Wear characterization of Ti–6Al–4V under fretting–reciprocating sliding conditions

Abstract

Wear affects life, vibrational characteristics, and dimensional stability of the components that slide against each other in small oscillatory movements. This study investigated the fretting–reciprocating wear behavior of a titanium alloy under cylinder-on-flat contact configuration (both Ti–6Al–4V) with bulk cyclic loading. Four series of tests were conducted to characterize the effects of sliding and loading parameters on the wear behavior. These were the relative slip, contact load, number of cycles, and bulk cyclic stress. The reciprocating wear produced “U-” and “W-”shaped (scar with multiple valleys) scars on the specimen as well as on the pad. Optical and scanning electron microscopy of the wear scars was conducted to study the wear mechanism(s). MATLAB-based computer programs were developed to determine the wear volume using the three-dimensional profilometry data, the product of contact load (P) and relative slip (δ), and the energy dissipated (E_d) per fretting cycle. Analysis of the experimental data showed that the cyclic stress increased the relative slip. The ratio of shear force to contact load (Q/P) increased with number of fretting cycles. The wear volume was proportional to the cumulative products of ΣPδ_n and the dissipated energy ΣE_d. The microscopy of W-shaped scars showed that the valleys were covered by abrasive grooves, whiles the peaks showed signs of adhesive wear, thus suggesting that the wear was caused by both the adhesion and abrasion processes.

Introduction

Fretting wear occurs when two contacting surfaces slide against each other with oscillatory motion. It results from undesirable vibration as well as misalignment between the mating bodies. Further, the surface damage is produced due to fretting action, which can be the result of several wear mechanisms acting individually or in combination, and these are corrosion, adhesion, and/or abrasion [1]. For instance, as the surfaces slide back and forth, adhesion will clean substrate and expose the underlying material to oxidation and corrosion [1]. Oxidized debris, broken free from the surfaces, can act as an abrasive and initiate three-body wear [1]. The surface damage produced by fretting is a function of the type of relative slip: partial or gross, however, the maximum wear damage is found to occur under gross slip [2], [3].

The capability to predict the geometry of fretting and reciprocating wear scars, such as the scar's width and depth, would be a valuable tool when designing mechanical components. For example, the prediction of the depth of a wear scar is important to estimate the life of a protective coating, which depends on the thickness of the coating [4], [5]. It may be noted that the contact geometry of the components undergoing fretting and sliding wear will be modified. It would also affect significantly the contact stresses (both normal and shear stresses) which could then reduce the fretting life. Therefore, the worn components may no longer be able to meet the design specifications due to changes in component geometry and stress state.

Therefore, the characterization of wear behavior for various materials under different fretting and sliding conditions is needed, and there have been several studies in this direction [2], [3], [6], [7], [8], [9], [10], [11], [12], [13], [14], [15], [16], [17], [18]. The present study investigates the fretting–reciprocating wear behavior of titanium alloy in contact with titanium alloy (both Ti–6Al–4V). This material is widely used in aerospace and biomedical industry due to its superior mechanical properties and relatively low density. One such application is the dovetail contact region of blade/disk attachments in gas turbines where fretting fatigue-induced damages, both cracking and wear, are commonly seen. There have been several studies on the fretting fatigue as well as fretting wear behavior of Ti–6Al–4V [3], [8], [14], [18], [19], [20], [21], [22]. The present study is an extension of previous similar studies, however, characterizes the wear under reciprocating conditions in the presence of cyclic load on the substrate (specimen) unlike the previous studies where substrate (specimen) had no direct load. This phenomenon is referred to as the fretting–reciprocating sliding condition in this study unlike the conventional plain reciprocating sliding condition. Furthermore, the present study involved a large amount of wear damage. These aspects were the main focus of the current study and are directed towards the design of the blade/disk attachments in gas turbines. For this purpose, four series of tests were conducted to characterize the effects of different sliding and loading parameters on the wear behavior under cylinder-on-flat contact configuration; and these parameters were relative slip, normal contact load, number of cycles, and bulk cyclic stress. In addition, the evolution of shapes of wear scar under these different test conditions were documented and analyzed.