Influence of carbon nanotubes and dispersions of SiC on the physical and mechanical properties of pure copper and copper‐nickel alloy

This paper investigates the physical and mechanical properties of copper‐nickel alloy (at 50 wt.%–50 wt.%) and pure copper, mixed with various types of reinforcement materials such as carbon nanotubes (0.5 wt.%–2 wt.%) as nanoparticles, silicon carbide (1 wt.%–4 wt.%) as microparticles. The acquired composite specimens characteristics were estimated such as microstructure, density, electrical and thermal conductivity, hardness, and compression stress properties to determine the suitable reinforcement percentage that has the best physical and mechanical properties with different main matrix material whether copper‐nickel mechanical alloying or pure copper powder. The micron‐sized silicon carbide and nanosized carbon nanotubes were added to improve the mechanical and physical properties of the composite. The electrical and thermal conductivity of pure copper alloy enhanced compared with the copper‐nickel alloy matrix material. The hardness and compression yield stress of both pure copper and copper‐nickel composites have enhancement values and for copper‐nickel base composites hardness and compression yield stress have enhanced with the most positive enhancement values to examined an optimum percentage of reinforcing material.     

Influence of carburising on distortion behaviour

Case hardening is a common process to produce steel components, which are characterised by a hard surface combined with a ductile core. As a result of this treatment the parts show a good wear resistance as well as an increase in fatigue strength. High strengthened gear wheels can be mentioned as important examples, being treated in this manner. The case hardening process consists of a carburising and a hardening treatment. The carbon gradient in the surface results in a gradient in the transformation behaviour. The transformation to ferrite, pearlite and bainite is affected as well as the martensite transformation. Distortion due to these chemical inhomogeneities is added to distortion caused by temperature gradients and temperature gradient implied transformation stresses. In this project the effect of the carburising process on distortion behaviour is investigated. The influence of the microstructure before carburising, the carburising depth, the surface carbon content, the course of the process, the carburising temperature, the hardening temperature and the interaction between these parameters are investigated. Design of experiment methods are used to receive the effect of interactions between the varied parameters and to reduce the number of experiments. Additional investigations concern parameters, which can not be varied completely with the other parameters. The influence of low pressure carburising treatments and carburising at high temperatures, for example, are analysed in a spot‐check. The aim of the investigations is to find the decisive parameters affecting distortion in the carburising process, which are later on varied in a design of experiments plan containing the whole production line.     

Effect of particle size on the electrochemical corrosion behavior of X70 pipeline steel

In this study, by using a standard quartz replace of sandy soil particles, the effect of soil particle size (0.1–0.25 mm, 0.6–1.0 mm) on the electrochemical corrosion behavior of X70 pipeline steel in sandy soil corrosive environment simulated by 3.5 wt% sodium chloride (NaCl) was investigated through polarization curve and electrochemical impedance spectroscopy (EIS) technology. The results indicated that the polarization resistance of X70 steel decreased with decreasing particle size. For all polarization curves, the right shift of cathodic branch with decreasing particle size, suggesting that the cathode oxygen reduction process is accelerated. The corrosion of X70 steel is controlled by the process of cathode diffusion and oxygen reduction. This can be attributed to the effect of gas/liquid/solid three‐phase boundary (TPB) zone on cathodic process of X70 steel, and the corrosion rate is mainly determined by the cathodic reaction. EIS of X70 steel consisted of two capacitive loops with 7, 60 and 90 days buried corrosion, and the charge transfer resistance of X70 steel increased with increasing particle size.     

Magnesium addition in copper‐chromium alloys: The refinement of aging precipitates and improvement on mechanical properties

Herein, we demonstrate the synthesis of copper‐chromium and copper‐ chromium‐magnesium alloys by melting and casting process and explore the effect of the magnesium addition on mechanical and electrical properties of the alloys. This article focuses on the variation of the precipitation sequence and the decrease of strengthening phase sizes induced by the addition of trace magnesium element. The results show that magnesium element has little effect on the hardness of copper‐chromium alloy, but it significantly improves the hardness of the aging alloy and maintains high conductivity. The addition of magnesium element inhibits the growth and structural transformation of the precipitated phase. The refinement impact of magnesium addition on precipitated phase and change in alloy precipitation sequence may be the main reasons for the high hardness of copper‐chromium‐ magnesium alloy. In addition, the magnesium addition shows a significant refinement effect on small size precipitation phase, but it does not present the same refinement effect on large size precipitation phase. This attributes to the presence of a semi‐coherent interface between the matrix and the large size of precipitates, which provides the dislocation‐based diffusion channels for high‐rate chromium diffusion and promotes the precipitate growth.     

Studies on sliding wear characteristics of aluminium LM25/silicon dioxide functionally graded composite and optimisation of parameters using response surface methodology

This paper deals with the study of dry sliding wear of LM25/silicon dioxide (10 wt.%) functionally graded composite. The composite was fabricated using stir casting technique and the melt was poured into a horizontal centrifugal die rotating at 1200 min^?1. After casting, the specimen (length 150 mm, external diameter 150 mm and internal diameter 130 mm) was subjected to microstructure and hardness tests at three different depths from the outer periphery (1 mm, 8 mm and 13 mm). The results of the respective tests revealed that the outer periphery of the specimen had higher particle concentration and hardness. Then, wear test was done on a pin‐on‐disc tribometer at room temperature with the experiments designed using response surface methodology and by taking specimens of size 8 x 8 x 15 mm such that the surface undergoing wear was at 1 mm from the outer periphery of the cast. The process variables of load (10 ‐ 40 N), velocity (1 ‐ 4 m/s) and sliding distance (400 ‐ 1200 m) were varied using a level 5 design and experiments were carried on for 20 different optimal combinations. From the regression equation generated for the wear response, it was found that load had maximum effect on the wear rate. The confirmation experiments proved that the regression model could serve well in predicting the wear rate for the given ranges of the continuous factors, for the given composite. Surface plots showed that the wear rate had an increasing trend with respect to load, which was the dominating continuous factor. Though the wear rate increased, severe delamination of the functionally graded composite was delayed. The optimum levels of the continuous factors to minimize the wear rate were found using response optimisation and found to be 10 N, 1.7576 m/s and 2000 m respectively. Scanning electron microscopy analysis of the worn surface of the specimens connected to the obtained trends and thus further validated the model developed. Thus, a functionally graded LM13 composite with silicon dioxid reinforcements is developed and a wear model to predict its wear rate under different process parameters is proposed with predictions of optimal performance conditions. This composite can increase life of components of wear applications in aerospace and automobile industry.     

A prediction method for endurance of carburized components based on fatigue of corresponding material states with different carbon contents

Carburizing generates different material properties and residual stresses in the carburized layer. A material mechanics based model for endurance prediction of carburized steel components has been developed, in which thin walled tubes are used under cyclic internal and monotonic external pressures allow the determination of the material properties of the carburized layers. An unexpected independence of endurance and mean stress sensitivity from carbon content and hardness has been found, contrary to existing scientific knowledge. Component like specimens were tested to validate the endurance prediction model. The deviations of the predictions from the experiment are in the range of ±8 percent.