Improving fretting fatigue behaviour of Al 7075-T6 clamped plates using surface coatings

Abstract

In this thesis, the fatigue behaviour of aluminium alloy 7075-T6 in bolted joints has been studied where the joint plates may suffer from fretting fatigue phenomenon under applied clamping forces and oscillatory displacements. The main aim was to improve the fretting fatigue resistance of the aluminium joint by means of surface coatings method. First, the isolated effect of the bolt clamping force on the fatigue strength of a bolt-filled hole was studied in the absence of fretting. The results showed beneficial compressive stresses around the aluminium bolted holes with localised maximum magnitudes at the critical edge of the hole where there is a high tensile stress concentration as a result of the applied tensile load. The presence of the clamping compressive stresses was confirmed at different environmental temperatures of -50, 25 and 60 °C as typical flight temperatures. The compressive stress clearly reduced the net effect of the damaging cyclic longitudinal tensile stress which causes fatigue crack initiations and propagation due to the applied loads. Therefore, the fatigue strength of the bolted plates was substantially improved using higher tightening torques at the three tested environmental temperatures. In the second part of this study, the fatigue behaviour of Al 7075-T6 plates coated with two different coating materials was investigated. Titanium nitride (TiN) and nickel-phosphorous (Ni-P) coatings were deposited onto the aluminium substrate using physical vapour deposition (PVD) and electroless nickel (EN) plating processes, respectively. These coating materials were selected because of their favourable characteristics mainly for fretting fatigue considerations such as high hardness, low friction and good wear resistance. However, high operating temperature of the PVD process reduced the mechanical properties as well as fatigue strength of the aluminium substrate coated with TiN. Therefore, a post heat treatment was developed to restore the lost properties. Although the applied heat treatment substantially improved the tensile and fatigue strengths of the coated material, the lost properties were not fully recovered to achieve the original properties of Al 7075-T6. A double-lap bolted joint specimen was then designed and manufactured from the aluminium plates and subsequently coated with Ni-P coatings with a high phosphorous content of 10-13 wt%. Different tightening torques were applied to clamp the plates together with the aim of studying the effect of clamping force on the fretting fatigue life of the coated joints. Ni-P coatings were successfully found to protect Al 7075-T6 clamped plates against fretting fatigue damages even at low fatigue loads where the uncoated joints showed a large reduction in their fatigue life due to the fretting effects. Fretting fatigue life of moderately and firmly clamped plates was improved by 35% and 59% respectively after the application of Ni-P coatings.