New method to prepare iron particles with different morphologies: a way to get high green strength metal compacts

Abstract

Metaliron powders of well controlled size and morphology were synthesised by thermal decomposition under hydrogen of precipitated ferrous oxalates. Green compacts were prepared by uniaxial pressing of metal powders at 290 MPa. The bending green strengths of compacts were measured.

The precipitation of β-FeC₂O₄.2H₂O oxalate from ammonium oxalate gives rise to the formation of spherical particles by aggregation ofelongated grains. Thermal decomposition of this oxalate from 400 to 500°C under hydrogen permits metal iron particles with a rough surface to be obtained. Decomposition occurring above 500°C induces a smoothness of the particle surface. Metal particles synthesised at 500°C show both surface roughness and micrometer sized primary grains.This specific microstructure has allowed the highest value ofcompact green strength (31·7 MPa) to be obtained.

Acicular shaping of the β-FeC₂O₄.2H₂O particles precipitated from oxalic acid involves, after decomposition, an increase in the surface roughness and shape irregularity of the metal particles, owing to an entanglement of the elementary grains. An exceptional value (about 60 MPa) for the metal compact green strength was thus obtained for this type of powder.     

Effect of exciting current of high frequency electromagnetic field on initial solidification of steel during electromagnetic continuous casting

Abstract

A three-dimensional finite element model of the electromagnetic field and temperature field of electromagnetic continuous casting (EMCC) process was developed. The aim was to investigate the effects of induction heat of high frequency electromagnetic field on the early solidification process of molten steel in mould under various conditions of exciting current parameters. The results show that the induction heat has significant effects on the early solidification process, which appear as increasing the billet surface temperature, thinning the initial solidified shell and lowering the starting point of the initial solidification. The increases in exciting current frequency and density make the effects of induction heat on solidification process increase remarkably. Especially, with the exciting current frequency increase, the early solidification process and shell growth become non-uniform in billet circumferential direction. Furthermore, if the exciting current density exceeds a certain value, there occurs a high temperature region in the top of molten steel column, and then the initial solidification rate is greatly decreased. As a conclusion, the effects of induction heat on initial solidification process must be considered when the exciting current frequency and density are adjusted during the electromagnetic continuous casting process.     

Titanium–hydroxyapatite composite biomaterial for dental implants

Abstract

The objective of the present study was to investigate high velocity compaction of titanium powder and to prepare a dense composite biomaterial of titanium and hydroxyapatite with the purpose of forming dental components with improved early healing properties. A high purity titanium powder was compacted using high velocity compaction to study the density distribution. Then, a titanium–hydroxyapatite composite was prepared by mixing titanium powders and hydroxyapatite grains. Dental implant components were formed from the high velocity compacted specimens, exposing the hydroxyapatite grains at the component surface. The green density reached more than 98·5% after more than one impact. The composite was heated to 500°C, enough to bind the titanium grains, but to avoid observable reactions. Compacted pure titanium could be sintered to full density. The heated composite material reached 99% density, no reaction was observed between titanium and hydroxyapatite, and the composite material could be formed into dental implants.     

Multi-phase piezoelectric structures through co-extrusion

Abstract

A novel processing route has been developed which will allow electrode structures to be applied around and within structures of lead zirconate titanate (PZT) before sintering, without the use of expensive metal electrodes such as palladium or platinum. The processing methods which have been applied to obtain these results have involved viscous processing and co-extrusion. Deagglomerated pastes of PZT and different levels of silver (I) oxide have been formed and co-extruded into cylindrical sections to demonstrate the feasibility of the technology. The microstructural and functional properties of these structures are shown, and the potential for the technology to generate complex novel piezoelectric devices illustrated.     

The effects of surface texture on natural rubber/metal friction at high pressures

Abstract

A technique is described that allows the friction of rubber on metal at high pressures (up to 15 MPa) to be measured. A series of experiments were performed investigating the behaviour of rubber under these conditions and the effects that the track surface texture had. It was found that Thirion's law remained effective for describing the friction of rubber under these conditions. It was also observed that, at ~5 MPa, a crossover occurred and at higher pressures, the smooth tracks resulted in lower friction.     

Studies on rate dependent damage evolution in polymer blends in high strain rates

Abstract

The dynamic damage evolution and the damage modified constitutive response for polypropylene-polyamide polymer blends with different compatibilisers are experimentally investigated by a new technique, namely by combining the split Hopkinson pressure bar technique with the back propagation neural network. The results reveal that the damage evolution including the threshold condition for the beginning of damage evolution, are dependent on both strain and strain rate. Thus for viscoelastic materials, particularly for polymer blends or composites, mechanical behaviour with damage evolution should be evaluated in a wide range of strain rates.     

Application of novel heat treatments for aluminium high pressure die castings to industrially produced components

Abstract

High pressure die casting (HPDC) is widely used as a cost effective way to mass produce metal components that are required to have close dimensional tolerances and smooth surface finishes, accounting for ～50% of the aluminium castings produced worldwide. These components are not considered to be heat treatable by conventional means because the high temperatures involved with solution treatment cause surface blistering and dimensional instability. A new heat treatment procedure involving a truncated solution treatment at lower than conventional temperatures alleviates this problem and can significantly improve mechanical properties, in many cases, doubling the 0·2% proof strength after artificial (T6) aging. This may enable current HPDC parts to be redesigned to use less metal while still achieving the required performance. The cost of heat treatment can be easily offset by the reduction in metal content and productivity improvements which result in an overall lower cost of the part. The new process also creates opportunities to substitute for some other cast or wrought products with aluminium HPDC parts of lower weight and lower cost. Application of this heat treatment technology to a range of industrially produced HPDC components is discussed and the cost advantages are briefly considered.     

Influence of production process chain on grain size stability of a microalloyed 20CrMo5 case hardening steel

Abstract

Within the scope of a research project in cooperation with steelmakers and steel users, the whole production process chain of gear parts was investigated concerning the influence on the grain size stability. Three different case hardening steels were investigated in a basic and a microalloyed state. This paper discusses the results gained with a case hardening steel with a new alloying concept, called 20CrMo5. The grain size stabilities of four different industrially produced states of the microalloyed material are compared to the standard material produced in a standard production chain. As a result, it emerged that additional annealing treatments above the eutectoid temperature within the production process chain reduce the grain stability of microalloyed steels.     

Oxidation of nickel base alloys strengthened by different hardening effects

Abstract

Three nickel base alloys strengthened by different hardening effects were investigated by thermogravimetry in air under isothermal conditions. The alloys investigated were γ′-Ni₃ (Al, Ti)-hardening alloy 80A (75Ni, 21Cr, 2·5Al, 1·7Ti, DIN No. 2·4952),solid solution hardened alloy C22 (59Ni, 21Cr, 13Mo, 3·5 Fe, 2·8W, DIN No. 2·4602) and a new high nitrogen containing and nitride hardening alloy N (61Ni, 27Cr, 10W, 1·4Ti, 0.2N). Tests were conducted in air between 900 and 1100° C for 48 h. Parabolic oxidationrates were determined and the formation of the oxide layer was investigated by optical microscopy and SEM. Oxidation data showed that the hardening mechanism has almost no influence on the oxidation kinetics. All of the alloys investigated formed chromia layers. After initial transient stateoxidation, the kinetics followed a parabolic law. Alloy 80A had the highest oxidation rate of the investigated alloys, which is attributed first to its lower chromium content and second to the formation of chromium carbides. At grain boundaries, internal oxidation, mainly of aluminium andtitanium, took place. The Al and Ti contents of alloy 80A were too low for the formation of a protective inner oxide layer of one of the two elements to take place. Alloy C22 showed the best resistance to oxidation since its chromium content of 21% is close to that for the minimum in the kineticsof oxide formation that has been found for binary Ni–Cr alloys. Additionally, there were no chromium rich precipitates to shift this chromium content to values that would result in higher oxidation rates. The nitride-containing alloy N contained a higher chromium content of 26%, whichled to a higher oxidation rate than that for alloy C22. A certain amount of inner oxidation took place, especially at coarse Cr₂N precipitates. Conclusions are presented about the optimised chemical composition of chromia laye-forming nickel base alloys for minimised oxidationrate.     

Modelling of electrically enhanced friction stir welding process using finite element method

Abstract

Conventional friction stir welding (FSW) of high strength and high melting point materials, such as steel and titanium, has the disadvantages of a serious tool wear problem and slow welding speed. A new friction stir welding process for such materials called 'electrically enhanced friction stir welding process (EHFSW)' has been suggested and analysed using finite element modelling. The basic idea of EHFSW is that electric current passes from the welding tool into the workpiece through the contact area in the welding region. Thus it results in more localised heating while welding is in progress and is not simply a preheating process. The temperature distribution in the workpiece during the pin plunge stage and the welding stage of the EHFSW process has been determined. The results show that EHFSW can reduce the plunge force significantly with the help of localised electrical heating during the pin plunge stage, which may imply lower tool wear when compared with conventional FSW. At the same time, in the welding stage, the simulation results indicate that the welding speed of the EHFSW process can be at least two times faster than that of the conventional FSW process. Thus, finite element analysis shows that EHFSW is a promising process and could reduce tool wear while improving the welding speed, especially for high melting/O point materials.