Metal finishing: enhanced performance achieved through plasma processing

Abstract

This paper provides an overview on how plasma deposited coatings can significantly enhance the surface properties of metallic components. Two plasma processing technologies known as physical vapour deposition (PVD) and plasma enhanced chemical vapour deposition (PECVD) are introduced. Examples of the coatings deposited using these two processes are discussed. Among the coating application areas outlined are their use to increase the wear life of tools, to improve decorative finish and to enhance medical device biocompatibility.     

Ecologically green conversion coating for zinc–cobalt alloy

Abstract

A phytic acid based conversion coating has been designed for Zn–Co alloys. The morphology of this coating was studied by SEM and showed that immersion of coatings for 15 min is smooth and compact. The composition of conversion coatings was analysed by X-ray photoelectron spectroscopy. The analysis data showed that the coating is composed of Zn, C, P, N and O. The presence of Co in the coating was not established. Corrosion protection provided by this coating was evaluated by electrochemical measurements (polarisation curves, electrochemical impedance spectroscopy) in a 0·5M NaCl solution. Electrochemical corrosion measurement showed that the corrosion resistance of the Zn–Co alloy has been improved by the conversion treatment with phytic acid.     

Towards industrialisation of microarc oxidation (MAO) for ceramic conversion surface treatment of magnesium and aluminium alloys

Abstract

Microarc oxidation (MAO), as a novel surface treatment, has demonstrated significant achievements both in academic research and in industrial application. Here, the electrical energy consumption and the role of the electrolyte during the MAO process are discussed. The corrosion resistance of MAO coatings on a magnesium alloy has been evaluated by salt spray test and electrochemical analysis. The wear resistance of MAO coating on an aluminium alloy has been tested by an abrasion technique. Finally, application prospects are discussed.     

Kinetics and dilatometric behaviour of non-isothermal ferrite–austenite transformation

Abstract

A model that describes the ferrite–austenite transformation during continuous heating in Armco iron and three very low carbon, low manganese steels with a fully ferritic initial microstructure is presented. This model allows calculation of the volume fractions of austenite and ferrite during transformation as a function of temperature, and hence knowledge of the austenite formation kinetics under non-isothermal conditions in fully ferritic steels. Moreover, since dilatometric analysis is a technique very often used to study phase transformations in steels, a second model, which describes the dilatometric behaviour of the material and calculates the relative change in length that occurs during the ferrite–austenite transformation, has also been developed. Both kinetics and dilatometric models have been validated by comparison of theoretical and experimental dilatometric heating curves. Predicted and experimental results are in satisfactory agreement.     

Estimation of resistance of filter media used for Prefil Footprinter tests of liquid aluminium alloys

Abstract

During the pressure filtration of molten metals, an essential parameter is the resistance of the filter medium, which is relevant to the calculation of flow velocity, pressure loss and distribution, and drag forces. The resistance has significant influences on the filtration behaviour and structure of the residual cake of inclusions retained on the filter. The Prefil Footprinter, a portable pressure filtration instrument, is usually used to determine the quality of molten aluminium alloys. To date, however, no data on the resistance of the filter medium used have been reported. The present work is an attempt to estimate the resistance. Experiments are described and analyses of flow behaviour are presented. Values of the medium resistance of the filters are determined and reported for the first time.     

Effects of addition of TiN nanoparticles on microstructure and mechanical properties of TiC based cermets

Abstract

Titanium carbide based cermets with additions of titanium nitride nanoparticles were fabricated by using ultrasonic dispersion and conventional powder metallurgy techniques. In order to characterise the microstructure, SEM and TEM have been utilised. The results show that the microstructure becomes finer with increasing additions of titanium nitride nanoparticles. The mechanical properties were also measured and the results show that the properties do not change monotonously with the amount of titanium nitride nanoparticle addition. Good properties can be obtained when the amount of TiN addition reaches 6 - 8 wt-%. The improvement of properties is mainly owing to the refinement of TiC grains as well as the deflection effect of titanium nitride nanoparticles on crack propagation.     

Factors affecting mechanical properties of resistance spot welds

Abstract

The present paper addresses the factors governing the mechanical performance of low carbon resistance spot welds. Correlations among the process parameters (welding current, welding time, electrode force and holding time), physical spot weld attributes and mechanical performance are analysed. Peak load and energy absorption of spot welds during the tensile shear test are used to describe spot welds performance. It is shown that weld nugget size, electrode indentation, failure mode and strength/ductility of the failure location are the main factors affecting peak load and energy absorption of spot welds.     

Interpretation of creep behaviour of a 9Cr–Mo–Nb–V–N (T91) steel using threshold stress concept

Abstract

The creep behaviour and the microstructural evolution of a 9Cr–Mo–Nb–V (T91) steel were extensively evaluated by means of short term constant load creep tests and TEM analysis. Statistical analysis of the microstructural data revealed that the precipitated phases M₂₃ C₆ (where M is a metal, mainly Cr or Fe) and MX (where M is Nb or V, and X is C and/or N) were subject to coarsening during creep exposure. The coarsening law and its dependence on applied stress were identified, and the model was used to predict the magnitude of the Orowan stress at the time corresponding to the minimum creep rate. The minimum creep rate dependence on applied stress at 873 K was described by incorporating the threshold stress concept in a power law with stress exponent n = 5. In the resulting phenomenological model, the strengthening effect of the dispersed phases was thus expressed by a threshold stress proportional to the Orowan stress.     

Investigation of quenching effect on mechanical property and abrasive wear behaviour of high boron cast steel

Abstract

The effect of quenching temperature from 900 to 1050°C on the microstructures, mechanical properties and abrasion resistance of modified high born cast steel containing 0·3 wt-%C and 3·0 wt-%B was studied by optical microscopy, scanning electron microscopy, X-ray diffraction analysis, tensile tester, impact tester, hardness tester and abrasion tester. Quenching at 900°C resulted in structures containing a small amount of pearlite. The existence of pearlite led to poor hardness and wear resistance of modified high born cast steel. Quenching at temperatures between 900 and 950°C resulted in the decrease in pearlite and the increase in hardness and abrasion resistance in comparison with 900°C quenching. The metallic matrix all transformed into the martensite quenching at 1000°C; the modified high born cast steel had high hardness, tensile strength, impact toughness and excellent abrasion resistance. The hardness, tensile strength and impact toughness of modified high born cast steel had no obvious change quenching over 1000°C. The increase in quenching temperature also led to the transformation of boride from continuous shape to isolated shape and promoted the coarsening of boride.     

Microstructure of electromagnetic stir cast grain refined iron base alloy

Abstract

Electromagnetic stirring using multiple induction coils has been investigated as a method of preparing semisolid FC20 alloy. The effects of additions of titanium, zirconium, and boron on the microstructure of the FC20 alloy have also been studied. The electromagnetic flux density increased with input voltage and was higher at the wall of the container than near the centre of the coil. The microstructure of the alloys varied with cooling rate: the higher the cooling rate, the smaller the particle size and the greater the degree of sphericity. The additions of titanium, zirconium, and boron also improved the alloy microstructure.