The Modification of Aluminium Silicon Casting Alloys

This paper reviews recent TEM studies of the structure of the Si phase in unmodified, quench modified and impurity modified Al-Si eutectic alloys and discusses the growth mechanism of the Si phase in the three eutectic structures. Initial results from kinetic measurements being made to provide additional information on the growth mechanisms are reported.     

Friction Skew-stir welding of lap joints in 5083–0 aluminium

Abstract

A variant of the friction stir welding technique developed at TWI, called Skew-stir (tradename) was applied to the welding of lap joints in Al alloy AA5083–0. This technique differs from the conventional method in that the axis of the tool is given a slight inclination, or skew, to that of the machine spindle. It is particularly advantageous in instances where a wide weld region is required, such as lap joints in which the interface is perpendicular to the machine axis. The microstructures and mechanical properties of welds made using both a conventional pin type probe and the rotary Skew-stir technique with an A-Skew (tradename) probe were studied. The joints made using the Skew-stir technique sustained significantly higher tensile loads and had longer fatigue lives compared with those made using the conventional pin type probe.     

Thermomechanical processing of 42CrMoS4 steel

Abstract

Billets of 42CrMoS4 steel were subjected to a programme including forging and rolling to different reduction ratios, followed by quenching and tempering to simulate online thermomechanical treatment (TMT) during rolling. The mechanical properties obtained were compared with those obtained by conventional heat treatment (CHT) (quenching from 860°C and tempering). It was found that increasing the hot reduction ratio from 18 to 60%, accompanied by a decrease in the finish rolling temperature from 900 to 750°C, enhances strength only at the expense of elongation, while rapid quenching instead of air cooling from the same finish rolling temperature yields improvements in both strength and ductility. It was also found that CHT provides higher hardness, whereas TMT provides higher impact toughness. TMT will confer major economic savings since the heat treatment is achieved online.     

Grain boundary engineering and the role of the interfacial plane

Abstract

Grain boundary engineering (GBE) involves the use of microstructural design to improve bulk material properties and enhance resistance to intergranular degradation. More specifically, the patented GBE procedure involves the design and control of fcc metallic microstructures using thermomechanical treatments and grain boundary characterisation based on the coincidence site lattice model. The phenomenon of multiple twinning is used to create a ‘twin limited’ microstructure, i.e. a microstructure composed entirely of special grain boundaries and triple junctions that is highly resistant to intergranular degradation. However, the theory behind GBE is not fully developed and therefore further study of the interfacial geometry, including the grain boundary plane and its role in GBE, is required to improve understanding of multiple twinning with the ultimate aim of improving the bulk and intergranular properties of metallic materials. An introduction to GBE is presented, including a number of cases where grain boundary design has improved the properties of fcc alloys for industrial applications. The theoretical characterisation of grain boundaries, including interfacial structure and geometry, is reviewed, highlighting the problems associated with microstructural characterisation based on limited knowledge of the grain boundary geometry. The importance of the grain boundary network is discussed: the grain boundary and triple junction character distributions are known to have a significant influence on bulk properties. Finally, the role of the interfacial plane is considered. It is concluded that although GBE has produced significant results, its theoretical basis and the ultimate creation of twin limited microstructures require further development.     

The Prediction of Shrinkage Cavities in Cast-Steel Rolls

Abstract

A 2D finite difference program has been written which enables the progress of solidification to be predicted in cylindrical castings with geometries typical of cast-steel rolls. A simple method for predicting the formation of gross shrinkage cavities has been introduced into the program, which assumes that liquid metal flow is instantaneous in regions where the solid fraction is below some critical value and feeding through regions above the critical value is not possible. Using metal/mould heat transfer coefficients determined previously for a variety of mould surface conditions (bare chill, coated chill, sand-lined chill), the computer model has been validated experimentally in terms of solidification times, position and shape of cavities and regions of porosity for a number of geometrical arrangements with different mould surfaces.     

Potential of semi-soft coals as replacement for hard coals in stamp charging blend

Abstract

In India the use of appropriate imported coals to supplement the inherent deficiencies of indigenous coals for cokemaking is in vogue. Stamp charging is very efficacious for inferior coking coals and has enabled Tata Steel to produce coke of international quality. It has been found to offer scope for the utilisation of less expensive imported semi-soft coals to replace expensive imported hard coals. Extensive tests were carried out to assess the blending potential of semi-soft coals for cokemaking by stamp charging in a 7 kg mini-oven, and a 600 kg pilot oven in the laboratory. This was followed by commercial scale trials. The test results clearly indicated that hard coals can be replaced with 30–35% of semi-soft coals in the stamp charging blend at Tata Steel without any compromise of coke strength. Semi-soft coals are now in regular use in all Tata's stamp charged batteries.     

Fatigue crack growth behaviour of 4130X steel in H2S environment

Abstract

Fatigue crack growth rates of 4130X steel used for compressed natural gas vessels were investigated in this paper. Considering the operating conditions, corrosion fatigue tests at a low frequency of 0·0067 Hz, in H₂S saturated, H₂S unsaturated and air environments were conducted on modified wedge opening load specimens by using a home made low cycle fatigue test system. Curve fitting was applied to the fatigue test data of da/dN–ΔK according to Paris formula. A correlation study between fracture surface and stress intensity factor range was conducted and K values for three stages in different environments were characterised quantitatively. The results show that da/dN in H₂S environment is more than 20 times faster than in an air environment. When the H₂S concentration reaches a certain range, the increase of da/dN becomes slower than that of the H₂S concentration. da/dN differs by 2·4 times while the concentration differs by 11 times. The corrosive environment accelerates the fatigue failure.     

Production of Si55 Al20 Fe10 Ni5 Cr5 Zr5 bulk amorphous alloy by hot pressing

Abstract

The temperature dependence of the relative density was examined for a Si₅₅ Al₂₀ Fe₁₀ Ni₅ Cr₅ Zr₅ alloy which was produced by hot pressing of the amorphous powder during heating up to various temperatures under a critical pressure of 1·5 GPa through a heating/pressing cycle. The density of the compacts increased with increasing temperature and reached a maximum near the crystallisation temperature of 698 K. The relative density of the compacts reached 98·3% at the critical condition of 1·5 GPa and 687 K. The hardness value of the bulk specimen was 940 HV(10 gf, 0·098 N), corresponding to that of the amorphous ribbon. Exposure to higher temperatures resulted in the precipitation of the crystalline phase. The present results indicate that Si based alloys can be produced in a compact form with a low fraction of voids by hot pressing the amorphous powder.     

Influence of nodule count on fatigue properties of ferritic thin wall ductile iron

Abstract

The present work focuses on the study of the influence of nodule count on the fatigue resistance of ductile iron. Fatigue tests were carried out on specimens taken from thin wall ductile iron plates of 2 and 4 mm thickness and standard Y blocks of 12·7 mm thickness, showing nodule counts ranging between 1800 and 300 nod mm^?2. All samples were ferritised before testing to obtain a homogeneous ferritic matrix. The results showed a large dispersion of fatigue strength values. Nevertheless, careful examination of the fracture surfaces showed the presence of very small casting defects on many test samples. When only the results measured on sound test samples were accounted for, a significant increase in fatigue strength was found as the nodule count increases. Casting defects, particularly microshrinkage, revealed by scanning electron microscopy on the fracture surfaces, were responsible for the premature failure, due to shortening of the crack initiation stage. The fatigue lives measured experimentally were compared with estimations based on the fracture mechanics theory.     

Toughness and fatigue behaviour of eutectic and hypereutectic Al–Si–Cu–Mg alloys produced through lost foam and squeeze casting

Abstract

The strength and toughness of four high silicon content Al–Si–Mg–Cu alloys have been studied at room temperature (RT), 200°C and 300°C. The fatigue behaviour has also been investigated. The alloys were produced using two very different processing routes: lost foam and squeeze casting. In the tensile tests, the ductility was low for alloys produced via both routes irrespective of the testing temperature. The strength was similar at RT and 200°C, but at 300°C it fell abruptly. The toughness followed the same trend with testing temperature. Direct observation of fatigue cracks revealed that the brittle silicon and intermetallic particles broke ahead of the crack tip; the fatigue crack advanced by linking the main crack with cracks formed ahead of it. The T6 thermal treatment improved fatigue resistance in the squeeze cast material, especially at high D K values.