An Investigation on the Microstructure and Wear Properties of TiB 2 Reinforced AA2014 Aluminium Alloy Produced by Vacuum Infiltration Şule Ocak Araz, Recep Çalın, Muharrem Pul, Osman Bican and Onur Okur* Metallurgical and Materials Engineering Department

1.INTRODUCTION Aluminium-based composites reinforced with hard ceramic particles have received considerable interest because they can exhibit ; High strength, Stiffness, Creep resistance, Superior wear resistance and also provide; Good electrical and thermal resistance. This suit of properties makes particle-reinforced AMCs attractive to a wide range of applications in;  Automative,  Aerospace,  Transport industries.

Ceramic particles, such as TiC, TiB 2, SiO, MgO, SiC and B 4 C, are generally preferred as reinforcement elements. Among these particles, Titanium Diboride, TiB 2, is particularly attractive because it exhibits; High elastic modulus, Strength and hardness, High thermal resistance. Titanium Diboride also has; High melting temperature with high chemical stability, It is compatible with an aluminium matrix, Does not react with the aluminium at low temperatures.

2. EXPERIMENTAL PROCEDURE 2.1 Infiltration of composites 2014 aluminium alloy (93.6% Al - 4.1% Cu - 0.4% Si % Mn % Mg) TiB 2 (d 50 : 32 μm) were used to produce the composites. The chemical composition of the Al 2014 alloy was determined by atomic absorption analysis method. The particle size of the TiB 2 was determined using laser diffraction (Malvern, model Mastersizer Hydro 2000e) connected to a computer. Fig. 1: A prototype vacuum infiltration test apparatus used to produce the composites

Vacuum infiltration tests were carried out by using the prototype apparatus shown in Fig. 1. The system consists of a vacuum unit, an infiltration tube (quartz), a crucible and temperature control system. Quartz tube has an outside diameter of 10 mm, a wall thickness of 1 mm and a length of 300 mm. Stainless steel filter and Al foil were placed at the bottom of the tube. Infiltration tests were performed with TiB 2 powder to form 50 mm height. A filter, alumina blanket and a balance weight were placed on the powders to prevent the oxidation of the powders. Fig. 2. Powder blend in quartz tube prepared for infitration.

The molten metal temperatures were kept at;  700, 750 and 800 °C ± 1, respectively.  505 ± 5 mmHg vacuum was applied to the tube and the tube was dipped in liquid metal. The vacuum was kept under these conditions for 3 minutes. After 3 minutes of vacuuming, the tubes were taken out and cooled to the room temperature. The tubes were broken and composites were removed from the tubes. Composites with a 2.0, 4.0 and 8.0% volume ratios were produced with this method. Samples for microstructural examinations were prepared using standard metallographic techniques and examined by optical (Nikon MA-100) and scanning electron microscopy (Jeol JSM-6060 LV). The theoretical density was calculated using the mixture rule according to the volume fraction of the TiB 2 particles. Porosity of the composites was determined using theoretical and experimental densities. The Vickers hardness (ASTM E-384) of the composites was measured using a load of 10 kgf. At least three readings were taken to determine the density, porosity and hardness of the composites.

2.2 Wear Tests The wear tests were carried out using a prototype pin-ondisc test machine. A schematic diagram of the test machine is shown in Fig. 3. The machine consists of a disc, a pin (specimen) and its mounting system, and a loading system. Specimens with 8 mm diameter and 30 mm length were prepared from Al 2014-TiB 2 (2%, 4% and 8%). Wear tests were performed at different loads (10–50 N) and a sliding speed of 0.5 m.s −1 for a sliding distance of 40 m using 140 mesh (105 μm) grain size Al 2 O 3 abrasive papers. Each specimen was ultrasonically cleaned and weighed before wear tests using a balance with an accuracy of 0.01 mg. Worn samples were removed after the test, cleaned in solvents (to eliminate the debris) and weighed to determine the mass loss. The surfaces of the worn samples were examined using SEM. Fig. 3: A schematic diagram of prototype pin-on disc test machine

3. RESULTS Fig. 4: Microstuctures of the Al 2014-TiB 2 composites infiltrated at 800 °C: (a) 2% TiB 2, (b) 4% TiB 2 and (c) 8% TiB 2

Optical images of the composites with 2%, 4% and 8% TiB 2 are given in Figure 4. It was observed that the microstructures of the composites consisted of Al matrix and TiB 2 particles. TiB 2 particles exhibit various shapes such as cubic, triangular and spherical. Fig. 5: The changes in average distance among the TiB 2 particles as a function of TiB 2 content The change in average distance among the TiB 2 particles as a function of TiB 2 content is given in Fig. 5. It was observed that average distances are decreased with increasing TiB 2 content.

The variation of the porosity of the composites as a function of TiB 2 is given in Figure 6. It was observed that porosity of the composites increased with increasing TiB 2 content and decreased with production temperature. Fig. 6: The variation of the porosity as a function of TiB 2 content

Hardness of the composites increased both the content of TiB 2 and production temperature, as shown in Fig. 7. Optical images of the composites showed that the porosity decreased with increasing production temperature. Fig. 7: Hardness of the composites produced under different production temperatures as a function of TiB 2 content

Fig. 8: The variation of weight loss of the composites as a function of TiB 2 content Fig. 9: The change in weight loss as a function of applied load The variation of weight loss of the composites as a function of TiB 2 content is shown in Figure 8. It was observed that the weight loss of the composites decreased with increasing TiB 2 content. The change in weight loss as a function of applied load is given in Fig. 9. The weight loss of the composites increased with increasing load. The lowest weight loss is obtained from Al % TiB 2 composite.

SEM images of the worn surfaces of the composites tested at different loads (10, 30 and 50 N) are given in Figure 10. The worn surfaces of the Al % TiB 2 and Al % TiB 2 composites tested at a load of 30 N are shown in Fig. 11a and b. The worn surfaces of the composites were characterized by smearing, grooves and scratches. It was observed that the number and size of the scratches increased with increasing load. However, the smeared layers on the surface of the composites were observed to be increased with increasing TiB 2 content on constant load. The EDS analyses performed on the worn surfaces of the composites showed that the percentage of the aluminium and titanium elements increased in the surface with increasing volume fraction of TiB 2 particles. Fig. 10: SEM images of the worn surfaces of the Al % TiB 2 composite tested at different loads, with a sliding speed of 0.5 m.s −1 for a sliding distance of 40 m: (a) 10 N, (b) 30 N and (c) 50 N

Fig. 11: SEM images of the worn surfaces of the Al % TiB 2 and Al % TiB 2 composites tested at a load of 30 N

3. RESULTS Those were reached from the results of expirements; 1.The production temperature and hardness are effective parameters on wear of produced composites. 2.Average distances are decreased with increasing TiB 2 content. 3.Porosity of the composites increased with increasing TiB 2 content and decreased with production temperature. 4.Hardness of the composites increased both the content of TiB 2 and production temperature. 5.SEM images of the composites showed that the porosity decreased with increasing production temperature. 6.The weight loss of the composites increased with increasing load. 7.The lowest weight loss is obtained from Al % TiB 2 composite. 8.The highest hardness was obtained from Al % TiB 2 composite with all production temperatures. 9.The worn surfaces of the composites were characterized by smearing, grooves and scratches. It was observed that the number and size of the scratches increased with increasing load. 10.The smeared layers on the surface of the composites were observed to be increased with increasing TiB 2 content on constant load. 11.The EDS analyses performed on the worn surfaces of the composites showed that the percentage of the aluminium and titanium elements increase in the surface with increasing volume fraction of TiB 2 particles.  Evaluation of microstructural and mechanical test results suggests that it would be beneficial to keep the production temperature around 800 °C.  Al % TiB 2 composite can be recommended for engineering applications where the hardness and wear resistance are considered to be significant factors.