Dry Sliding Wear Behavior of Cast A7075 and A7075/SAF 2205 Composite Material

Materials

A7075 aluminum alloy and SAF 2205 duplex stainless steel chips were selected as matrix and reinforcing materials, respectively. Tables 1 and 2 represent the chemical compositions of A7075 and SAF 2205 alloys.

Casting and heat treatment applications

The solid investment casting mold was prepared with a cylindrical wax pattern having 21 mm diameter and 50 mm height. The wax pattern was fastened into a rubber flask base and a stainless steel perforated flask was placed on the base. Perforated flask holes were covered with an adhesive band. Plaster bonded (plaster/silica) commercial investment powder was mixed into water using 0.40 powder/water ratio. The slurry was filled into the flask under vibration and the flask was placed into an electrical furnace for dewaxing and burnout processes. To obtain successful burnout regime, the mold was heated up to 700°C gradually. The steel-based preform was placed into mold just 10 min before casting for preheating of the preform without excessive oxidation. Casting process illustrated in Figure 1 was carried out at 830°C using 10⁻⁵ Pa pressure. Age hardening was carried out on the composite specimens to eliminate hardness difference between the matrix and reinforcing materials. Solution heat treatment was performed at 460°C for 2 h and age hardening was performed at 190°C for 5 h.

Figure 1:

Casting process used in this study.

Hardness test

The effect of the SAF 2205 duplex stainless steel chips on the mechanical properties of A7075 alloy was investigated with Vickers hardness tests using hardness tester Bulut HVS1000. Hardness tests were carried out under 300 g test load and average of five measurement was reported. The hardness values were determined as 162.01 ± 10 HV and 269.67 ± 9 HV for cast and composite materials, respectively.

Wear test under dry sliding condition

For tribological characterization wear tests were performed under dry sliding condition and wear characteristics were then determined based on the appearances of worn surfaces and wear-induced loss amounts. Wear tests were realized using a “ball-on-disc” type tribometer (Nanovea). Related test parameters are listed in Table 3. Volume loss and specific wear rate values were calculated according to ASTM G99-05 standard using optically measured wear track dimensions for both of the tested materials.

Metallographic sample preparation and microscopic examinations

All samples for the microstructural characterization were prepared by grinding with 320, 600 and 1,000 mesh size SiC abrasives, respectively, and then ground surfaces were polished with 3 μm and then 1 μm diamond solution. Etching was carried out with Keller reagent in order to determine the phases within the matrix. Zeiss Axiotech 100 light microscope (LM) and Jeol JSM 6060 scanning electron microscope (SEM) were used for both metallographic and worn surface examinations. IXRF model energy dispersive x-ray spectrometer (EDS) was used for characterization of the phases.

Microstructure

Figure 2 shows the microstructures of A7075 material where the matrix consists of typical dendritic structure with several intermetallics formed in interdendritic spaces (Figure 2(a)). The Al-Zn-Mg precipitates provide the basis for the 7xxx wrought and cast alloys. Two phases can form by eutectic decomposition in commercial Al-Zn-Mg alloys: MgZn₂ (ɳ) and Al₂Mg₃Zn₃. Depending on the zinc/magnesium ratio, copper-free alloys are strengthened by metastable precursors to either MgZn₂ or Al₂Mg₃Zn₃ (ɵ). In Al-Zn-Mg-Cu alloys, copper and aluminum substitute for zinc in MgZn₂ to form Mg(Zn,Cu,Al)₂. Particles of Al₂CuMg can also form in these alloys by eutectic decomposition and solid-state precipitation [23–25]. In addition to these intermetallics, Al₇Cu₂Fe, Al₂₃CuFe₄, Al₆Fe, and Mg₂Si particles have been observed in A7075 alloy in other studies [26, 27]. EDS analyses of the sample show that the dark particles marked by black arrows in the image contain Mg, Zn and Si, suggesting that they are Mg₂ (Zn, Si). The matrix has two types of bright particles marked by white arrows: (i) the particles in the form of Chinese script contain Al, Fe, Mg, Zn and Cu, (ii) the particles having globular morphology contain Al, Mg, Zn and Cu (Figure 2(b)). The variation of microstructural features during aging has not been studied; however, it was well known that the precipitation sequence in A7075 alloy is described by GP zones→ ɳ’ → ɳ and the high mechanical strength developed by the alloy in the T6 temper is associated with high density metastable precipitates, distributed homogeneously in the aluminum matrix [24, 28–31].

Figure 2:

(a) LM and (b) SEM micrographs showing the cast structure of A7075 alloy. The matrix consists of dendritic α phase including several intermetallics.

The composite material was also characterized by LM and SEM examinations (Figure 3). Two principal precipitates are formed during the solidification of the cast samples due to the interdiffusion of aluminum and steel. Figure 3(a) and 3(b) shows one of these precipitates formed in plate/needle-like shape. The presence of chromium as a major alloying element in the steel matrix leads to the formation of ternary phases and EDS analyses indicate that the phase is Fe₂Al₁₃Cr₂. Another precipitate is determined as an intermetallic layer around steel matrix and EDS analyses point to Fe₅Al₄Cr₂ intermetallic (Figure 3(c)). As it is well known, these intermetallic phases are harder and more brittle than the matrix and the nature of the interface between the steel and Al-matrix in the cast composite metal was found to have a prominent effect on their deformation behavior [32].

Figure 3:

(a) LM and (b and c) SEM micrographs showing the composite structure.

Evaluation of wear test data

Figure 4 shows the variation of friction coefficient values as a function of distance for two different counterpart materials for cast A7075 and A7075/SAF 2025 composite materials. With both 100Cr6 and ZrO₂ interaction, A7075 alloy has a higher friction coefficient than that of A7075/SAF 2025 composite material. The diagram given in Figure 4(a) indicates that the friction coefficient value for A7075 alloy is in the range of 0.10–0.20; however, this value is in a narrower range like 0.12–0.15 for the composite material. As mentioned in Section “Microstructure”, both cast and its composite structures consist of several intermetallics giving strength to the matrix. Composite material exhibits lower friction coefficient than the cast alloy due to its higher hardness. However, the effect of hardness on the wear is complicated by the fact that different wear mechanisms can prevail depending on the microstructure and also operating conditions. The wear intensity may decrease by increasing the hardness of the materials in contact [33]. On the other hand, when a harder counterpart material is selected like ZrO₂, the friction of coefficients reaches to higher values and this concept suits for both materials studied, as illustrated in Figure 4(b). As it is well known, in sliding of a hard ball on a plate, frictional forces become a function of increased shear strength and decreased contact area making the friction coefficient higher [34]. Figure 5 shows the change of volume loss in wear and specific wear rate of A7075 and composite materials as a function of counterpart material. The diagram clearly indicates that values of wear loss and wear rate measured for cast alloy are higher than those of composite material in interaction against both counterparts; however, the harder counterpart (ZrO₂ ball) causes more wear. Some characteristics such as roughness, plastic deformation, grooving, tendency to adhesion during sliding, surface fatigue, tribochemical reactions affect the wear [34]. The examination of worn surfaces reveals the wearing features which will be evaluated in Section “Examinations on worn surfaces”.

Figure 4:

The diagram showing the variation of friction coefficients as a function of distance with the interaction of 100Cr6 (a) and ZrO₂ counterpart (b).

Figure 5:

The diagram showing the variation of wear loss and specific wear rate of A7075 and A7075/SAF 2025 composite materials with the interaction of 100Cr6 and ZrO₂ counterpart in dry sliding conditions.

Examinations on worn surfaces

In order to understand the wear characteristics under dry sliding condition, the worn surfaces of the materials were investigated using SEM. In sliding contact, surface damage is caused by abrasion, adhesion, surface fatigue and also tribochemical reactions [29]. Figure 6 shows the surface degradation on the materials. The materials exhibit typical grooves and adhesion layers on the worn surfaces contacted with 100Cr6/ZrO₂ ball. Grooves resulting from micro-cutting are evident in both non-reinforced and reinforced structure indicating abrasive wear (Figure 6(a) and 6(c)). In abrasion, micro-cuttings, fatigue due to repeated ploughing, fracture due to microcracking (Figure 6(b)) of the base body (A7075 or A7075/SAF 2025) caused by the counterpart’s hard asperities or by hard particles such as intermetallics removed from the matrices in the interfacial medium lead to wear. During the test, oxidation occurs and micro-crack coalescence causes fracture of the oxidized surfaces. Figure 6(a) and 6(c) gives good evidences to understand the wear resistance of two different materials. In Al-matrix, the width of wear track is wider than that of steel part. As given in Figure 5, wear rates were evaluated as a function of both matrix and counterpart type. A7075 has higher wear rate than that of its composite contacted with harder metal/ceramic counterpart and ceramic-based counterpart increases the rate by a factor of 2. The worn surfaces also indicate that ceramic counterpart causes severe wear by forming adhesion layers (Figure 6(b) and 6(d)). In adhesion, after possibly extant protective surface layers have been broken through, microwelds form above all on the plastically deformed microcontacts between the base body and counterpart. If the strength of the adhesive bonds is greater than that of the softer friction partner, material eventually detaches from the deformed surface of the softer friction partner and is transferred to the harder one (metal/ceramic ball). The transferred material can either remain on the harder friction partner or detach or even return as given in Figure 6.

Figure 6:

SEM micrographs showing the worn surfaces of cast A7075 (a and b) and A7075/SAF 2025 composite (c and d) materials contacted with 100Cr6 (a and c), ZrO₂ counterpart (b and d).