Waterjet Machining and Peening of Metals | Part1 Introduction

The development of high-performance materials, such as composites and advanced ceramics, presents a variety of manufacturing challenges. Many of these materials cannot be effectively or economically machined by conventional methods, and therefore require methods of shaping and/or postmold processing with unique sources of material removal. Various methods are currently used in machining and surface treatment of structural monolithic alloys.

Apart from economics, the means for process selection is often based on the machined surface integrity. The high-pressure waterjet with abrasive additives known as abrasive waterjet ~AWJ! is one viable alternative to conventional processing and has been suggested for use in postmold shaping of composite and other hard-to-cut materials @1–4#. Omni-directional cutting potential as well as minimal thermal and mechanical loading are just a few of the advantages realized when cutting with a water-driven abrasive slurry.

Despite current and a continuing development of interest in this machining process, only a limited understanding of material removal mechanisms in WJ and AWJ cutting exists. Furthermore, the influence of material properties on the mechanisms of material removal and change in mechanisms with cutting depth has not been well understood. Hence, a comprehensive analysis of the surface integrity resulting from high-pressure WJ machining of metals was necessary.

Mechanisms of material removal present in AWJ machining are often described with terminology from the studies of solid-particle impact-induced erosion. Hashish @3,5# suggested from a visualization of the cutting process in plexiglas that material removal occurs by cutting wear and deformation wear, terms used by Finnie @6# and Bitter @7,8# in describing abrasive-induced erosion. Cutting wear defines erosion at small angles of particle impact and occurs when the shear strength of the material is exceeded due to abrasive particle shear loading. The ‘‘cutting wear zone’’ on an AWJ machined surface is regarded as the uppermost portion of the eroded kerf which exhibits high-quality surface texture with limited macroscopic variation. Deformation wear erosion as defined by Bitter @8# is material removal by repeated particle bombardment at large impact angles ~greater than 20 deg!. In this mode of abrasive removal, the parent material is plastically deformed, locally work-hardened with continual bombardment, and eventually removed due to plastic embrittlement. The ‘‘deformation wear zone’’ in AWJ cutting exists below the cutting wear zone and is typically identified by waviness or striation patterns caused by severe jet deflection. Waviness patterns have been noted not only in AWJ machining, but also in other beam cutting processes like plasma, electron, and lasers. A variety of materials with uniquely different mechanical properties, both ductile and brittle, exhibit these characteristics.

Distinction between the cutting wear and deformation wear mechanisms of material removal in AWJ studies are commonly differentiated by waviness patterns on the kerf. Based on our extensive experimental work at the University of Washington for the past ten years, we found the formation of waviness patterns are a function of the energy of the impinging jet @9–11#, not the mechanisms of material removal @12–14#. An understanding of the mechanisms of material removal in high-pressure WJ and AWJ machining process is extremely important due to their effect on structural performance. External forces applied to create a new surface through mechanical work can result in a near sub-surface stress field. Burnham and Kim @15# found that the cutting force during AWJ machining of alumina and steel increased with jet penetration depth. An increase in cutting force may influence the size and distribution of the residual stress field near the surface in a material susceptible to work hardening. For materials where
plastic deformation or brittle fracture occurs, surface properties of the workpiece material may change during material removal.

In this study, we will review our investigations on common ductile metallic materials, which were treated and machined with WJ and AWJ. Machining tests were conducted with numerous
parametric combinations to promote varying degrees of cutting wear and deformation wear zones on the machined surfaces. A thorough visual analysis was carried out with scanning electron microscopy to distinguish and differentiate mechanisms of material removal as a function of cutting depth and material properties. Material removal is discussed with regard to machine parameters, material properties, and as a function of the standoff distance in WJ impacting and depth of cut in AWJ machining, respectively.

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