Strain measurement of forming process using digital imaging

Abstract

An understanding of local strain behaviour during a metal forming operation can be important but maybe difficult to measure. A non-invasive digital imaging procedure has been developed to quantify local strain behaviour in metal forming. From digital images obtained during the forming process, discrete spatial data of the object profile was acquired. B-spline curve fitting was then applied to obtain a continuous profile. The deformation strains were calculated from the curvature of the continuous profile. The procedure was applied to an electric resistance welded pipe forming process. Strain gauge data was used to validate the proposed procedure.     

Interfacial reactions between AZ91D magnesium alloy and plaster mould material during investment casting

Abstract

Reactions at the mould/metal interface play an important role in determining the quality of investment castings. They are particularly critical in the case of magnesium alloys cast in industrial plaster moulds. In this work, reactions of molten magnesium alloy with plaster mould were studied. First the potential interactions with mould materials (including gases) were examined using thermodynamic considerations. Then thin sheets of AZ91D magnesium alloy were cast in industrial plaster moulds using vacuum assistance: the surface of sheets and plaster mould were characterised. The analysis of reaction products indicates that magnesium vapours diffuse through the plaster and reduce the silica present in the investment material according to the following reaction: 4Mg + SiO₂=2MgO + Mg₂Si. The extent of reactions is controlled by mould temperature and thickness of castings.     

Investigation of deformation and heat treatment effects on properties of PM 92.5W–5Ni–2.5Fe heavy alloys

Abstract

This paper presents the effects of vacuum heat treatment under different cooling conditions on mechanical and structural properties of forged heavy alloys, such as 92.5W–5Ni–2.5Fe and 92.5W–5Ni–25Fe microalloyed with Co. The tungsten composition in the c phase has proved to be higher and more homogenous in the rapidly cooled alloys than in the slowly cooled ones. The effects of chemical composition inhomogeneity on mechanical and structural properties of alloys were also analysed and discussed. The results of tensile and toughness testing have shown an increase in ductility and toughness, while the strength of heat treated alloys decreased in comparison with the strength of forged alloys. The fracture analysis has shown that in the sintered and rotary forged alloys, intergranular fracture of the tungsten phase and transgranular fracture of the γ phase occurred, respectively. The fracture of these phases after heat treatment was characterised by transgranular morphology.     

Large undercooling,rapid solidification,and nucleation mechanism during multistage atomisation

Abstract

The large undercooling and rapid quenching that can occur during multistage atomisation are studied both theoretically and experimentally. Results show that these two effects are closely related to and promoted by each other. The level of undercooling for droplets is dependent on alloy composition, powder particle size, and atomisation condition. The cooling rates of droplets depend heavily on their particle size. Both experiments and theoretical analysis on Sn-Pb alloys reveal that a certain function of undercooling has a linear relationship with the logarithm of the mean powder particle particle size. Based on the present results, a mechanism of nucleation preferred on surface oxide is proposed to give a quantitative interpretation of the experimental observations.     

Physical properties of selected brazing filler metals

Abstract

A suitable selection of the filler metal is vital for producing satisfactory brazed joints. The wettability of brazing alloys with base metals depends on physical properties such as surface tension, density, melting point, and viscosity. Thermal conductivity and electrical resistivity are also important since the filler metal is frequently required to have similar values to those of the base metal. In the present paper, the physical properties of liquid alloys relevant to brazing have been evaluated. Six different filler metal systems were analysed, comprising alloys based on Ag, Al, Au, Cu, Ni, and Ti. Results show that the viscosity values for most binary brazing filler alloys are of the order of 2–8 mPa s, with Cu and Al alloys exhibiting the lowest viscosities. The surface tensions of brazing alloys vary from 800 to 1800 mN m^-1, with the lowest surface tension values corresponding to the Ag and Al alloys. Thermal conductivity and electrical resistivity values fall in the range 10–200 W m^-1 K^-1 and 17–300 μΩ cm, respectively.     

Joining stainless steel metal foams

Abstract

Metallic foams produced from stainless steel are one of the most recently developed ultralightweight materials. These foams have very low densities and high energy absorption capacities and are therefore expected to have widespread applications in the manufacture of ultralightweight structural components. The fabrication of load bearing structural components such as implants, and high temperature air or fluid filters, are potential application areas depending on the form of the cell structure of the foam. Closed cell metal foams are typically suitable for structural uses whereas open cell foams tend to be preferred for functional applications. Development of adequate joining technologies for these materials is an essential step for their widespread industrial utilisation. The present paper describes a brazing method that is capable of providing excellent joints between 316 stainless steel foams and a conventional 316 stainless steel bulk alloy. Having optimised the bonding conditions and using a Cu–Ti alloy as the filler metal, bonds between a foam and a bulk alloy were produced. No apparent plastic deformation of the metal foam occurred in the course of the 10 min length brazing process, and the resulting bonds had tensile strengths higher than that of the stainless steel foam.     

Metal transfer instability in gas metal arc welding

Abstract

This work presents a simplified model of metal transfer in gas metal arc welding. The model incorporates key features of metal transfer including the change in droplet diameters as welding moves from the globular into the spray metal transfer region, and the increase in welding voltage that is observed to occur as the pendant droplet grows. The model predicts that an instability arises in the globular metal transfer region, which leads to deterministic chaos and complex limit cycles with many droplet sizes. The instability also causes deterministic chaos with a characteristic gap in droplet diameters at the transition to spray mode metal transfer. The model explains observed features of metal transfer in some detail, including the existence and location of preferred bands of droplet sizes. Whether the instability is present or not defines the boundary between chaotic globular metal transfer and the stable drop spray transfer mode. The identification of deterministic chaos in gas metal arc welding metal transfer opens the way for new approaches to welding control.     

Study on ac-PMIG welding of AZ31B magnesium alloy

Abstract

Welding of AZ31B magnesium alloy is carried out using alternating current pulsed metal inert gas (ac-PMIG) welding with 1·6 mm diameter of filler wire. Typical current waveform is used to make sure arc given an accurate energy input into filler wire. The arc characteristics, metal transfer forms, microstructure and mechanical property of ac-PMIG welding of AZ31B magnesium are investigated. The results show that a stable welding procedure and continuous joints can be obtained easily under a wide range of welding parameters. The most important factors for ac-PMIG welding are negative electrode (EN) ratio and pulse rework current, which give an accurate energy input into filler wire. The grain in fusion zone is much finer and more uniform, and grain size does not grow significantly in the heat affected zone compared with base metal. The average ultimate tensile strength of weld beads is 97·2% of base metal.     

Effects of base and filler chemistry and weld techniques on equiaxed zone formation in Al–Zn–Mg alloy welds

Abstract

Equiaxed zone (EQZ) formation in Al–Zn–Mg alloy welds as affected by base metal, filler metal chemistry and weld techniques is studied. Filler metal chemistry and welding techniques have great influence on the formation of EQZ microstructure as base metal composition has. In an effort to characterise the equiaxed grain zone formation in Al–Zn–Mg alloy welds two commercial Al alloys AA7018 and RDE40 were selected. Gas tungsten arc welding in continuous current, pulsed current and arc oscillation mode were applied to weld the base materials. The influence of Sc containing fillers have been studied and compared with the commercial filler material. Mechanical and metallurgical characterisation were carried out in the EQZ. Intergranular corrosion in EQZ was studied according to ASTM G 110-92. Results reveals that RDE40 with low solute contents showed wider EQZ but relatively better corrosion and mechanical properties compared to AA7018 EQZ. Gas tungsten arc welding in pulsed and arc oscillation mode fusion boundary region exhibits better corrosion and mechanical properties compared to continuous current mode welds. Addition of Sc to the AA5556 filler combined with pulsed mode resulted in elimination of EQZ, better corrosion and mechanical properties compared to welds made with conventional AA5556 filler and also the presence of Sc within the EQZ so called unmixed zone has been observed.     

Study on metal transfer modes and welding spatter characteristics of self-shielded flux cored wire

Abstract

The metal transfer behaviour of self-shielded flux cored wire and the resultant welding spatter were investigated by using a high speed camera. Three modes of metal transfer were found, i.e. bridging transfer without arc interruption in explosive form or by surface tension, globular repelled transfer and droplet transfer, while the former two modes played a key role in the weld metal transfer. Correspondingly, the bridge explosion, discontinuous globular repelled process and the misalignment of droplets transfer were the main factors causing the welding spatter.