Development and characterisation of direct laser sintering multicomponent Cu based metal powder

Abstract

Recent advances in direct metal laser sintering (DMLS) have improved this technique considerably; however, it still remains limited in terms of material versatility and controllability of laser processing. In the present work, a multicomponent Cu based metal powder, which consisted of a mixture of Cu, Cu–10Sn and Cu–8·4P powder, was developed for DMLS. Sound sintering activities and high densification response were obtained by optimising the powder characteristics and manipulating the processing conditions. Investigations on the microstructural evolution in the laser sintered powder show that liquid phase sintering with partial or complete melting of the binder (Cu–10Sn), but non-melting of the cores of structural metal (Cu) acts as the feasible mechanism of particle bonding. The additive phosphorus acts as a fluxing agent to protect the Cu particles from oxidation and shows a concentration along grain boundaries owing to the low solubility of P in Cu and the short thermal cycle of laser sintering. A directionally solidified microstructure consisting of significantly refined grains is formed, which may be ascribed to laser induced non-equilibrium effects such as high temperature gradient and rapid solidification.     

Effect of heat treatment on compressive properties of open cell Al/Al2O3 composite foams

Abstract

The sintering and dissolution process was used to produce open cell Al/Al₂O₃ composite foams with a relative density of 0·25–0·40 and a pore size of 112–400 μm. The compressive properties of as fabricated and T6 heat treated Al/Al₂O₃ composite foams were investigated. After T6 heat treatment, the yield strength of the open cell Al/Al₂O₃ composite foams is increased relative to the untreated material. T6 heat treatment gives rise to a mean 36% increase in the compression yield strength of the open cell Al/Al₂O₃ composite foams. The yield strength of as fabricated and T6 heat treated Al/Al₂O₃ composite foams shows a significant dependence on the relative density and also exhibits a distinct dependence on the pore size. After T6 heat treatment, the influence of the relative density and pore size on the compressive behaviour of Al/Al₂O₃ composite foams becomes more distinct.     

A basic study for a robotic transfer aid system based on human motion analysis

A metal hydride (MH) actuator uses the reversible reaction between heat energy and mechanical energy of a hydrogen-absorbing alloy and moves smoothly without any noise. Moreover, the actuator is compact, light and has adequate compliance. In this paper, some experiments concerning human motion patterns were carried out in order to adopt the MH actuator to the transfer aid system. The results were evaluated from a biomedical engineering viewpoint in order to obtain the optimal parameters for transferring the disabled from a bed to a wheelchair and vice versa (transfer aid). Sagittal plane kinematics, surface electromyographic signals (EMG) obtained from lower limbs, ground reaction forces and foot pressure distribution were measured using a three-dimensional motion analyzer. From the experimental results, it was found that the motion of the transfer aid was smooth when the initial angle of the ankle was 70°. The best initial angle of the trunk was also found to be about 45° while standing from a bed. However, the initial angle of the trunk was best at 60° when a person was in the process of sitting down on a wheelchair. Furthermore, a knee pad which supports the lower limbs requires flexibility in the range of 0.1-0.2 kgf/mm.     

Recent developments in machine translation

This paper describes developments in the area of machine translation (MT). First, the paper gives an overview of developments in Germany in general; then, special problems are discussed. The system taken as an example is METAL (Machine Translation and Analysis of Natural Language), where recent development work has centered around two main topics. (i) Efforts have been made to make the system really multilingual. The German-to-English prototype had to be expanded, some system components had to be readjusted, and additional problems had to be solved. Currently, analysis and synthesis components for German, English, French, Spanish, and Dutch are under development. All these languages use a common system kernel and a standard interface structure. (ii) The system had to be made user-friendly. This was an even more important task as, up to now, MT systems have not been well accepted by users. METAL tries to be more realistic, and also tries to support the main user interfaces in a much better way than has been done before. This is based on the conviction that there are several parameters which determine the real success of an MT system. It is not just translation quality which is decisive, it is also the integration of an MT system into the whole process of preparing and translating documents.Gregor Thurmair is head of the Linguistics Department at Siemens Nixdorf Information Systems and project leader of the machine translation group, METAL. He is involved in projects in information retrieval (morphological analysis), speech understanding (parsing, semantics) and machine translation (METAL system). He has presented papers on morphology, semantics in speech understanding, transfer problems in MT, and grammar checking.     

Electrochemical behaviour of 254SMO stainless steel in comparison with 316L stainless steel and Hastelloy C276 in HCl media

Abstract

The electrochemical behaviour of 254SMO stainless steel in HCl solutions has been studied and compared with that observed for other alloys using open circuit potential measurements, potentiodynamic polarisation curves, and SEM analysis techniques. 254SMO does not display pitting corrosion at room temperature in HCl solutions having concentrations ≤5·0M. Its performance is comparable with that of Hastelloy C276 in HCl media in solutions of strength ≤1·0M. The critical pitting temperature for 254SMO exceeds 76°C in 3M HCl solutions.     

Natural and artificial aging response of semisolid metal processed Al–Si–Mg alloy A356

Abstract

International standards for aluminium alloys often permit significant fluctuations in the content of alloying elements. This allows metal suppliers more freedom in preparing these alloys. It is shown that the magnesium content of semisolid metal processed Al–Si–Mg alloy A356 has a significant influence on the natural and artificial aging behaviour of the alloy. Furthermore, natural aging before artificial aging causes the time to peak hardness (T6) to be longer compared to the time when only artificial aging is used. The optimum quality index in this study was obtained using a short solution heat treatment of 1 h at 540°C, no natural aging and artificial aging at 180°C for 1 h. An increase in the magnesium content of the alloy resulted in an increase in the quality index for all the T6 heat treatment cycles studied.     

Effect of hydrogen on ductility of stable austenitic steel

Abstract

Straining in hydrogen at a pressure of2 bar, or in air after thermal charging in high pressure hydrogen, produces only minimal loss in ductility at particular strain rates for as received or solution treated samples of a stable austenitic stainless steel. However, cathodically charging at 80°C introduces a significant embrittlement that is associated with theformation during straining of brittle surface cracks whose depth increases with charging time. The latter effect is related to the depth of penetration of hydrogen and the transformation to martensite that is induced by the combined effect of hydrogen and stress. Removal of the hydrogen and the associated α martensite by subsequent vacuum annealing produces recovery of ductility. The application of a sensitising treatment before charging or testing in hydrogen gives results that are not significantly different.     

Current PM literature for engineers and users

Abstract

Fully dense composite materials of M3/2 tool steel reinforced with 5 and 8 vol.-% of niobium carbide were developed using the powder metallurgy route. The consolidated materials exhibited a fine and uniform microstructure consisting of a fine dispersion of carbide particles in a matrix of ferrite/martensite. The 0·2% yield strength up to 600°C was evaluated in compression. The slight increase observed after reinforcement by NbC particles in as hipped materials suggests that the martensite has the major contribution to the strength. After tempering, the reinforced materials showed a moderate increase in yield stress at room temperature with respect to the unreinforced M3/2. This increase is attributed to reinforcement mechanisms associated with niobium carbide. PM/1175     

The humping phenomenon during high speed gas metal arc welding

Abstract

A commonly observed welding defect that characteristically occurs at high welding speeds is the periodic undulation of the weld bead profile, also known as humping. The occurrence of humping limits the range of usable welding speeds in most fusion welding processes and prevents further increases in productivity in a welding operation. At the present time, the physical mechanisms responsible for humping are not well understood. Thus, it is difficult to know how to suppress humping in order to achieve higher welding speeds. The objectives of this study were to identify and experimentally validate the physical mechanisms responsible for the humping phenomenon during high speed gas metal arc (GMA) welding of plain carbon steel. A LaserStrobe video imaging system was used to obtain video images of typical sequences of events during the formation of a hump. Based on these recorded video images, the strong momentum of the backward flow of molten metal in the weld pool that typically occurred during high speed welding was identified as the major factor responsible for the initiation of humping. Experiments with different process variables affecting the backward flow of molten weld metal were used to validate this hypothesis. These process variables included welding speed, welding position and shielding gas composition. The use of downhill welding positions and reactive shielding gases was found to suppress humping and to allow higher welding speeds by reducing the momentum of the backward flow of molten metal in the weld pool. This would suggest that any process variables or welding techniques that can dissipate or reduce the momentum of the backward flow of molten metal in the weld pool will facilitate higher welding speeds and productivity.     

Size distribution of oxides and toughness of steel weld metals

Abstract

Experiments have been conducted to modify the size distribution of oxide particles in steel weld metals which have predominantly martensitic microstructures. It is found that the toughness improves significantly when the particles are refined, even when the total concentration of oxygen is unchanged in the process.