UCTEA Chamber of Metallurgical & Materials Engineers Proceedings Book

504 IMMC 2016 | 18th International Metallurgy & Materials Congress

Solid Particle Erosion Behavior of Cast CuCrZr Alloy Gülşah Aktaş Çelik, Ş. Hakan Atapek, Sinan Fidan, Şeyda Polat

Kocaeli University - Türkiye

Abstract

CuCrZr alloys are a group of precipitation hardenable high temperature copper alloys having good conductivity with moderate mechanical properties and can be used as liner material for engines. In service conditions, these alloys are subjected to solid particle impingements causing surface erosion, crack initiation and fracture. Thus, in this study, it was aimed to evaluate the solid particle erosion behavior of cast CuCrZr alloy against aluminum oxide particles at 30°, 60° and 90° impact angles. Surface degradation of tested alloys were characterized using light and scanning electron microscopes and optical profilometer. It was concluded that weight loss decreased with increasing impact angle since the mechanism of material removal changed from ploughing to work hardening.

1. Introduction

Precipitation hardenable CuCrZr alloys are used as main combustion chamber liners of regeneratively cooled rocket engines, lead frame integrated circuits, contact wire of high speed railways, heat sink of thermonuclear reactor divertor, spot welding electrodes, due to their high conductivity, good high temperature strength, creep resistance, low cycle fatigue resistance, good corrosion and tribological properties [1-6]. In service conditions, these alloys are exposed to elevated temperatures, high mechanical and tribological stresses. Especially when used as main combustion chamber liners, CuCrZr alloys are subjected to high temperature cycles and flow of various gases. This gas flow causes surface erosion, crack initiation and fracture due to solid particle impingements. Thus, in this study, it was aimed to evaluate the solid particle erosion behavior of a cast CuCrZr alloy against aluminum oxide particles at 30°, 60° and 90° impact angles.

2. Experimental Study

2.1. Material

Cu-0.97Cr-0.09Zr (wt. %) alloy was provided from Sa lam Metal Co. in as cast condition. Its initial microstructure is given in Fig. 1. Elemental Cr in globular morphology is observed in blue contrast in the light microscope (LM, Olympus BX41RF-LED) image (Fig. 1a) and marked with a circle in scanning electron microscope (SEM, Jeol JSM-6060) image (Fig. 1b). Needle like Cu5Zr phases exist also in the Cu matrix as shown in Fig. 1.a and b.

(a)

(b)

Figure 1. (a) LM, (b) SEM micrographs showing the microstructure of cast CuCrZr alloy. Etchant: 10g FeCl3, 50 ml HCl, 10 ml HNO3, 100 ml H2O.

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2.2. Solid particle erosion test

Solid particle erosion tests were performed at room temperature, on polished and cleaned surfaces, using a home-made solid particle erosion test setup illustrated in Fig. 2 [7]. The parameters of solid particle erosion tests are given in Table 1. Al2O3 particles (Fig. 3) were accelerated from nozzle to the sample surface using dry compressed air at three different impact angles. Specimens were weighed prior and following erosion tests, in order to determine weight losses. After the solid particle erosion tests, surface degradation of tested alloys were characterized using SEM and Nanovea PS50 model optical profilometer.

Figure 2. A schematic illustration of the setup used in erosion tests.

Table 1. Parameters of solid particle erosion tests.

Erodent particle

Size of erodent particle (mesh)

Impact angle

(°)

Pressure (bar)

Time (second)

30 60 Al2O3 120 90

2 10

Figure 3. SEM image of Al2O3 particles used as erodent material.

3. Results and Discussion

3.1. Effect of impact angle on wear

It is known that the erosion behavior of materials depend on not only whether they are brittle or ductile but also on the type, size and velocity of erodent particle, impingement angle, distance between nozzle and target and environmental conditions [7-9]. Several studies focusing on the effect of impact angle on erosion behavior, concluded that the erosion rate varied as a function of the plastic deformation capability of materials [7-12]. The erosion of ductile materials is higher at lower impact angles since the erodent particles produce a ploughing effect and cause material loss. Fig. 4 shows the effect of impact angle on weight loss of specimens, occurred in solid particle erosion tests. The diagram indicates that, the weight loss values increase by changing the impact angle from 30° to 90°. This can be explained by the change of wear mechanism from ploughing to work hardening as the impact angle increases. This effect will be discussed in Section 3.2.

Figure 4. Weight loss values of specimens occurred in solid particle erosion test.

3.2. Worn surfaces investigations

In erosion of ductile materials, a series of mechanisms take place resulting in removal of material from the surface. At the impingement of erodent particles, craters are formed in different locations on the surface and then material is displaced from the crater to form a raised lip. These displaced metals are deformed by subsequent impacts leading to lateral displacement of the material and causing some ductile fracture in heavily strained zones. After cyclic impingement of hard particles, the detachment of material from the strained surface continues causing weight loss [13].

Worn surface images of the samples tested at different impact angles are given in Fig. 5. The images exhibit

0 5

10 15 20 25 30 35

30 60 90

Wei

ght l

oss (

mg)

Impact angle ( )

UCTEA Chamber of Metallurgical & Materials Engineers Proceedings Book

506 IMMC 2016 | 18th International Metallurgy & Materials Congress

typical eroded features like ploughing, craters and lips, etc. expected from a ductile material and formation of these features can be clarified by the erosion mechanisms mentioned above. It is observed that erosion occurs at 30° impact angle with ploughing (Fig. 5.a). At 60° impact angle, ploughing is lower and formation of craters is higher than 30° case (Fig. 5.b). The erosion occurs by the platelet mechanism rather than ploughing at 90° impact angle as seen in Fig. 5c.

(a)

(b)

(c)

Figure 5. SEM micrographs showing the worn surfaces at (a) 30°, (b) 60° and (c) 90°.

The erosion occurs by creating three zones; the first zone is the central area where the material loss and depth of eroded region are maximum. In the second zone, material loss and depth are less and in the third zone, they are minimum due to the decrease in the

energy of impacting particles [8, 9]. Optical profilometric images showing the worn surfaces reveal clearly both the effect of impact angle on the eroded region and the transition from one zone to the other (Fig. 6). Specimen eroded at 30° impact angle has the highest depth and widest eroded area (Fig. 6.a) while specimens eroded at 60° and 90° impact angles have lower depths and narrower eroded areas (Fig. 6.b and c). These observations are in good agreement with the weight loss values given in Fig. 4.

(a)

(b)

(c)

Figure 6. Optical profilometric images showing the worn surfaces (a) 30°, (b) 60° and (c) 90°

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4. Conclusions

In this study, solid particle erosion behavior of cast CuCrZr alloy was investigated and it was concluded that (i) in cast microstructure CuCrZr alloy had elemental Cr and needle-like shaped Cu5Zr precipitates in Cu matrix, (ii) weight loss and depth of eroded region decreased with increasing the impact angle due to the change of erosion mechanism on the surface, (iii) while ploughing was the main mechanism at 30° impact angle, plastic deformation and platelet mechanism became dominant as the impact angle increased.

ACKNOWLEDGEMENT

The authors are grateful to Sa lam Metal Co. for providing the samples used in the study.

REFERENCES

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