Cavitation erosion of metals and alloys

Several high purity metals (aluminium, cobalt, chromium, copper, iron, magnesium, molybdenum, nickel, tungsten and zinc) and some commercial copper-, iron-, nickel- and cobalt-base alloys were investigated by weight loss measurements and by surface analysis with scanning electron microscopy and X-ray photoelectron spectroscopy. The results lead to the conclusion that the cavitation erosion resistance (CER) is decisively determined by the binding energy and the crystal structure of the base metal. The ability of alloys made of base metals with a high CER to deform and to transform allotropically is the second important factor.     

A comparison of liquid impact erosion and cavitation erosion

A comparison has been made between the response of several metals and alloys to multiple liquid jet impact and their response to ultrasonically induced cavitation. For the single-phase metals and alloys, namely aluminum, iron, Al-1%Cu, Al-1%Mg and Cu-30%Zn, the failure mechanism is the same for the two techniques. However, there are significant differences for Al-4%Cu and Al-9%Mg. The former alloy fails in cavitation by the growth of macroscopic fatigue-like striated pits, whereas under liquid impact the fracture appears more like transcrystalline cleavage. The differences in the Al-9%Mg alloy are even more dramatic: under cavitation the surface becomes covered with macroscopic striated pits, whereas under liquid impact failure is intercrystalline. Although the magnitude and duration of the pressure pulses produced by the two techniques are similar, all the materials studied required three to four orders of magnitude more impacts by avitation than by liquid impact for a given amount of erosion. The greater erosive power of the liquid impact is attributed to the shearing effects of lateral flow of the liquid after impact.     

The effect of stacking fault energy on the erosion behaviour of copper alloys at oblique impact

The major aim of this investigation was to determine the effect of the stacking fault energy (SFE) on the solid particle erosion rate at oblique impact angles. Such an investigation is essential to establish whether erosion is a deformation- or a fracture-controlled process. Towards this purpose, experiments were carried out at an impact angle of 30° on a series of copper alloys with a wide range of stacking fault energies using spherical steel shot travelling at a velocity of 40 m s^?1 as the erodent. The experimental results clearly show that the erosion rate decreases with decreasing SFE, thereby indicating that erosion is a deformation-controlled process. An inverse relationship between the erosion rate and the strain-hardening exponent is also observed. In addition, the erosion rate at 30° is lower than the erosion rate at 90°. Finally, it is demonstrated that the localization model for erosion is capable of rationalizing all these observations.     

Liquid impact erosion characteristics of martensitic stainless steel laser clad with Ni-based intermetallic composites and matrix composites

NiAl-Ni₃Al intermetallic composites (IC) and intermetallic matrix composites (IMC) with TiC and WC reinforcement were laser clad to increase the liquid impact erosion resistance of AISI 420 Martensitic stainless steel. Laser process parameter optimisation and pre- and post-heat treatment of the laser clad specimens were carried out to minimise porosity and sensitivity to crack formation. The coatings were characterised by optical microscopy (OM), scanning electron microscopy (SEM), X-ray diffraction (XRD) and energy-dispersive spectroscopy (EDS). The erosion resistance of the substrate material at a water droplet exit velocity of up to 150 m/s was improved from 116.9 to 838.7 min/mm³ for the nickel aluminide IC coating and from 855 to 1446 min/mm³ for the IMC coating with TiC and WC reinforcement. The pseudo-elasticity combined with the high work hardening ability was attributed to the excellent erosion resistance of nickel aluminide IC coatings. The IMC coatings with ceramic reinforcement extended significantly the initial resistance against liquid impact erosion. However, once damage occurred the erosion accelerated rapidly. No direct correlation could be established between the erosion resistance and the mechanical properties. The influence of hardness, elastic modulus, strain-hardening coefficient and the reversible penetration ratio on the erosion resistance was discussed.     

Adiabatic shear localization and erosion of strong aluminum alloys

A high strength aluminum alloy, 7075-T651, was eroded by ball-bearing impact at high velocities (about 180 m s^t?1) at normal and oblique incidence. Scanning electron microscope studies of the target and ejected chip show that the fracture surface is covered with myriad small globules (roughly 1 μm in dimension) whose form was clearly established by the effect of surface tension on a fluid metal. It is argued that for high strength age-hardened alloys like this, chips are formed by fracture across molten adiabatic shear bands which then quickly solidify after separation to give the globular surface. Such structures were not found on eroded surfaces of the much softer commercially pure aluminum.     

Resistance to wear and cavitation erosion of bearing alloys

Sliding wear and vibratory cavitation erosion tests in paraffin oil were carried out on bearing alloys, i.e. tin-based and lead-based white metals, Cu-Pb alloy and leaded bronze. In lubricated wear under mild conditions the surface is worn smooth and a slight difference exists between the wear resistances of the four alloys. In cavitation erosion an eroded surface which is much rougher than the worn surface is formed. Cavitation erosion is affected strongly by the composition and crystal structure of the alloy and thus the erosion resistances of the four alloys differ greatly, the ranking of resistance being lead-based white metal < Cu-Pb alloy < tin-based white metal < leaded bronze. The surface damage, which is caused by the joint action of cavitation erosion and wear, was also investigated by rubbing the eroded surfaces which had been exposed to cavitation erosion for various times. This damage becomes larger with increasing cavitation damage. The resistance to this damage differs much more in the four alloys tested and tends to correlate with the results of the erosion tests rather than those of the wear tests. Therefore, it is clear that the cavitation erosion resistance should be considered in the selection of bearing materials.     

An impact and erosion study of polyetheretherketone

Single-particle impacts by 4 and 5 mm diameter steel spheres on polyetheretherketone (PEEK) at various angles and speeds were studied using high speed photography and scanning electron microscopy. The following two material removal mechanisms were identified:

• 1.(1) the drawing out of filaments in oblique impact;
• 2.(2) material jetting in normal impact for speeds of approximately 480 m s⁻¹. A “deformation map” was constructed giving the types of crater formed by these spheres at given angles and speeds. Low speed (about 30 m s⁻¹) damage by quartz sand grains, sieved to give the size range 300–600 μm, was also studied. Various types of damage site were identified and the proportion of each for angles of impact ranging from 20° to 90° determined. The compressive stress-strain curve at a strain rate of 10⁴ s⁻¹ has been measured and a high speed photographic sequence of the deformation of a PEEK disc at a strain rate of approximately 2 × 10³ s⁻¹ is presented.

     

Liquid drop impact on solid surface with application to water drop erosion on turbine blades,Part II: Axisymmetric solution and erosion analysis

Based on the nonlinear mathematic model and computational method of liquid drop–solid impact established in Part I, in this paper, the quasi-3-D (axisymmetric) impact process is simulated and the results are specified for water drop impact on 1Cr13, with impact speed varying from 10 to 500 m/s. When dimensionless parameters are used to describe the impact procedure, the effect of water drop size is normalized. Both the transient pressure distribution in the liquid (including shock wave) and the transient stress distribution in the solid are obtained, and the magnitude and position of the maximum equivalent stress are given. The relationship between the most important parameters characterizing impact and incident speed is established, and simple formulae are fitted for the maximum stress, influence duration time, and influence zone size. Next, water drop erosion is analyzed for repetitive impact. With the statistics of water drop impact in a typical blade channel upon full and reduced load conditions, a simple fatigue model is employed to obtain the lifetime map on the blade surface, in terms of both impact times and operation hours. The most dangerous water drop erosion regions and operating conditions of the steam turbine blade are deduced. These results are useful to evaluate the water drop erosion mechanisms based on the fundamental solution of liquid–solid impact.     

Prediction of impact erosion in valve geometries

In this paper, the capability of computational fluid dynamics techniques is investigated to predict the rate of solid particle erosion in industrially relevant geometries. An Eulerian–Lagrangian model of the flow is used, in combination with empirically developed equations for the mass removal, to examine erosion in valve components for aqueous slurry flows. Two types of geometries were used: (i) a relative simple geometry with basic geometrical features similar to real valves and (ii) a geometrically complex valve (a choke valve). Predictions of flow coefficients and mass removal rates were directly compared with measurements from a parallel experimental programme. While flow characteristics and erosion locations were identified satisfactorily, erosion rates were seriously underestimated.     

Comparative tendencies for metal loss by abrasive wear,impact erosion and arc erosion

It has been shown that metals exhibit similar relative tendencies to metal loss by abrasive wear, impact erosion or arc erosion. High metal-metal bond energies in metals indicate high resistance to metal loss by any of these three processes.     

Cavitation erosion of Ti–Ni base shape memory alloys

Cavitation erosion tests were carried out by using Ti–Ni base SMAs (shape memory alloys) with the addition of third elements (Co, Fe, V and Cu) to Ti–Ni SMA. The erosion resistance and its mechanism are discussed. Erosion resistance of Ti–Ni base SMAs is about 1/4 to 1/2 as compared with that of Ti–Ni SMA, but it is about 6–16 times higher than that of SUS304. It was found that the erosion resistance of the martensite phase is superior to that of the austenite phase. We conclude that the erosion resistance of Ti–Ni base SMAs mainly depends on the defect density and the removal rate at the eroded area.     

Liquid drop impact on solid surface with application to water drop erosion on turbine blades,Part I: Nonlinear wave model and solution of one-dimensional impact

Water drop erosion is regarded as one of the most serious reliability concerns in the wet steam stage of a steam turbine. The most challenging aspect of this problem involves the fundamental solution of the transient pressure field in the liquid drop and stress field in the metal substrate, which are coupled with each other. We solve the fundamental problem of high-speed liquid–solid impact both analytically and numerically. In Part I of this paper, the governing equations based on a nonlinear wave model for liquid are derived. Analytical and approximate solutions of one-dimensional liquid–solid impact are given for both linear and nonlinear models, which provide critical insights into the water drop erosion problem. Both continuous and pulsant impacts on rigid and elastic substrates are analyzed in detail. During continuous impact, the maximum impact pressure is always higher than the water hammer pressure. Upon pulsant impact and at a particular instant related with the impact duration, the maximum tensile stress appears at a certain depth below the solid surface, which can be readily related with the erosion rate. In Part II of this paper, two-dimensional (axisymmetric) liquid–solid impact is solved numerically, from which the most dangerous impact load/duration time and the most likely crack positions are deduced. Based on our recent solution of the water drop impact statistics (associated with the fluid flow in the blade channel), a comprehensive numerical study of the water drop erosion (fatigue) on a turbine blade is carried out.     

The influences of macrosegregation, intermetallic particles, and dendritic spacing on the electrochemical behavior of hypoeutectic Al-Cu alloys

The purpose of this research is (1) to investigate the influence of Al(2)Cu intermetallic particles associated with the dendritic arm spacing on the corrosion resistance of different hypoeutectic Al-Cu alloys and (2) to evaluate the electrochemical behavior of a hypoeutectic Al-Cu alloy directionally solidified under unsteady-state heat flow. The as-cast samples were produced using vacuum arc remelting and vertical upward water-cooled solidification. Microscopic examinations were carried out with optical microscopy and scanning electron microscopy + energy dispersiveX-ray analyses. To evaluate the surface corrosion behavior of such alloys, all corrosion tests were performed in a 0.5-M NaCl solution at 25 degrees C using an electrochemical impedance spectroscopy technique and potentiodynamic polarization curve analysis. Based on the tests, corrosion rate and impedance parameters were obtained. The present research has underlined the use of appropriate techniques of characterization for determining Al(2)Cu distribution, morphology, and fraction within the typical microstructures of Al-Cu alloys. The experimental results have established correlations between the Al-rich phase dendritic arm size, the intermetallic particles distribution in the eutectic mixture, the macrosegregation profile, and the resulting corrosion resistance.     

Single solid particle impact erosion damage on polypropylene

The literature on the erosion of polymers is reviewed. Single-particle impacts by spheres of 4 mm diameter have been carried out at various angles and speeds on polypropylene. The crater types are classified and plotted on a “deformation map”. High speed photography was used to record the impacts and any material loss. Both ductile (the drawing out of filaments) and brittle (the fracturing of blocks of polymer) erosion processes were observed. The surface finish of the specimens is an important variable in the latter mechanism.     

Correlation between solid particle erosion of cermets and particle impact dynamics

Since the erosion rate depends on energy exchange between particle and material, a reformulation and solution of the equations of two solid bodies collision is presented and adapted to the calculation of the energy absorbed by a material surface during impact of a spherical particle onto a plain target. It has been observed that energy loss is a strong function of dynamic coefficients named as coefficient of velocity restitution after impact, k, and coefficient of dynamic friction, f. The new method and experimental equipment for the coefficients determination are described. It was shown that energy consumption during application may be an appropriate guide for the material selection in the conditions of erosive wear.     

Scaling laws governing the erosion and impact resistance of thermal barrier coatings

Airfoils coated with columnar thermal barriers have been removed from aero-engines and characterized. Observations of deformation and cracking have been used to identify three material removal mechanisms. These are: (i) sub-surface, trans-columnar cracking attributed to the initial elasto-dynamic response (nanosecond timeframes), (ii) segmented cone cracks when the ensuing penetration (millisecond timescale) remains elastic, (iii) local densification, accompanied by either kink bands or lateral cracks when the projectile penetrates plastically, especially at high temperature. Basic impact mechanics, combined with fracture criteria, have been used to devise scaling laws for these mechanisms. The results are combined to derive relations that characterize the thresholds for material removal and the transitions between major mechanisms (expressed in terms of a mechanism map). Some aspects of the material removal at kinetic energies above the thresholds are examined.     

Solid particle erosion of diamond coatings under non-normal impact angles

This paper describes an erosion study, which examines the effect of impact angle on the erosion behaviour of diamond coatings deposited on tungsten substrates by chemical vapour deposition (CVD). The coatings were 37–60 μm in thickness and were erosion tested using angular silica sand with a mean diameter of 194 μm at a particle velocity of 268 m s⁻¹. The impact angles used were 30, 45, 60 and 90°. The results show that the damage features, termed “pin-holes” are generated at all angles, though the number of impacts required for pin-hole initiation is significantly increased at lower angles. This work provides useful information in attempting to explain the mechanism by which damage is generated during the high velocity sand erosion of CVD diamond.     

Transmission and scanning electron microscopy studies of deformation at erosion impact sites

Scanning and transmission electron microscopy methods have been employed to study topographic features and subsurface damage associated with erosive-particle impact craters in annealed 310 stainless steel surfaces. Angular Al₂O₃ and spherical glass particles approximately 50 μm in diameter were projected at a velocity of 59 m s^?1 to impact the surface at attack angles of 90° and 20°. Under these conditions, material was found to be displaced but not removed from the surface at isolated impact sites. A comparison was made with damage produced at diamond pyramid hardness indentations. Substantial differences were not observed. In general, a high dislocation density zone a few microns wide was found to surround both impact craters and hardness indentations. The width of this zone varied according to the size and shape of the crater and the direction of particle motion. Deformation twinning occurred at some impact sites. The plastic strain associated with impact craters in 310 stainless steel and copper was also determined by a method that is based on an analysis of selected-area electron channelling patterns.