Application of Magnetic Field Treatment to Enhance the Durability of Metallic Alloys under Cyclic Loads

Abstract

This research studies the effect of alternating magnetic field treatment on fatigue and wear properties of EN8 steel, 70/30 brass alloy, nickel-aluminium bronze, and aluminium alloy 2014-T6. Cavitation erosion, fatigue and sliding contact wear tests were used as a means to compare the performance of the untreated and treated materials. This thesis describes the effect of the treatment on the following; (i) cantilever rotating fatigue behaviour of EN8 steel and AA2014-T6 aluminium alloy, (ii) friction/wear behaviour of EN8 steel, nickel-aluminium bronze and AA2014-T6 aluminium alloy and (iii) cavitation erosion behaviour of EN8 steel, nickel-aluminium bronze, 70/30 brass and AA2014-T6 aluminium alloy. Within this research, an alternating magnetic field treatment rig was built to treat metallic alloys.

The application of an alternating magnetic field (0.54 T) was observed to lead to an improvement in the cantilever rotating fatigue endurance of both EN8 steel and aluminium AA2014-T6 alloy. An increase in Vickers microhardness and tensile strength of both alloys in the treated condition was also observed. Fractography by using scanning electron microscopy showed evidence of more ductile fracture features after treatment in contrast to the untreated samples. The results of X-ray diffraction indicated the formation of more compressive residual stresses following treatment, while examination by transmission electron microscopy showed evidence of fewer dislocations compared to the untreated state. In the case of the AA2014-T6 alloy, Guinier–Preston (GP) zones and theta prime were also generated by the alternating magnetic field. However, the recorded increase in temperature during the treatment range between 12-14 °C and this was not high enough to explain these observations. Therefore, the results were attributed to a non-thermal effect of the treatment, due to the change in the magnetic field direction and the change in polarity leading to depinning and movement of dislocations.

Application of an alternating magnetic field treatment (1.24 T) led to improvement of the wear/friction properties of EN8 steel, nickel-aluminium bronze and aluminium alloy AA2014-T6. To investigate the wear and friction properties, pin-on-disc wear tests were conducted. The pin-on-disc tests were under lubricating conditions using a AISI52100 steel ball bearing as the counterface material and showed a reduction in the width and depth of wear scars as well as lower values of the coefficient of friction following the treatment. In addition, nanoindentation tests, before and after treatment, revealed an increase in the hardness and Elastic Recovery Parameter. Examination by means of transmission electron microscopy attributed these results to increased dislocation mobility and migration to the surface as a result of alternating magnetic field treatment leading to an increase in the Vickers microhardness and to a change in the residual stress state at the surface. In addition, transmission electron microscopy also revealed increased precipitation of κIV for nickel-aluminium bronze and increased precipitation of GP zones and theta prime for Aluminium Alloy 2014-T6 in the treated conditions.

The final part of the thesis investigated the effect of alternating magnetic field treatment (0.76 T) on the cavitation erosion properties of EN8 steel, nickel aluminium bronze, 70/30 brass and aluminium alloy 2014-T6. Cavitation erosion testing (ASTM G32 10) was fulfilled at a frequency of 20 kHz in deionized water. The results showed a significant improvement in the cavitation erosion resistance for all samples treated by alternating magnetic field treatment. In the analysis of the results, the different magnetic nature of the tested alloys was taken into consideration. Scanning electron microscopy of the eroded surfaces were studied. Furthermore, microhardness measurements before and after treatment were also studied. The results of X-ray diffraction indicated the formation of more compressive residual stresses following treatment, while examination by transmission electron microscopy showed evidence of increased dislocation mobility and in some alloys increased precipitation.

From the research work demonstrated in this PhD project, the use of this alternating magnetic field treatment is a promising method for improving the mechanical properties of metallic alloys.