Development and characterization investigations of labratory synthesized and commercial LaB6 reinforced Al-7% wt. Si powders and sintered hybride composites

Metal matrix composite (MMC) technologies are developed day by day to produce advanced materials. Metal matrix composites have superior mechanical, thermal and physical properties over the conventional alloys due to presence of the reinforcement materials such as; carbides, nitrides, oxides and borides (TiC, ZrC, SiC, ZrO2, Al2O3, B4C and Si3N4 etc.) Reinforcement materials can be short fibers, whiskers, and particulates forms. Particulate reinforcements have higher distribution inside the composite than the other forms. Therefore, homogeneous microstructure formation enhance the final properties of the products. Aluminium (Al) and its alloys are widely used for aerospace, automotive and many other industrial applications due to their lightweight and high specific strength. Silicon (Si) is the most important and main alloying element of aluminium alloys. Al-Si based particulate reinfoeced composites are used in many applications due to their advanced properties such as low density, light weight, high strength, high elastic modulus, high wear and corrosion resistance. MMC's are usually fabricated liquid phase production techniques like stir casting, squeeze casting, etc. However, it is possible to fabricate MMC's by solid state production techniques especially powder metallurgy. Mechanical alloying (MA) is a high energy ball milling technique is a part of the powder metallurgy. In MA, components are blended and then welded due to the collisions of the balls in a high-impact vial environment. After the repeated cold welding, fracturing and rewelding of powder particles homogeneous materials can be produced. Welded structures are repeatedly fractured and crushed to form new alloys which are difficult to be produced by conventional methods. Some variables effect the MA'ing process such as, composition of the powders, the type of materials of the milling balls and vials, milling medium, ball-to-powder ratio, milling atmosphere, milling time and process control agent (PCA) usage. In this study, Al wt. 7% Si based metal matrix alloy powders were produced via mechanical alloying process using different MA times ( 1,2, 3, 4, 6, 8 hours). The starting powders were mixed with 2 wt% zinc stearate which is a process control agent to prevent excessive cold welding and agglomeration of the powders and they were place inside a hardened steel vial in a glove-box filled with high purity argon using a 7/1 ball to powder weight ratio. As particulate reinforcement, LaB6 hybrid boride powders were mechanochemically synthesized (for 5 h) and leached with HCl from the La2O3-B2O3-Mg powder blends. After the leaching, pure LaB6 were synthesized without side products of the reaction like magnesium oxide (MgO). Both of the, commercial and synthesized LaB6 reinforced wt.2% to (Al-7 % wt. Si) powders were produced via mechanical alloying process for different durations (1,4 and 8h) in a high energy ball mill environment with a 7/1 ball-to-powder weight ratio. In addition to LaB6, commercial NbB2, VB and synthesized NbB-NbB2-Nb3B4 and VB-VB-V3B4 were added as reinforcement material to the Al7Si matrix mechanically alloyed only 4 hour. After the milling, all hardened steel vials open in a glove-box filled with high purity argon. Differences between non- reinforced metal matrix powders and 2%wt. different type of borides reinforced metal matrix composites were characterized. After the milling, phase anaylsis, particle size analysis with laser technics, surface area analysis and microstructral characterizations of the unmilled/milled Al7Si matrix and boride reinforced powders are conducted. XRD results verified the existence of the only Al, Si and different boride phases at the powder systems. Therefore, there were no secondary phases and intermetallics occurred during the mechanical alloying between the Al, Si and boride phases, or Fe impurity from the milling vials/balls due to collisions. Crystallite sizes and % lattice strains of Al were calculated by the TOPAS software. Crystallite sizes of the particles decreased, lattice deformation increased with increasing MA time. Microstructural evolution of the powder constituents during mechanical alloying was determined with scanning electron microscope (SEM). Particle sizes of the powders were characterized by laser diffraction techniques. During milling, particle size gets reduced continuously. There is no remarkable corelation between the particle size distribution and adding reinforcement material into the structures. On the other hand, Brunauer, Emmet ve Teller (BET) surface area analysis shows that the unmilled powders have smaller surface area than the milled powders (1 hour MA). However, increasing milling time decrease the surface area values becouse of the aglomeration of the powders. Thermal analysis of the hybride composites are conducted via differential scanning calorimetry (DSC). Sintering temperature is determined regard of the DSC analysis. DTA analyses showed a endothermic peak belonging to unmilled powders nearly at 655ᵒC which is the melting temperature of aluminum. In a comprasion with the unmilled powders, endothermic peak shifted to nearly 577ᵒC at the temperature where Al- Si point is. This endothermic peak belonging to the eutectic point of Al- Si, was seen even at the powders milled for 1 hour. This means, mechanical alloying is actualized even if in a 1 hour milling timeDTA curves showed no remarkable difference between reinforced and unreinforced powders. After the characterization investigations of powders, milled and unmilled composite powders are compacted in uniaxial press with a pressure of 450 MPa. At the debinding stage; stearic acid removed out of the green bodies under Argon atmosphere at at 420ᵒC with a 2 ◦C/minute heating rate and 2 hours holding time. Then, samples sintered at 570oC for 2h under argon gas flowing conditions. Microstructures of the Al7Si based MMC's are characterized via optical microscobe (OM) and scanning electron microscobe (SEM). No remarkable effect of the reinforcement materials on the microstructure was noticed with regard tothe microstructral investigations. There are cracks and porosities at the unmilled samples, but mechanically alloyed samples have less porosities than unmilled ones. Besides, mechanically alloyed samples have homogeneous mictostructures. Density measurements of the sintered bodies were conducted to determine the physical properties. Al7Si matrix composites had average 95% relative densities. Unmilled composites had lower relative densities becouse of porosities and cracks on the microstructures. At the final stage of the characterization investigations, mechanical tests including microhardness and siliding wear tests were conducted to determine the mechanical properities of the produced MMC's. Microhardness measurements were conducted on the as-blended and MA'd powders to determine the effects of mechanical alloying and boride reinforcements. According to results of the Vickers microhardness tests, increasing mechanical alloying time and boride reinforcements increase the hardness of the sintered materials. Especially commercial borides are increased the Vickers microhardness values. Microhardness value increase from 0,933 GPa to 1,012 GPa with LaB6 reinforcement amonst the 4h MA'd samples. As a results of the sliding wear test boride reinforcements reduce the wear rate of the materials. Commercial LaB6 reinforced sample has minimum wear rate value (0,98 mm3 /mm). In conculusion, mechanical and physical properties of the hypo-eutectic Al-Si metal matrix hybride composites were enhanced by mechanical alloying.