Solidification and microstructure of ZA27/SiCp composite fabricated by mechanical–electromagnetic combination stirring process

Abstract

The solidification behaviours and microstructural characteristics of both ZA27/SiC_p composites and monolithic ZA27 alloy were studied by using differential scanning calorimetry, scanning electron microscopy, transmission electron microscopy, electron probe microanalysis, and X-ray diffraction. It was found that there were differences in the transformation temperature and volume fraction of the phases, although the solidification process was almost identical for the composite and the monolithic alloy. The incorporation of SiC particles in the ZA27 alloy led to slight refinement of primary grains and reduced volume fraction of eutectic-like phase. The SiC particles obstructed Zn diffusion in the residual melt during the formation of proeutectic β phase, but promoted Zn diffusion from (Al) to η (Zn) phase during eutectoid transformation. During solidification, Cu was mainly segregated in the final solidification regions; Mg was present not only in the matrix but also on SiC particles; and oxide inclusions were mainly distributed around SiC particles. The matrix microstructure for both materials mainly consisted of primary cores of Al rich +η eutectoid; β′ phase resulting from the eutectoid transformation of the proeutectic β phase; and Zn rich +η eutectoid resulting from the eutectoid transformation of the eutectic-like phase. The SiC particles were mainly distributed around the primary grains. Several new phases based on the Al–Zn–Mg–Cu system and interfacial reaction products, including Al₂₁Fe₃Si, Cu₅Zn₈, Mg₆Cu₃Al₇, MgAl₂O₄, and amorphous oxide inclusions, were identified in the final solidification regions. The nucleation of both primary phase and eutectic-like +η phase at the surface of SiC particles and their crystallographic orientation relationships were investigated theoretically and experimentally. No distinct crystallographic orientation relationship between the matrix and SiC has been identified, although the mismatch between (0001)_SiC and (111)_ was calculated to be as small as 7·6%.     

Examination of HA230 turbine transition duct

Abstract

The present paper reports the results of an examination of a service run transition duct from one of Siemens' industrial gas turbines and the correlation between the findings for this, and those from the studies undertaken on laboratory exposed Haynes alloy 230 material. The studies have shown that there are changes in the microstructure and hardness of the material that can be related to temperature and time of exposure, and the information gained can lead to estimations of the average component operating temperature. Metallographic studies revealed a high level of heterogeneity of the grains in this transition duct.     

Effect of powder size on microstructure and mechanical properties of Ti(C,N) based cermets

Abstract

Effect of the particle size of TiC and TiN on the microstructure and mechanical properties of Ti(C,N) based cermets has been evaluated. Ti(C,N)–WC–Co cermets made from four groups of mixed raw powders of different sizes were manufactured by vacuum sintering. The microstructure and composition were studied using X-ray diffraction (XRD), scanning electron microscopy (SEM) and energy dispersive X-ray spectrum (EDX). The result shows that the four samples have the typical microstructures of 'black core/grey rim'. The mechanical properties of the cermet manufactured from submicron TiC and nano TiN are the best among the four samples.     

Solidification of high Nb containing TiAl based alloys

Abstract

Casting of titanium aluminides is an attractive processing route for production of near net shape components: turbocharger wheels, valves and aero-engine components are presently at the heart of casting developments. Among the casting alloys under consideration are a number of niobium rich TiAl based alloys that contain low boron additions for grain refinement and minor additions of other elements to enhance creep resistance. An essential condition that must be met to achieve grain refinement is a solidification pathway competed via β-(Ti), e.g. a pathway that avoids peritectic growth of α-Ti. In this contribution we describe the microsegregation analysis of a unidirectionally solidified sample from the ternary alloy Ti–45Al–8Nb. The corresponding solidification path is discussed on the basis of thermodynamic calculations and is shown to closely follow Scheil predictions with some amount of back-diffusion for aluminium. The analysis indicates that the nucleation undercooling for peritectic α (Ti) in the deep mushy zone is significant.     

Effect of chloride ions on passivity of Mg based materials

Abstract

The effect of chloride ions on passivation of pure Mg, Mg–0·6 vol.-%Mo composite, Mg–0·6 vol.-%Cu composite and Mg alloy AZ91D has been studied in 0·1M NaOH solution by cyclic polarisation and electrochemical impedance spectroscopy (EIS). An addition of even 0·05M of chloride ions was sufficient to cause breakdown of passivity. Cyclic polarisation curves revealed a positive hysteresis loop in the presence of chloride ions. Breakdown potentials decreased continuously, for all materials, with increasing addition of chloride. Electrochemical impedance spectroscopy studies revealed that the film resistance of all Mg based materials continuously decreased with the addition of chloride ions. The film resistance of Mg–0·6%Cu and Mg–0·6%Mo composites was lower than that of pure Mg and AZ91D. Mg–0·6%Mo composite showed the lowest film resistance in all the solutions. The increase in film capacitance, for pure Mg and Mg based composites, with the addition of chloride ions, was attributed to surface roughening. Mo reinforcements were more detrimental than copper reinforcements.     

Gas tungsten arc welding of titanium using flux cored wire with magnesium fluoride

Abstract

Grade 2 Ti–CP was gas tungsten arc welded using flux cored (FC) wires and flux pastes that contained various MgF₂ contents. The effects of MgF₂ on bead morphology, chemical composition and hardness of weld bead were investigated and interpreted. With an increase of MgF₂ content in the flux paste, depth/width ratio of weld bead increased gradually with little variation in interstitial element contents and hardness. Weld bead made with cold FC wire feed showed even deeper and narrower bead, indicating the greater effectiveness of wire feed than flux paste on weld penetration. While the 50% MgF₂ FC wire produced complete slag coverage and smooth weld bead surface, 85% MgF₂ wire resulted in incomplete slag coverage and rough weld surface. Arc spectroscopy revealed that the 50% MgF₂ FC wire produced plasma spectrum with atomic and ionised titanium peaks, which is an indication of a high temperature arc and a larger amount of flux vapours in the arc. Therefore, it is believed that deep weld penetration associated with high MgF₂ fluxes in this experiment is caused by arc constriction, resulting from the greater amount of flux vapours owing to high arc temperature.     

Mechanism of Activation of Polymer Surfaces by Mixed Stannous Chloride/Palladium Chloride Catalysts

Mixed stannous chloride/palladium chloride catalysts are often used to activate etched ABS surfaces prior to electroless nickel or copper deposition. Little is known of the nature or activation mechanism of the catalyst. The nature of the catalyst was investigated using nephelometry and electron diffraction. No evidence for the existence of colloidal metallic palladium was found. The active component of the catalyst is thought to be a complex chloride of tin (II) and palladium (II). The major species present on the ABS surface at each step of the pretreatment cycle were identified using electron diffraction. A mechanism of activation was proposed involving hydrolysis of an adsorbed chloride complex within the ABS surface. On subsequent treatment with ammonium bifluoride accelerator, a surface redox reaction is presumed to occur between the hydrolised species forming active palladium nuclei: Sn (II) + Pd (II) = Sn(IV) + Pd (0). The process variables were also investigated. It was found that immersion times in all pretreatment solutions including rinses had a direct influence on induction period.     

Peak temperatures during friction stir welding of Ti–6Al–4V

Abstract

Experimental measurements were made to determine the peak temperatures during friction stir welding of Ti–6Al–4V alloy as a function of the processing conditions such as tool rotation speed and feedrate. It was found that the spindle speed has a dominant effect on peak temperatures, while feedrate controls exposure time. Low spindle speed conditions lead to peak temperatures near, or below, the beta transus temperature of the material, 1000°C (1800°F), while high spindle speed welds result in peak temperatures above 1200°C (2100°F). Weld microstructures were also evaluated as a function of the weld parameters. Higher spindle speeds and lower federate lead to increased grain size.     

Microstructure,mechanical properties and tribological behaviour of PM Fe-Cu-Zn alloys containing solid lubricants

Abstract

Fe-Cu-Zn alloys containing solid lubricants of graphite and talc produced via cold pressing and sintering technology are investigated. The influence of composition and sintering temperature on open porosity, density, strength and hardness of these alloys is studied. The microstructure is correlated with the mechanical properties of the sintered materials. The tensile tests showed that the peak strength was observed for samples sintered at ≤1000°C. Above this temperature the bending and tensile strength values decrease. Microstructural analysis of sintered materials revealed three phases in the structure: α-Fe, α-brass and talc. Results indicate that the microstructure of sintered samples is sensitive to process variables such as brass and talc contents and sintering temperature.     

Vacuum hot roll bonding of titanium alloy and stainless steel using nickel interlayer

Abstract

Vacuum hot roll bonding of titanium alloy and stainless steel using a nickel interlayer was investigated. No obvious reaction or diffusion layer occurs at the interface between stainless steel and nickel. The interface between titanium alloy and nickel consists of an occludent layer and diffusion layers, and there are the intermetallic compounds (TiNi₃, TiNi, Ti₂Ni and their mixtures) in the layers. The total thickness of intermetallic layers at the interface between titanium alloy and nickel increases with the bonding temperature, and the tensile strength of roll bonded joints decreases with the bonding temperature. The maximum tensile strength of 440·1 MPa was obtained at the bonding temperature of 760°C, the reduction of 20% and the rolling speed of 38 mm s^–1.