Helen V. Atkinson

Overview no. 65: Theories of normal grain growth in pure single phase systems

Abstract The development of theories for normal grain growth in pure single phase systems is reviewed. A major theme which emerges is the interplay between the topological requirements for space filling and the kinetics of change in mean grain size with time. Computer simulation is now playing a key role in exploring this interplay.

Characterization of inclusions in clean steels: A review including the statistics of extremes methods

The application of new secondary refining techniques and non-metallic inclusion reduction techniques in steel production processes has greatly reduced the size and amount of non-metallic inclusions remaining in molten steels and steel products. This makes the inspection of inclusions difficult. Here the main methods used for the characterization of inclusions in clean steels are reviewed. The influences of inclusions on the properties of steels are discussed. Statistical methods for the prediction of the maximum inclusion size in a large volume of steel are introduced. Methods based on the statistics of extremes are described in detail. The methodology for the practical application of these methods is described and the factors affecting the precision of the estimation are discussed.

Semi-solid processing of novel MMCs based on hypereutectic aluminium-silicon alloys

Abstract This paper reports on the semi-solid processing (thixoforming) of spray-formed Al Si metal matrix composites (MMCs). In these novel MMCs, silicon—in quantities of up to 45 vol.%—was the reinforcing phase, rather than a ceramic. The project investigated all steps in the processing of the materials; mechanical tests on the products were carried out and demonstrator components were manufactured. It was shown that cylindrical billets of the MMCs could be formed in one step into intricate shapes at temperatures well below the liquidus. This confirmed that thixoforming is a safe, clean and practical method for the near net shape forming of components from materials that could be shaped only with difficulty by other techniques. The thixoformed MMCs exhibited very high specific stiffness, low thermal expansion, high specific strengths and high wear resistance.

Semi-solid processing of alloys

EVOLUTION AND DESIGN OF MICROSTRUCTURE IN SEMISOLID ALLOYS.- Fundamental Aspects.- Characterization of Microstructure in Semisolid Slurries.- Evolution of Microstructure in Semisolid Alloys During Isothermal Holding (Soaking).- Recent Developments in Slurry Formation.- RHEOLOGY AND MODELING.- and Definitions for Rheology and Modeling.- Experimental Determination of Rheological Behavior.- Modeling of Semisolid Processing.- General Conclusions on Rheology and Modeling.- INDUSTRIAL APPLICATIONS OF SEMISOLID PROCESSING.- to Industrial Applications of Semisolid Processing.- Raw Material.- Process Control in Die Filling and Die Design.- Component Design Rules.- Practical Applications in Use Today.- The Future.

Thixoforming of an automotive part in A390 hypereutectic Al-Si alloy

Abstract Hypereutectic aluminium–silicon alloys offer the possibility of an in situ natural composite (the silicon acting as the reinforcing phase) with properties that make them attractive for a number of automotive applications. However, conventional casting techniques result in excessive growth of the silicon particles in the melt, which adversely affect the mechanical properties. Thixoforming allows hypereutectic Al–Si alloys containing 40–50% fraction liquid to be shaped into complex near net shape components, whilst keeping the silicon particle size quite fine. This paper describes the development of a series of hypereutectic alloys based on the A390 composition (17%Si, 5%Cu, 0.5%Mg), their thixoforming, their resulting microstructures and mechanical properties. Finally the thixoforming of an automotive component using an A390 alloy is also described.

Application of the Generalized Pareto Distribution to the estimation of the size of the maximum inclusion in clean steels

Abstract Predicting the maximum inclusion size in a large volume of clean steel from observations on a small volume is a key problem facing the steel industry. The maximum inclusion size controls fatigue behaviour and other mechanical properties. A statistical method based on the Generalized Pareto Distribution, which has not previously been applied to inclusions, is described and used to analyse results from two steels with different cleanness levels. The predictions of maximum size in larger volumes are compared with those from extrapolating the log-normal curve which fits the size distribution of inclusions observed on the surface of cold crucible remelted buttons of the steels. If the predicted maximum inclusion size is plotted against increasing volume of steel, there is a continuous increase for the log-normal extrapolation but for the Generalized Pareto Distribution method the curve tends towards an upper limit. This is of great significance to steelmakers because the existence of an upper limit is more in accord with the expectation from steelmaking practice.

QFD in new production technology evaluation

Abstract The large number of new technologies being developed means it is vital for organisations to make appropriate selections that will translate limited capital resources into maximum competitive advantage. However, a thorough economic evaluation of a technology requires considerable time and effort. This paper presents a tool developed from the techniques of quality function deployment. This tool allows a rapid evaluation of the feasibility of using the thixoforming process to manufacture products. The paper describes the semi-solid metal processing technology of thixoforming, the relevant quality function deployment techniques and the approach used to develop the tool.

Thermodynamic predictions of wrought alloy compositions amenable to semi-solid processing

Abstract Semi-Solid Metal (SSM) Processing or Thixoforming is now a commercially successful manufacturing route producing millions of near net-shape parts per annum for the automotive industry. Currently, effort is directed towards developing wrought performance alloys that are amenable to the thixoforming process. Here, the 7000 series Al–Zn–Mg–Cu system has been studied by means of thermodynamic modelling to identify those compositions likely to be most suitable. The non-equilibrium solidification has been determined by using the Scheil equation and the liquid fraction/temperature relationship obtained. The results show the effect of chemical composition and Zn/Mg ratio on the slope of the liquid fraction/temperature curves. As the Zn+Mg+Cu content increases and the Zn/Mg ratio decreases, the slopes of these curves become less steep. The effect of chemical composition, Zn/Mg ratio and temperature on equilibrium phases is also considered. The phases in equilibrium at a given temperature are strongly affected by the Zn/Mg ratio and Cu content.

The precision of methods using the statistics of extremes for the estimation of the maximum size of inclusions in clean steels

The maximum inclusion size in clean steels influences fatigue behaviour and other mechanical properties. Hence, its estimation and the uncertainties associated with the estimation are important issues for steel makers and users. Here, two methods based on the statistics of extremes, one termed the Statistics of Extreme Values (SEV) method and the other the Generalized Pareto Distribution (GPD) method, are used for the estimation. Both methods use data on the size of inclusions revealed on the surface of sampled areas. The influence of the number of sample areas and the way the sample areas are grouped on the estimated result and its confidence limits is determined and compared. For both the SEV and the GPD methods, the estimated largest inclusion size is relatively insensitive to the number of sample areas but, as might be expected, the width of the confidence interval decreases steeply as the number of sample areas increases. A key point is that the SEV method has a narrower confidence interval than the GPD method for a given number of sample areas, because the SEV method makes an extra assumption about the form of the distribution of large inclusions. The particular assumption is difficult to justify on the basis of the data alone, and leads to a potentially over-optimistic estimate of precision. For practical application of the GPD estimation procedure, the number of sample areas needed for estimation depends on the confidence interval required and the volume of steel of interest. It is suggested on the basis of the GPD size distribution that fatigue failure initiation in a component is unlikely to be caused by the single largest inclusion, but rather by more frequently occurring inclusions near the top of the size range. This provides the conceptual basis for a statistically based design approach in which the estimated distribution of inclusion sizes is used in defect tolerance design of steel components and in control of steel production processes.

The wettability of silicon carbide by liquid aluminium: the effect of free silicon in the carbide and of magnesium, silicon and copper alloy additions to the aluminium

Results from the sessile-drop method are reported for the effects on wetting angle, θ, of free silicon in the silicon carbide substrate and of alloy additions of silicon, copper or magnesium to the aluminium drop for the temperature range 700–960 or 1040 °C in a titanium-gettered vacuum (10−4/10−5 torr; 1 torr=133.322 Pa). Wetting angle, θ, was reduced by a factor as large as 2.8 for pure aluminium on reaction-bonded, compared with sintered silicon carbide, attributable to partial dissolution by the aluminium of the 18 wt% free silicon present in the reaction-bonded material. For wetting of reaction-bonded silicon carbide, the addition of 5 wt% silicon, copper or magnesium to the aluminium gave contact angles that decreased in the sequence Si→Cu→Mg, with the magnesium addition being the only one to result in wetting (i.e. θ<90 °) for all conditions studied. These results may have implications for design of conditions for joining or promotion of infiltration of silicon carbide parts, preforms or arrays with aluminium alloy melts.