Predicting surface deformation during mechanical attrition of metallic alloys

Shan Cecilia Cao,Yongli Wang,San-Qiang Shi,Xiaochun Zhang,Robert O Ritchie,Jian Lu

doi:10.1038/s41524-019-0171-6

Abstract

Extensive efforts have been devoted in both the engineering and scientific domains to seek new designs and processing techniques capable of making stronger and tougher materials. One such method for enhancing such damage-tolerance in metallic alloys is a surface nano-crystallization technology that involves the use of hundreds of small hard balls which are vibrated using high-power ultrasound so that they impact onto the surface of a material at high speed (termed Surface Mechanical Attrition Treatment or SMAT). However, few studies have been devoted to the precise underlying mechanical mechanisms associated with this technology and the effect of processing parameters. As SMAT is dynamic plastic deformation process, here we use random impact deformation as a means to investigate the relationship between impact deformation and the parameters involved in the processing, specifically ball size, impact velocity, ball density and kinetic energy. Using analytical and numerical solutions, we examine the size of the indents and the depths of the associated plastic zones induced by random impacts, with results verified by experiment in austenitic stainless steels. In addition, global random impact and local impact frequency models are developed to analyze the statistical characteristics of random impact coverage, together with a description of the effect of random multiple impacts, which are more reflective of SMAT. We believe that these models will serve as a necessary foundation for further, and more energy-efficient, development of such surface nano-crystalline processing technologies for the strengthening of metallic materials.

Highlights

Deformation processes based on the impact of particles have been utilized extensively to harden surfaces, by imparting residual compressive stresses and/or by creating a work-hardened layer for the purpose of improving surface properties, such as resistance to fatigue or wear.1,2 More recently, these techniques have been used to make stronger engineering metal alloys,3–5 through the use of techniques such as air blast and ultrasonic shot peening,6 surface mechanical attrition treatment,7 particle impact processing,8 and surface nano-crystallization and hardening,9 to develop surface hardness and structural gradients
We further provide an analogous size, ball density, impact velocity and kinetic energy, and of the model to describe the phenomenon of full coverage by random resulting depths of the impact plastic zones; theoretical results are multiple impacts, which is most representative of SMAT
The following numerical relations are found: (a) the indent diameter D has a linear relationship with ball diameter dB, and a power-law relationship with both the normal impact velocity νn and ball density ρB, as described in Eq [3]; (b) the indent depth h, which describes the roughness of the component surface, has a linear relationship with the ball diameter dB and normal impact where dB, vn, ρB and Ek are the ball diameter, normal impact velocity, ball density and impact kinetic energy, respectively, with units listed in Table 3. k1, k2, k3 and k4 are material constants

Summary

Introduction

Deformation processes based on the impact of particles have been utilized extensively to harden surfaces, by imparting residual compressive stresses and/or by creating a work-hardened layer for the purpose of improving surface properties, such as resistance to fatigue or wear. More recently, these techniques have been used to make stronger engineering metal alloys, through the use of techniques such as air blast and ultrasonic shot peening, surface mechanical attrition treatment, particle impact processing, and surface nano-crystallization and hardening, to develop surface hardness and structural gradients. Deformation processes based on the impact of particles have been utilized extensively to harden surfaces, by imparting residual compressive stresses and/or by creating a work-hardened layer for the purpose of improving surface properties, such as resistance to fatigue or wear.1,2 These techniques have been used to make stronger engineering metal alloys, through the use of techniques such as air blast and ultrasonic shot peening, surface mechanical attrition treatment, particle impact processing, and surface nano-crystallization and hardening, to develop surface hardness and structural gradients. The technique was first introduced by Lu and Lu, who exploited an ultrasonic set-up to vibrate spherical balls actuated by high-power ultrasound.15 During this process, hard spherical balls are randomly impacted on surface of target materials, to establish nano-sized crystalline layers on a variety of metallic materials, including steels, nickel-based alloys and Mg-based alloys.. A comparison in Fig. 1i, j with other highstrength steels, namely dual-phase steels and TRIP-steels, of the tensile and yield strength properties of the SMAT-treated stainless steel as a function of their tensile ductility, clearly reveals that SMAT treatments have the capacity to radically enhance the damage-tolerant properties of medium- to high-strength steels in terms of much improved combinations of strength and ductility

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Conclusion