Optimising grey iron powder compacts

Abstract

The grey iron microstructure Fe–2C–2Si powder based compact is tailored by different kinds of in situ and post sintering processing. This has been achieved by combining thermodynamic and kinetics modelling of microstructure development with sintering and controlled heat treatment experiments of tensile test specimens die compacted at 600 MPa. Applying optimised sintering conditions led to a grey iron like microstructure with 95% relative sintered density. Sinter hardening the compacts led to 500 MPa in yield strength and 600 MPa in ultimate tensile strength in combination with ductile fracture. Quenched and tempered condition showed the same strength values, but combined with brittle fracture due to martensitic structure. Pore rounding and partial pore filling by graphite were obtained by austenising isothermal hold during the cooling of the sintering cycle.     

Effect of tool rotation rate on microstructure and mechanical properties of friction stir welded copper

Abstract

Defect free copper welds were achieved by friction stir welding (FSW) carried out at a constant welding speed of 100 mm min^?1. The influence of tool rotation rate on microstructure, mechanical properties and fracture location was investigated. As the tool rotation rate increased, the grains of nugget zone grew significantly, the thermomechanically affected zone became indistinct and the grain size increased, but the effect of tool rotation rate on the grain size of heat affected zone was limited. Both ultimate tensile strength (UTS) and elongation increased first and then decreased with increasing rotation rate and the UTS achieved a highest value of 282 MPa at the rotation rate of 400 rev min^?1 together with the welding speed of 100 mm min^?1, which was on the level of the base metal. The fracture occurred at the cavity defect on the advancing side of the joint when the FSW was performed at a low tool rotation rate, while it occurred on the retreating side when the tool rotation rate was relatively high.     

30—THE EFFECT OF VARIABILITY OF FIBRE DIAMETER ON WORSTED PROCESSING: PART I. THE USE OF COMPARISON SAMPLES FROM A SINGLE MERINO FLOCK

Two greasy-wool bulks were made up from sound fleeces selected from a single medium-merino flock so that their mean diameters, mean staple lengths, and mean crimp frequencies would be the same or similar, but the coefficients of variation of diameter (V _d) would be different. The tops made from these bulks showed closely similar mean fibre diameters (20·6 and 20·8 (μm), lengths, and length variabilities, with the V _d values 18·3% (L) and 21·4% (H). In conventional Bradford spinning, L was slightly favoured over H at the spinning limit, but the differences in ends-down per 100 spindle-hours at commercial counts were negligible. Yarn properties, such as tensile strength and elongation at break, and yarn evenness did not disclose differences of any consequence. Fabrics made from 25-tex yarns gave closely similar properties. Ambler Superdraft spinning supported these conclusions.

The difference of 3% in V _d values did not indicate significant worsted-manufacturing differences between the tops. Since these tops were made from single-quality sources, the V _d values are low compared with the range of values for European tops of the same diameter, the majority of which would occur in the 20–26% V _d range.     

32—SUPERMOLECULAR STRUCTURE AND FIBRE PROPERTIES

Various theories of fibre structure are briefly reviewed, and it is shown that the original ‘fringed-micelles’ theory is no longer considered valid. The influence of the genesis of fibres on their structure is demonstrated, and it is shown that, because of this difference in fibre formation, it is difficult to fit a single model to all fibres. The architectural models proposed for various fibres are discussed, and it is pointed out that the validity of these models can be tested by reference to the fibre properties.

It is emphasized that there is still insufficient knowledge of the behaviour of fibres subjected to transverse stresses, and it is shown that some of the phenomena that occur under the action of such stresses can be explained by the comparatively recent ‘fringed-fibrils’ theory of fibre structure. Finally, attention is drawn to some aspects of the structure and behaviour of fibres that are still unexplained, and towards which it is proposed that further research should be directed.     

38—THE EFFECT OF SPINNING CONDITIONS ON THE TENSILE PROPERTIES OF CORE-SPUN YARNS

A critical assessment of the merits of core-spinning by means of comparisons of the strength and extension of core-spun yarns with all-staple-fibre yarns forms the basis of this paper. Core-spinning was done by introducing the filament under tension into the drafted strand as the latter entered the front rollers of the ring-frame. A 20-den (2.2-tex) nylon monofilament was used as core, and three types of staple-fibre covering were investigated. The effects of twist, pre-tension, and feeding arrangements on the geometrical disposition and tensile properties of the yarns were examined. Improvements in strength are, in general, realized at low twists by the incorporation of the filament and by further constraining it to occupy the core. The contribution of the components to the yarn strength and the process of breaking were also subjected to a detailed study. The studies reported provide an interesting insight into the extent of cohesion developed between the fibres during the straining of the yarn and its influence on the process of breaking and ultimately on yarn extension.     

On the diffusion behaviour of Os in the binary Ni–Os system

Abstract

The diffusion of Os in Ni is characterised by measuring interdiffusion coefficients in the binary Ni–Os system, in the temperature range 1200–1350°C. The results are compared with data for Ni–Re. Theoretical calculations indicate that Os should display a diffusion coefficient comparable with Re, among the lowest displayed by elements from the d-block transition elements. The authors' experimental findings confirm these predictions, and suggest that alloying with Os is likely to improve the high temperature properties of the nickel-based superalloys.     

Microstructure evolution and mechanical properties of nickel based honeycomb sandwich

Abstract

A nickel based superalloy honeycomb sandwich was manufactured by high temperature brazing. The microstructure evolution and the out of plate mechanical properties were investigated for honeycomb sandwiches aged at 1000&degC in the present paper. The maximum tensile stress was 28·5 MPa and the compressive yield strength was 29·6 MPa for the original specimens. These parameters decreased to 22·9 and 20·5 MPa for specimens aged for 2 h, to 18·2 and 12·2 MPa for specimens aged for 5 h and to 20·2 and 14·3 MPa for specimens aged for 10 h respectively. With increasing aging time the tensile elongation decreased, the intermetallic compounds and the eutectic structure in the brazing region disappeared, and the solid solution approaching the matrix gradually increased.     

Effects of Ca combined with Sr additions on microstructure and mechanical properties of AZ91D magnesium alloy

Abstract

Weight reduction to improve automobile fuel economy has triggered renewed interest in magnesium. The effects of Ca/Sr separate and composite additions to AZ91D magnesium alloy on its microstructure and mechanical properties have been investigated. The results indicate Ca can refine both the grain and eutectic phase of AZ91D magnesium alloy. Sr hampers microstructure refinement when composite Ca/Sr additions are made. In addition, separate Ca additions to AZ91D magnesium alloy increase yield strength but decrease elongation of this alloy. By adjusting the Ca/Sr composite proportions, additions to AZ91D magnesium alloy are able to improve both microstructure and mechanical properties of the alloy.