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.     

Effects of cold and hot densification on the mechanical properties of a 7XXX series powder metallurgy alloy

Abstract

The objective of this work was to investigate the effects of hot and cold swaging on the density and mechanical properties of a commercial Al–Zn–Mg–Cu powder metallurgy alloy known as Alumix 431D. To do so, as sintered samples of the PM alloy were swaged under a variety of conditions and characterised. For comparison purposes, equivalent characterisation tests were completed on the chemically similar wrought alloy 7075-T6. Cold swaging was moderately successful provided the as sintered billets were annealed or solutionised before densification. Here, modest improvements in density and tensile properties were noted. Hot swaging proved to be a more effective approach. Optimal properties were achieved when samples were preheated to 470 ± 10°C. When processed in this manner, a density of 99·6% of theoretical was realised while the tensile and fatigue properties exceeded those of the wrought 7075-T6 alloy tested for comparison purposes.     

Design criteria for rolling contact fatigue resistance in back-up rolls

Abstract

This research centres on surface initiated damage on back-up rolls whereby rolling contact fatigue cracks can propagate into the rolls potentially reaching the internal stress fields and leading to catastrophic failure and has sought to establish design criteria for avoiding such failures. The project objectives have been achieved by examining field evidence, determining the loading and tribological conditions at the work roll/back-up roll interface and investigating both theoretically and experimentally the mechanisms involved in rolling contact fatigue in this case. The presented rolling contact, fatigue and fracture mechanics model includes criteria for crack branching either upwards (i.e.relative safety) or downwards (i.e.potentially catastrophic) and the link between these two cases is related, quantitatively, to the properties of the roll material. After linking mechanics to microstructure, the influence of work roll test disc surface roughness on both the surface wear of and the interaction between wear and rolling contact fatigue at the surface of back-up roll test discs has been quantified using the results obtained from experimental simulations carried out on a rolling–sliding testing machine. Finally practical quantitative recommendations are made for the mechanical and microstructural design of bainitic back-up roll materials, back-up roll redressing procedures and the surface roughness of both work rolls and back-up rolls presented to the mill.     

Flux shielded arc butt welding of concrete reinforcing steel

Abstract

The present paper reports on a new variant of flux shielded arc butt welding without the addition of a filler material, applied to steel bars for concrete reinforcement. The welding process is described, including the arc ignition and forcing together of the workpieces, via the application of a mechanical force, to form an upset. A new portable machine is described, using which it is possible to apply flux shielded arc welding on site. Experiments were carried out, from which optimum welding parameters such as welding current, weld time, and upsetting force were found. The strength properties of the welded joints obtained were determined via mechanical testing, i.e. a bend test, a tensile strength test, and an impact toughness test. Macroscopic sections of the welded joints were also produced. A quality analysis of the butt welded joints showed that the mechanical properties of the joints were equivalent or even superior to the properties of the parent metal.     

Crack initiation and kinking behaviours of spot welded coach peel specimens under cyclic loading

Abstract

The present study is carried out to investigate fatigue crack initiation and kinking behaviours of spot welded coach peel (CP) specimens of low carbon steel sheets subjected to cyclic loading by experimental and finite element analysis methods. Evaluations of fatigue crack growth stages were performed by crack tip plastic strains and J integral analyses and also by microhardness measurements on process zone. According to the experimental and analytical results, fatigue crack initiation and growing stages in the spot welded CP specimens can be divided to three stages. Stage I corresponds to 'gap sharpening stage' observed at the beginning steps of cyclic loading with crack growing on the interface plane between the overlapped sheets. Stage II corresponds to kinked crack initiation and propagation through the sheet thickness observed after applying a certain number of loading cycles. Stage III corresponds to crack propagation through the width of the specimens observed at the final step of the fatigue crack propagation. The FE results of the crack paths and crack locations are in good agreement with those of experimental observations.     

EFFECT OF FRICTIONAL FORCE ON GEARING CONTACT FATIGUE STRENGTH

The model for computing frictional coefficient between two teeth faces at the state of mixed elastohydrodynamic lubrication is established. And then more than 80 sets of numerical calculations and six sets of disc fatigue tests are completed. The results show that when the film thickness ratio λ＜1.6, frictional coefficient μ is drastically decreased as λ rises; Thereafter it decreases smoothly until λ=4.5. When λ＞4.5, however, it goes up again with λ, which indicates that the excessive film thickness ratio will deteriorate gearing contact fatigue strength. At the end, the formulae for determining the frictional coefficients are formed.     

VOLUME DEFECT FATIGUE FAILURE OF CERAMIC BALLS UNDER ROLLING CONDITION

A newly developed pure rolling fatigue test rig with three contact points is used to test the rolling contact fatigue properties of silicon nitride ceramic balls. Ball surfaces are examined after failure with optical microscopy and scanning electron microscopy. The failure cause,fatigue phenomenon and mechanics are analyzed. The research shows that subsurface cracks play a dominant role in the formation of spalling failure. These cracks originated from volume defects of the material and propagate,to form elliptical fatigue spalls under the action of principal tensile stresses. The principal tensile stress increases with increasing contact load,causing spall formation and reduction of rolling contact life. The greater the principal tensile stress is,the more severe the peeling of near surface is. Under the same condition,the closer volume defects are to the surface,the more likely failure occurs,the shorter the rolling contact life is.     

BAM guideline draft for lid and trunnion systems: contents,structure, compatibility and use

Abstract

The Federal Institute for Materials Research and Testing (BAM) intends to provide a new guideline for the analysis and assessment of lid and trunnion systems. This guideline is discussed in the present paper with respect to its contents and structure, its compatibility with other guidelines and selective technical issues to demonstrate its use. Particularly, the motivation, the principles and the background of the guideline are addressed. Therefore, the present paper discusses current conclusions drawn from experimental results and the wide use of advanced analysis methods such as finite element analysis as well as the incorporation of these aspects in the guideline draft.     

Microstructural characterisation of sand cast aluminium alloy A356–SiC particle metal matrix composite

Abstract

The microstructure of a cast aluminium alloy A356 reinforced with 15 vol.-%SiC particles has been investigated using analytical microscopy. It is shown that the morphology of the silicon phase, as well as that of the Al–Si eutectic structure, which are the features of this alloy system, are dramatically changed by the presence of the SiC particles. Significant effects of grain refinement werefound to occur not only on the primary aluminium grains, but also on the primary and eutectic silicon phases. Twinning and dislocations were often observed within that silicon phase which was situated adjacent to the SiC particles. Microcracks were also observed at the SiC/silicon interface. The presence of such microcracks suggests that a stress concentration had developed at the silicon/SiC particle interfaces, probably as a result of the thermal expansion mismatch occurring between the silicon and SiC particles. The microcracks which consequently develop are formed as a result of the poor silicon–SiC particle bonding. Other intermetallics, notably Mg₂Si and FeSiAl₅, which exhibit cubic and tetragonal symmetries respectively, were also identified as being present in the microstructure.

MST/1460     

Effect of plasma nitrocarburising on fatigue resistance of low alloy Cr–Mo sintered steel

Abstract

A nitrocarburising treatment was carried out on a low alloyed Cr–Mo sintered steel (1·5%Cr, 0·2%Mo) to evaluate its influence on the bending fatigue resistance of the material. Fatigue performance was compared with that obtained by a conventional through hardening and a plasma carburising. Two different green densitise (7·1 and 7·4 g cm^?3) and two different sintering temperatures (1120 and 1250°C) were adopted during preparing the as sintered steels. Fatigue strength after nitrocarburising on 1120°C sintered steels considerably increases. It is comparable with that obtained by through hardening and slightly better than that obtained by plasma carburising. On samples sintered at 1250°C the fatigue resistance of the nitrocarburissed is worse than expected, because the microstructure is softer and less homogeneous.