Microstructure and Metallurgical Properties of Aluminium 7075 – T651 Alloy / B4c 4 % Vol. Surface Composite by Friction Stir Processing

Abstract

Friction stir processing (FSP) is an emerging novel, green and energy efficient processing technique to fabricate surface composites which is based on the basic principles of friction stir welding. The distinct advantages of friction stir processing are microstructural refinement, densification, homogeneity, accurate control and variable depth of the processed zone. Metal matrix composites, particularly Aluminium based metal matrix composites, reinforced with ceramics were developed as an alternative to materials with superior strength to weight ratio and strength to cost ratio, high stiffness, and thermal stability, which effect on improving wear, creep and fatigue resistance. However, these composites also suffer from low ductility and toughness due to the incorporation of ceramic reinforcements. For many applications, a combination of high surface wear resistance and high toughness of the interior bulk material required. In those situations, it is desirable that only the surface layer of components is reinforced with ceramic particles while the bulk retains the original composition and structure with higher toughness. In this paper, the details about the fabrication of Al 7075-T651 B4C surface composite by FSP to have improved surface hardness are provided. A tool made of high carbon high chromium steel and hardened to 62 HRC, having cylindrical profile was used for FSP. By using SYSTAT software regression model has been developed for predicting microhardness of processed surface composite. The fabricated surface composites were examined using optical microscope and found defect free friction stir processed zone. It was also observed that Boron Carbide particles were uniformly distributed and well bonded with the matrix alloy fabricated at high heat input condition. It was found that the average hardness of friction stir processed surface composite was 1.5 times higher than that of the base metal aluminium 7075 – T651. The increase in hardness was attributed to fine dispersion of B4C particles and fine grain size of the aluminium matrix. Friction Stir Processing [1] is a new, solid state processing technique for microstructural modifications, which was developed based on the principle of friction-stir welding (FSW). A non-consumable rotating tool with a pin and shoulder is inserted into the material and travelled along the desired path. Because of this the frictional heat is generated and the material undergoes severe plastic deformation, resulting in significant microstructural modification in the processed zone. FSP creates a region called the Nugget or Stir zone, where the refinement of microstructure takes place producing equiaxed fine grains with high grain boundaries. Figure 1 shows the step by step procedure of Friction Stir Processing. Fig.1 Friction Stir Processing (a) Rotating tool prior to contact with the plate; (b) Tool pin contacts plate creating heat; (c) Shoulder of tool contacts plate restricting further penetration while expanding the hot zone; (d) Plate moves relative to rotating tool creating a fully re-crystallized fine grain microstructure. [1] Composites [2] represent a combination of at least two chemically distinct materials with a distinct interface separating the constituents. Their high strength to weight ratio, enhanced resistance to environmental hazards, lower density, high fatigue