phases, but no agglomeration is observed. The tensile strength of the MMNCs with 1wt.% nanoSiC_P is increased by 19%, compared to the matrix A356 alloy.

Abstract: This paper describes the application of Cold Spray to the deposition of a diamond grade pre-coated with Cu and Ni. Diamond powders are therefore used as the sole feedstock without the addition of separate binders (ductile phases) in cold spraying. The experimental results showed that it was possible to manufacture thick metal-diamond composite coatings with high diamond fraction in the coating but without phase change or graphitization. Results presented in this paper demonstrate the feasibility of a new methodology for the deposition of metal-diamond/ceramic composite coating with the cold spray technique.

Abstract: Al-Li 2198-T8 alloy sheet was processed by friction spot welding (FSpW). The microstructures and tensile properties of FSpW 2198-T8 alloy were studied by means of optical microscopy (OM), transmission electron microscopy (TEM), scanning electron microscopy (SEM), X-ray diffraction (XRD) and tensile testing. The results show that the grain size of Al-Li 2198-T8 alloy processed by FSpW was refined with the strengthening precipitates dissolved into Al matrix and dislocation density decreased. Hence, the plasticityin thermo-mechanical coupling zone (TMCZ) of FSpW 2198-T8 alloy was improved, while the yield strength (YS) of TMCZ zone was lower than the original material (239 MPa <470 MPa). In addition, the strengthening mechanisms of different zones of FSpW 2198-T8 alloy were estimated.

Abstract: A typical multiscale simulation consists of numerous fine scale models, usually one for each computational point of a coarse scale model. One of possible ways of limiting computing power requirements is replacing fine scale models with some simplified and speeded up ersatz ones. In this paper, the authors attempt to develop a metamodel, replacing direct thermodynamic computations of precipitation kinetic with an advanced approximating model. MatCalc simulator has been used for thermodynamic modelling of precipitation kinetic. Typical heat treatment of P91 steel grade was examined. Selected variables were chosen to be modelled with approximating models. Several attempts with various approximation variants (interpolation algorithms and Artificial Neural Networks) have been investigated and its comparison is included in the paper.

Abstract: Many pharmaceutical industries all around the world are facing the problem of dust mobilization during the productive process of medicines. This mobilization can be dangerous for the safety of the operators working in the factory and for the safety of the factory itself. It is therefore necessary to develop predictive models to simulate and forecast dust mobilization. The Quantum Electronics and Plasma Physics (QEP) Research Group of the University of Rome Tor Vergata has developed a facility to experimentally replicate dust mobilization in different critical conditions in an enclosed environment. The measurements performed with diagnostics available in the facility, provide the boundary conditions to run numerical simulations and to validate mobilization models . Even if the initial field of application of this novel facility is dust mobilization is nuclear fusion, the methodology developed can be used for the medicine industry, for the agribusiness and others. The authors will present the experimental and numerical results discussing new applications.

Abstract: Hydrogen in aluminum has been known to be the cause of blister and pore. Some aluminum alloy is susceptible to stress corrosion cracking, which is based on intergranular cracking arisen from hydrogen embrittlement. The behavior of hydrogen in aluminum has not been fully understood yet. Hydrogen gas plasma enables to introduce high hydrogen concentrations into specimen without Al (OH)₃ layer on the surface of specimen. In this paper, we have investigated the behavior of hydrogen in a plasma charged aluminum by means of thermal desorption spectroscopy, a method to evaluate the amount and trap states of hydrogen. Cold-rolled pure aluminum were annealed, electro-polished and charged with hydrogen gas plasma. Immediately after hydrogen gas plasma charging, TDS tests were performed under ultra-high vacuum. The hydrogen desorption spectrums obtained by TDS tests had three peaks corresponding to the co-diffusion of hydrogen-vacancy pair, dislocation and pore. Compared to a sample without charging, in a plasma charged sample, the amount of hydrogen trapped in vacancies especially increased.

Abstract: The aim of the present study is to identify the ternary eutectic Mo-Si-B composition to produce directionally solidified materials, which are expected to have excellent high-temperature properties due to the well-defined microstructure. Different alloy compositions in the respective primary solidification areas of the phases were chosen to investigate the microstructural evolution. The results were compared to thermodynamic calculations of the liquidus projection and isopleth phase diagrams using the software FactSageTM. By carrying out these experiments the eutectic point was found to have a nominal composition of Mo-17.5Si-8B (at.%). In the next step, the eutectic alloy was directionally solidified by a zone melting (ZM) process. The evolution of a typical eutectic microstructure due to the growth of lamella-like structures is shown by microstructural investigations. Furthermore, we present a eutectic phase field model for the eutectic Mo-Si-B alloy. The equilibrium interface geometries and interface mobility were calculated using an isotropic model. The results are shown to be in an adequate conformity with the experimental observations.

Abstract: Friction stir welding (FSW) is a solid state joining process in which metals are joined together using frictional heat and severe plastic deformation. The heating and the mixing of the metals is performed using a hardened tool with a shoulder and pin. FSW of lightweight metal alloy Al6061 has been carried out in the present study. For welding aluminum the parameters used were a constant tool rotation speed of 1600 rpm and varying tool translation speeds of 250, 500, 750 and 1000 mm/min. The welded coupons were characterized for microstructural observations and mechanical properties such as tensile and Charpy impact properties. The tensile and impact properties were studied at two different temperature namely, room temperature (RT) and 300°C. The FS welded aluminum specimens showed 86% – 98% tensile yield strength, in comparison to the base material at RT. At 300°C, the yield strength was observed to be 85% to 93% of the base material value. For the impact properties, the Al specimens showed 60% – 140% specific impact energy, in comparison to their respective base materials. Based on the mechanical properties and microstructural examination, the optimal weld parameter was identified as 1600 rpm and 250 mm/min which is dependent on the tool pin and shoulder design utilized in the study.