Advanced Materials Research Vol. 445

Abstract: Dense precipitation of various intermetallic compounds is a common feature in the microstructure of Al-Si piston alloys. In this investigation, microstructure of LM13 alloy and three high Cu-containing Al-Si piston alloys with different amounts of Ni, Fe, and Mn were studied by means of optical microscopy (OM) and scanning electron microscopy (SEM). Chemical composition of the phases was determined by using energy dispersive X-ray analysis (EDX). The precipitation of the phases was studied through thermal analysis of the solidifying samples. Also, tensile properties and hardness of the samples were measured. The results showed that the various intermetallics such as Al₁₂(FeMn)₃Si₂, Al₃Ni, Al₉FeNi, and Al₃CuNi precipitated during the solidification. The high Cu-containing alloy with optimum levels of Ni (1.8 wt.%), Fe (0.75 wt.%), and Mn (0.3 wt.%) has the highest tensile strength (250 MPa) and hardness (110 BHN) among the other alloys.

Abstract: A simplified thermomechanical growth instability model for early stages of solidification porcess is obtained. The model consists of a thermally deformable mold of finite thickness. The inclusion of the mold into the model introduces a parameter known as distortivity which plays an important role in the thermoelastic instability problems. The results show that the perturbation in contact pressure tends asymptotically to a maximum value at larger values of time. The magnitude of the contact pressure perturbation decreases as the thickness of the mold increases whereas it increases as the distortivity of the mold increases.

Abstract: Biodegradable implant is an alternative to metallic implant and has the advantage of not being necessary to remove once the fracture has healed. Magnesium is particularly desirable since it is biocompatible and has a modulus of elasticity closer to bone. In addition, it shows ability to biodegrade in situ, when used as an implant material. In this research, different percentages of calcium were added to magnesium during melting of the alloy. A selected alloy was forged at different parameters. Both as cast and forged alloys were subjected to polarization test performed in Kokubo simulated body fluid. Immersion test in the fluid was conducted for 96 hours to investigate the formation, growth and morphology of the hydroxyapatite on the surface of the alloys. The results showed that similar electrochemical behaviour took place in the alloys regardless of the calcium content. However, an increase in corrosion rate was observed with increasing calcium content. It was also observed that forging process decreased the corrosion resistance of the alloy. Furthermore, increasing calcium content accelerated the growth of bone-like apatite in the alloy.

Abstract: A single roll caster equipped with a scraper was devised to cast the strip with sound free solidified surface. A property of this single caster is equipment of the scraper. Semisolid metal on the free solidified surface was scribed by a scraper and the surface became flat. Center line segregation did not occur as the strip was solidified from single side. The AA5182 aluminum alloy could be cast at speeds up to 40m/min. Thickness was about 3mm. Pressure of the unit width from the scraper was ranging from 0.1N/mm to 1.0N/mm, and these pressure was enough to make the free solidified surface flat. Roll cast strip could be cold rolled down to 1mm. There was not difference between roll contact surface and the free solidified surface after cold rolling by a visual examination. Result of tension test of the roll cast strip was as same as that of D.C. cast and rolled strip. Deep drawing test was operated at the conditions both of the roll contact surface and the free solidified surface was outside. LDR (limited drawing ratio) was same at both conditions and they were 1.8. Thickness of the strip was controlled by the roll speed, solidification length (length of the melt pool) and pressure of the scraper. The single roll caster is simpler than a twin roll caster. Rigidity for rolling was not needed for the single roll caster. Cost of the roll is half comparing a twin roll caster. The equipment cost of the single roll caster is more economy than that of the twin roll caster.

Abstract: Nowadays polymer based nanocomposites are very interesting to manufacture products of less weight and higher mechanical properties and specific performance depending on the morphology of nanoscaled reinforcement. Most of these potential improvements are focused to the challenges newer products require like HEV (hybrid or electrical vehicles) for example. The development of these new products requires the full characterization of the rheological and mechanical behavior of the materials and the correct preparation of the raw material for further processing. As an example two nanocomposite blends were prepared letting down a masterbach of PA6+30% HNT (Halloysite nanotubes) to 3% and 6% of HNT content in a PA6 matrix of (BADADUR). The letting down process was developed in an extrusion-compounding machine (COPERION ZSK 26) and the rheological behavior was determined in a capillar rheometer obtaining the viscosity curves of the material needed for injection molding simulation. The products obtained were used for injection molding of test specimens in an electrical injection machine (JSW EL II 85). In addition, the letting down process was done directly in the injection machine in order to establish the relevance of the previous extrusion process. The probes obtained were analyzed by DSC and FTIR to determine the functional groups of the resultant product and SEM and TEM to determine the quality of the dispersion of the nanotubes. The probes were finally tested to determine its stiffness and tensile properties. The results showed the feasibility to develop parts made of nanocomposite with improved performance with scaled industry equipment with natural reinforcements..

Abstract: Microcellular foaming of injected plastics offers the possibility to manufacture parts with reductions in costs and weight if compared with conventional injection molding. For this reason there is an increasing interest in challenging applications such as HEV (hybrid and electrical vehicles) and lightweight material applications in general. Complexity of microcellular injection molding is very high because the final properties of the material obtained depend largely on the processing conditions and these in turn unalterable factors such as mold design and manufacturing. The shrinkage of the molded part must be applied as an oversize of the mold cavity in the design phase. Shrinkage of a microcellular foam depends on the reduction of foam density. Moreover, the piece is designed to get a mechanical performance and meet the dimensional tolerances. Knowing that the reduction of foam density implies a reduction of the mechanical properties and influences the final piece dimensions the conclusion is that the microcellular injection process has a very small process window to fit all these factors. This research focuses on two objectives. First is the variation of post-molding shrinkage in terms of reduction of weight to determine the process window. Second is the determination of mechanical properties which do not show a proportional reduction but exponentially with weight reduction components. The results obtained with a 750 Tons. injection moulding machine equipped with a MuCell plastication unit and a large spiral mold have shown small variations in the dimensions for a predetermined process window and smaller reduction of mechanical properties with weight reductions for 20% talc filled polypropylene. The goal of this applied research is that all experiments have been developed with scaled-industry tools (large injection molding machine, Mucell unit and mold and test parts) comparing with conventional injection molding.

Abstract: The mold-shell interface plays a very important role during the earliest stages of metal casting processes. Heat is extracted from the molten metal through this interface resulting in the growth of the shell. Small spatial variations in heat extraction can lead to shell morphological instability where certain regions of the solidification front grow preferentially over others. This may cause cracks in the shell which can greatly reduce the integrity of the final cast product. Mold coating at the mold-shell interface is one of the most important factors controlling the heat transfer and, hence, it has very important role on the solidification rate and development of microstructure. In this paper, a linear perturbation method is used to solve a two-dimensional heat conduction problem in which a liquid, becomes solidified by heat transfer to a planar mold of finite thickness. The influence of physical parameters such as the coating material thickness, conductivity and thermal contact resistance on the growth of solidified shell thickness is investigated. The present work can form the thermal part of a subsequent investigation of related thermo elastic stress problems.

Abstract: In this study, usability of boron as an alloy element in gray cast iron and its effect to abrasive wear behaviour were investigated. Pin-on-drum wear tests at the room temperature carried out for seven low nickels alloyed gray cast irons with different boron addition. The mass losses, hardness values and microstructures for gray cast iron specimens with different boron alloyed were investigated for determining wear behaviour. The pin for the wear tests was manufactured from X210Cr12 cold work tool steel with material number of 1.2080. Abrasive pin-on-drum wear tests were carried out at a 165 N constant load and two different sliding speeds that are closely related to the appropriate operating conditions in rolling mills. The experimental studies have shown that wear rate decrease with increasing boron amount in chemical composition of the alloy and the wear rate at high sliding speed has decreased more rapidly than the rate at the low sliding speed with increasing boron amount.

Abstract: A simple one dimensional model has been introduced to investigate the morphological instability observed in many solidification processes. It is shown that the solidified shell material with higher thermal conductivity might result in planar shell growth, whereas the mold material with higher thermal conductivity may cause irregular growth of the shell which, generally, causes cracking near the surface, and the thicker mold causes faster growth of the shell, and the higher thermal contact resistance leads to faster growth of the shell.

Abstract: A simple experimental setup has been used to investigate the morphological instability observed in many solidification processes. It is shown that the mold with smooth surface exhibits prominent thickness nonuniformities. Periodic grooves in the mold surface results in more uniform contact along the mold-shell interface, and therefore leads more uniform growth of the shell. Experimental results for the moving solidification front have also been compared to the results of a simple theoretical model introduced in Part I, and a good agreement has been obtained.