Electrodeposition of high performance multilayer coatings of Zn–Co using triangular current pulses

Abstract

Compositionally modulated alloy (CMA) coatings of Zn–Co were electrodeposited on to mild steel from an acid chloride bath containing thiamine hydrochloride, as an additive. Electroplating was carried out galvanostatically from a single bath containing Zn²⁺ and Co²⁺ ions. Gradual change in composition in each layer was effected by triangular current pulses, cycling between two cathode current densities. Compositionally modulated alloy coatings were developed under different conditions of cyclic cathode current density and number of layers, and their corrosion resistances were evaluated by potentiodynamic polarisation and electrochemical impedance spectroscopy. The formation of multilayer and corrosion mechanism was analysed using scanning electron microscopy. The corrosion resistances of CMA and monolithic alloy coatings were compared with that of the base metal. Compositionally modulated alloy coating at optimal configuration, represented as (Zn–Co)_{2·0/4·0/300}, was found to exhibit ～80 times better corrosion resistance compared with monolithic (Zn–Co)_3·0 alloy, deposited for the same length of time from the same bath. Improved corrosion resistance was attributed to the formation of n-type semiconductor film at the interface, supported by Mott–Schottky plots. Decrease in corrosion resistance at high degree of layering was found, and is due to lower relaxation time for redistribution of solutes in the diffusion double layer, during plating.     

Corrosion resistance of Ni/Al2O3 coatings in NaCl solution

Abstract

This paper presents results of a research on the corrosion resistance of composite Ni/Al₂O₃ coatings electrochemically deposited from Watts baths containing different amounts (0, 20, 100 g dm^?3) of Al₂O₃ particles. Potentiodynamic tests and electrochemical impedance spectroscopy (EIS) measurements were carried out in a 3% solution of NaCl. The coatings with about 6 wt-% of corundum, deposited from a bath with 100 g dm^?3 of a powder, showed the best protective properties. The rate of corrosion of such coatings after 7 days of exposure in the NaCl solution was over two times slower than that of coatings containing 2 wt-% of Al₂O₃ and six times slower than that of a standard nickel coating. Two equivalent circuits: one consisting of two RC circuits and the other one made up of three RC circuits were used for the analysis of the impedance spectra. Regardless of the presence and amount of the Al₂O₃ particles in the nickel coating, during first day of exposure in the NaCl solution a layer of nickel oxides and hydroxides forms on the surface of the coatings increasing their corrosion resistance. In the case of coatings with 6 wt-% of Al₂O₃, the passive layer is least vulnerable to the aggressive action of Cl^? ions.     

Mechanical and fatigue behaviour of laser and resistance spot welds in advanced high strength steels

Abstract

A study was carried out on laser and resistance spot welds in overlapped sheets of dual phase advanced high strength steel (DP780) and deep drawing steel (DC04) of 2˙0 mm in thickness. The aim of the study was to investigate the fatigue performance of these joints under tensile shear loading as well as the monotonic performance for applications in the automotive industry. The mechanical properties, failure behaviour and fatigue life analyses of spot welds in similar and dissimilar joints were investigated by experimental and numerical methods. The structural stress concept was used to describe the fatigue lives of spot welded specimens. The results revealed different failure types with different fatigue behaviours for laser and resistance spot welds under the application of cyclic loads at 'high load' and 'low load' levels.     

Numerical simulation of spot welding for galvanised sheet steels

Abstract

Galvanised sheet steels are now widely used to be the substrate for body in white (BIW) construction in the automotive industry. Weldability of galvanised sheet steels much worsened compared to spot welding of low carbon steels. The present paper develops a 2D axisymmetric model and employs an incremental coupled thermal–electrical–mechanical analysis to predict the nugget development during resistance spot welding (RSW) of galvanised sheet steels. Temperature dependent contact resistance for faying surfaces was determined to take into account of the influence of zinc coat for spot welding galvanised sheet steels. The effect of dynamic contact radii on temperature distribution was studied and compared with results under constant contact area assumption. The predicted nugget shape and size agreed well with the experimental data. Higher current and longer welding time should be applied for galvanised sheet steels compared to low carbon steel spot welding. The proposed model can be applied to predict weld quality and choose optimal welding conditions for spot welding galvanised sheet steels.     

Effects of electrical current on transport processes in resistance spot welding

Abstract

The effects of alternating current (ac) and direct current (dc) on cooling rate, solute distribution and nugget shape after solidification, which are responsible for microstructure of the fusion zone, during resistance spot welding, are realistically and extensively investigated. The computer program developed by Wang and Wei is used to predict transport variables in workpieces and electrodes during heating, melting, cooling and freezing periods. The model accounts for electromagnetic force, heat generations at the electrode/workpiece interface and faying surface between workpieces, and dynamic electrical resistance including bulk resistance and contact resistances at the faying surface and electrode/workpiece interfaces, which are functions of hardness, temperature, electrode force and surface condition. The computed results show that in contrast to dc, using ac readily produces the nugget in an ellipse shape. Deficit and excess of solute content occur in a thin layer around the boundary and interior of the nugget respectively.     

Microstructure characteristics of resistance spot welds of AZ31 Mg alloy

Abstract

The experimental investigation was carried out to study the weld microstructure of resistance spot welding of AZ31 Mg alloy 1 mm thick. A fine and homogeneous non-equilibrium microstructure of globular α grains, surrounded by eutectic mixtures of α and β (Mg₁₇Al₁₂), was achieved. The thermal–electrical–mechanical analysis model was employed to simulate the thermal history and the temperature gradient. It was found that a combination of the welding conditions and the particular thermophysical properties of the AZ31Mg alloy established a uniform temperature distribution throughout the weld pool and this thermal condition is ideal for nucleation throughout the melt metal and equiaxed grain structure forming.     

Alternative methodology for on site monitoring of corrosion and remediation of reinforced concrete

Abstract

Difficulties associated with the interpretation of site data collected over long periods of time from commonly used corrosion monitoring techniques for steel reinforcement in concrete, such as corrosion potential measurements and linear polarisation resistance, often make it difficult to assess accurately the extent of corrosion. An alternative methodology for the interpretation of data is proposed, based upon a model of the quality of the passive film on the steel surface. This model leads to a representation of the corrosion state by means of the relationship, over a long period of time, between the corrosion potential and the logarithm of the linear polarisation resistance. For the reinforced concrete panels tested in this study, data points representing this relationship closely fitted a family of results. This led to the development of a so called 'monitoring control diagram', MCD, in which for a fixed geometry and fixed experimental conditions, a relationship between the corrosion potential and polarisation resistance of steel exists. The establishment of an MCD may enable the development of a useful monitoring tool.     

Synthesis and some properties of Ti3SiC2 by hot pressing of Ti,Si and C powders Part 2 – Mechanical and other properties of Ti3SiC2

Abstract

Some properties of the remarkable Ti₃SiC₂ based ceramic synthesised by hot pressing of elemental Ti, Si, and C powders have been investigated. Its flexural strength by using three point bending tests and fracture toughness by using single edge notched beam tests were measured at room temperature to be in the range 310–427 MPa and about 7·MPa m^1/2, respectively. This material is a relative 'soft' ceramic with a low hardness of 4 GPa. Ti₃SiC₂ is similar to the soft metals and is a damage tolerant material that is able to contain the extent of microdamage. An oxidation test has been performed in the temperature range 1000–1400°C in air for 20 h. The oxidation resistance below 1100°C was good. Two oxidized layers were formed, the outer layer consisting of pure rutile-type TiO₂, and the inner layer a mixture of SiO₂ and TiO₂. The average coefficient of thermal expansion (CTE) of Ti₃SiC₂ was measured to be 9·29 × 10^?6 K^?1 in the temperature range 25–1400°C. The thermal shock resistance of Ti₃SiC₂ was evaluated by quenching the samples from 800°C, 1200°C, and 1400°C, respectively. The retained flexural strength drops dramatically at quenching temperature, but shows a slight increase after quenching from 1400°C compared with quenching from 800°C and 1200°C.     

Directional solidification of Ni base superalloy IN738LC to improve creep properties

Abstract

The high Cr, Ni base superalloy IN738LC has been directionally solidified on both laboratory and industrial scales using Bridgman and liquid metal cooling (LMC) methods respectively. In the Bridgman experiments, cylindrical rods were grown using a graphite chill with temperature gradient G = 5·0 K mm^-1 and a water cooled copper chill with G = 8·5 K mm^-1, and a wide range of withdrawal rates of R = 60, 120, 240, 600, and 1200 mm h^-1. In the LMC rigs, several turbine blades were grown using a wide range of withdrawal rates of R = 120, 225, 330, 420, and 630 mm h^-1. Grain and dendritic structures in both cylindrical and turbine blade specimens were evaluated in longitudinal and transverse directions. Dendritic segregation of rods was determined with SEM as a function of processing parameters. Some specimens were given a two stage heat treatment followed by tension tests at 25 and 650°C and creep tests at 152 MPa and 982°C, 340 MPa and 850°C, and 586 MPa and 760°C. It was shown that at R = 600 mm h^-1 with water cooled copper chill, directionally solidified rods with a well orientated dendritic structure and better segregation pattern gives higher tensile properties at 25°C and creep properties at 340 MPa and 850°C. Tension and creep tests of turbine blades showed that although the yield and tensile strength of directionally solidified specimens are in the range of conventionally cast ones, the creep properties of the blades have been significantly improved using the LMC process.     

Phenol–formaldehyde resins modified by copper nanoparticles

Abstract

Phenol–formaldehyde (PF) resins modified by copper nanoparticles were synthesised by in situ polymerisation process. X-ray diffraction (XRD), transmission electron microscopy (TEM) revealed that nanosized copper particles were well dispersed in the resulting PF resins. The thermal properties of the prepared PF resins were investigated by thermogravimetric analysis (TGA). It was indicated that copper nanoparticles remarkably improved the thermal stability of the PF resins at lower temperature. However, the copper nanoparticles increased the rate of the degradation of the PF resins at the elevated temperature. The toughness and the tribological properties of the friction materials based on the prepared PF resins were also studied. The results showed that copper nanoparticles obviously improved the brittleness and the tribological performances of the friction materials.