A. Tiziani

Influence of matrix hardness on the dry sliding behaviour of 20 vol.% Al2O3-particulate-reinforced 6061 Al metal matrix composite

Abstract In the present investigation the wear behaviour of the 6061 Al alloy reinforced with 20 vol. % Al 2 O 3 particles dry sliding against a tool steel counterface was studied as a function of load and with reference to different values of the matrix hardness, obtained by submitting the extruded composite to thermal and forging treatments. The obtained wear rates were interpreted on the basis of the analysis of the surface and subsurface damage to the composite due to sliding. If the hardness of the materials is increased by a treatment, their wear rate also increases, because of the occurrence of subsurface softening and the formation of surface mixed scales prone to leaving the tribological system. On the other hand, the as-extruded composite is characterized by a lower matrix hardness (and thus a higher ductility) which prevents the conditions for subsurface softening and formation of the mixed scale from being reached. In this case a transfer layer mainly consisting of iron oxides then forms and protects the composite, thus reducing its wear rate.

The influence of phase transformations on residual stresses induced by the welding process—3D and 2D numerical models

In this work, a numerical study of laser beam welding of steel was performed. In particular, phase transformation effects were considered, which consist mainly of volume change and transformation plasticity. Thanks to the possibilities of numerical modelling, additional analyses were performed (a) without taking into account phase transformations and (b) considering only the transformation plasticity phenomenon.The aim of this study was to examine the influence of phase transformation on the residual stress induced by the welding process, by comparing the results obtained with the described differences in the analyses. Finally, the residual stress field computed by the three-dimensional (3D) model was compared with the one computed by a two-dimensional (2D) model in order to estimate the grade of reliability of the more efficient 2D analyses, also in the presence of phase transformations. It was found that both volume changes due to phase transformations and transformation plasticity have a great influence on the residual stress induced by the welding process. 2D numerical models can be used with good accuracy instead of 3D models, if the in-plane stresses are of primary interest. All analyses in this investigation were performed with the finite element code SYSWELD®.

AZCS refractories prepared from gels

Abstract The material of chemical composition 1% Na2O, 28% Cr2O3, 28% Al2O3, 15% SiO2 and 28% ZrO2 was obtained by the gel route; this product is usually prepared by electrocasting and it is a well recognized refractory. The starting species dissolved in methanol/formic acid form the gel upon hydrolysis; this, on heating to 1500°C, forms a product of tetragonal and monocline ZrO2, ZrSiO4, Al2O3/Cr2O3 solid solution and vitreous phase. The evolution from gel to the final material was followed by X-ray analysis and DTA experiments: the separation and modification of the crystalline phases as a function of the temperature is demonstrated and discussed. Chemical analysis and scanning electron microscopy results allow a complete comparison to be made with the same composition obtained by electrocasting.

Dental Implants of Complex Form Coated by Nanostructered TiO2 Thin Films via MOCVD

This paper concerns with the study of an innovative process to be employed for the covering of dental implants with the aim to improve their characteristics. Dental implants were covered with TiO 2 that permits to increase the biocompatibility and the speed of osteointegration, which is the speed at which the bone begins to re-grow in direct contact with the implant. Good-quality, polycrystalline, extremely uniform TiO 2 thin films were deposited on glass and titanium (dental implants) by employing a hot wall LP-MOCVD (low pressure metal-organic chemical vapour deposition) reactor in 350-430°C range and using titaniumtetraisopropoxide Ti(i-OPr) 4 as precursor. The films resulted anatase oriented in (200) direction, nanostructured with uniform crystallite dimension (50-90 nm). The purpose of this paper is either to show the importance of using simultaneously various theoretical and experimental approaches in the investigation of MOCVD processes, and to obtain very uniform thin layers on substrates of complex form. A study of the growth rate was carried out in order to derive the kinetic control regimes and the corresponding kinetic laws. It was therefore possible to find the best conditions for high uniformity in the reactor.

Methodologies and experimental validations of welding process numerical simulation

The development of numerical codes for the simulation of metallurgical processes, such as welding, assumes a great importance in the industrial and research field. The prediction of residual stresses, deformations, phases proportion and temperature can be a fundamental step for a good planning of a welding operation. Due to the complexity of the problem, such numerical simulations require specialised tools and qualifications. The aim of this work is to summarise the methodologies currently used in the numerical simulation of welding process, with a particular attention paid to the strategies used to reduce the computational time. Some examples, mainly concerning high power source welding process will be described; numerical simulations, performed with the code SYSWELD and experimental results will be compared and discussed.

Metallurgical and mechanical characterization of electron beam welded DP600 steel joints

Several new commercial advanced high-strength steels exhibit high strength and enhanced formability. These materials have the potential to affect cost and weight saving while improving performance. However, welding, by modifying the microstructure of the steel, has in general a detrimental effect on the mechanical properties of structural components. If high power density technologies are used, the result is that the mechanical properties of such kind of joints can be improved. This article presents a metallurgical and mechanical characterization of electron beam welded joints in advanced high-strength steel DP600. The experimental analysis was supported by a thermal numerical model obtained through the Sysweld® code. Results show that mechanical properties of the electron beam welded joints are comparable with those of parent metal both in terms of static strength and ductility.

CO2 laser welding of aluminium matrix composites

The development of suitable welding techniques is a key-point for the use of Metal Matrix Composites in engineering applications. High energy welding techniques, such as laser beam, may have both positive and negative effects (high cooling rates, so minimum time is available for matrix-reinforcement reactions, but also high temperature, with strong beam-reinforcement interactions). This work is focussed on the set up of CO2 laser welding processes for discontinuously reinforced Aluminium Matrix Composites. Both unalloyed Al and an Al-Si casting alloy (A354) have been used as matrices, reinforced by particulate Silicon Carbide (5 and 15%vol). The composites have been submitted to penetration tests (traverse speed a 3.7 m/min, laser beam power a 4 kW). Two kinds of filler wires (ER4043 and ER4047) have been also employed. Microstructural characterisations have been carried out on the beads, leading, together with an analytical thermal model of the process, to the definition of guidelines for obtaining good welded joints on such composites.

Effects of aging on impact behaviour of different stainless steel weldments

Abstract Instrumented impact testing was used to investigate the effects of aging on the impact deformation and fracture of different weldments of an AISI type 316L stainless steel plate and a superduplex stainless steel plate. Aging at 800C of metal inert gas MIG and laser beam LB weldments of the 316L stainless steel plate produced a precipitation of sigma phase, which was more intense in the MIG weldments. Correspondingly, the MIG weldments showed a noticeable decrease in impact energy and in impact yield and maximum loads as the aging time was increased. The LB weldments, on the other hand, showed an initial decrease in absorbed energy followed by a noticeable increase, because of the appearance of delaminations during the fracture event. Aging at 900C of the parent metal and of LB and plasma arc PA weldments of the superduplex stainless steel plate produced a precipitation of sigma phase which was much more intense in the parent metal. Both the parent metal and the weldments showed a noticeable decrease in impact energy and in impact yield and maximum loads as the aging time was increased. These weldments were found to be more sensitive to the precipitation of sigma phase than the austenitic stainless steel weldments.

Correlation between microstructural and SIMS analyses of cast irons inoculated with CG alloy

A microstructural and SIMS (secondary ion mass spectrometry) characterization has been performed on cast iron castings, inoculated with the CG alloy and with different titanium and calcium contents. The influence of the above elements on the graphite morphology has been studied. Regarding titanium, its effect results to be the combination of two concomitant mechanisms: the reaction with oxygen and sulphur, and the subsequent reduction of their amount in the matrix; and the reaction with carbon to form carbides, which act as inoculating of the austenite during the solidification. Titanium content has to be optimized in order to avoid the formation of undesirable forms of graphite and the excessive precipitation of carbides (or carbonitrides), which induce brittleness in the material. The main effect of calcium, besides the well known inoculating action for graphite, is to reduce the oxygen and sulphur amounts in the solidifying liquid.The microanalytical characterization has been performed by SIMS, which appears to be very helpful in studying the segregations in the metallic materials and in particular in the cast irons, where the chemical segregations have a strong influence even on the technological properties.

Fracture morphology in compacted graphite cast iron

ABSTRACT Results of an investigation on the fracture morphology of as cast ferritic-pearlitic iron with compacted graphite are presented. Materials were characterized by tensile and impact tests. Generally the crack begins in the compacted graphite particles. The fracture surface of the matrix has mainly a ductile morphology. In the iron inoculated by higher-titanium alloy crack propagation in the matrix occurs more frequently by a brittle cleavage mechanism. In this case brittle particles of TiN were detected in the initiation areas of the cleavage. The graphite and matrix fracture modes appear to be independent of strain rate.