Research Of Fabrication And Weldability Of Boron Carbide Particulates Reinforced 2024 Aluminum Matrix Composites

The modern industry and energy in short supply stimulating the improvement of materials science and engineering. High specific properties, wear resistance and severe environment-suitable composites can be obtained by incorporating low specific density, super hard and neutron absorbing boron carbide into aluminum. The practicable application of these kinds of composite materials is closely related to its'processing. The weldability of the aluminum based composites hampering its'application, therefore research on fabrication technique and subsequent weldability is vital to improve its'application.This thesis aim at the potentially neutron absorbed, aeronautical and space-oriented boron carbide reinforced aluminum matrix composites. Research the influence of optimized casting and powder metallurgy technology on microstructure and mechanical performance, establishing the feasible and practicable fabrication routine. Two types of fusion welding techniques and one solid state welding technique were selected as welding method to study the weldability of B4CP/2024Al composites. X-ray diffractometer, optical microscope, scanning electron microscope, electron probe micro analysis, transmission electron microscope were employed to study the microstructure of B4CP/2024Al composites before and after welding. Tungsten inert gas welding machine, continuous waved CO2 Laser welding apparatus and computer-controlled friction stir welding machine were selected to evaluate the weldability of B4CP/2024Al composites. All the specimen made by above fabrication route and subjected to three welding method were tensile tested at ambient temperature. The main results listed as follows:Ultimate tensile strength of 10vol%B4CP/2024Al based composite by stir casting method is unfortunately lay below the mechanical property of unreinforced 2024 alloy, which due to the introduction of two borides generated through the reaction: between B4C and Al during casting process and then two brides were identified using TEM and XRD. The reaction between boron carbide and aluminum can be eliminated by powder metallurgy technique, and the results showed that specific properties of composites by above PM method have an advantage over their counterparts traditional structural materials such as alloyed steel and titanium alloy even SiCP/Al composite with exact reinforcement volume fraction. Good tensile elongation are obtained by above fabrication method, totally 9.6% in 10vol%B4CP/Al matrix composites and over 4% in 24vol%B4CP/Al matrix composites combined with ultimate strength 548.4MPa and 626MPa, respectively. Failure of above composite materials were controlled by the void initiation-growth-coalesce in matrix alloy, and no cracked reinforcement and decohesion were detected in broken samples; reversely, failure of interface and broken SiC particles observed universally in fractured surface. The shape transition of boron carbide from sharp corner to round and formation of MgO near interface owing to the boron oxide react with Mg during the hot consolidation process; the involved stacking faults in silicon carbide responsible for the breakage of silicon carbide particles during tension load applied.Vacuum degassing technique before TIG welding can decrease the formation of pore to a low level. Bead on plate experiment indicated that boron carbide can reacted with aluminum to generate two kinds of compounds- AlB12 and Al3BC, which degraded the mechanical performance of joint. When adding Al-10Ti master alloy as filler metal, TiC layers and fined TiB particles were formed in-situ during welding, and related joint strength is 234MPa. The strength of joint formed by adding ER2319 as filler metal is 233.2MPa, and the strength of this joint were enhanced through post welding heat treatment to a T6 condition, and the responding value is about 370MPa (joint efficiency over 70%, a high value compared with other's report). The research results showed that: the protective layers'in-situ'formed on the surface of boron carbide particles by preferred interaction between filler metal and boron carbide. The presented layer suppressing the formation of harmful needle shape compounds, meanwhile the contact angle between boron carbide and aluminum 125 degree decreased to 118 degree by introduction of titanium carbide layers on the reinforcement. Good bead configuration obtained compared with TIG welding, two needle shaped compounds- AlB12 and Al4C3 introduced after laser beam welding, for the boron carbide particles preheated by the interaction with laser beam, and B4C decomposed firstly and element B and C combined with Al to form AlB12 and Al4C3, respectively.According to the welding character of boron carbide particles reinforced aluminum matrix composites, research on joint forming and flow behavior by choosing different stirrer. H13 tool steel stirrer were not fitted to join B4CP/Al composites with higher volume fraction of reinforcement, many welding defects were introduced after welding and tunnel typed defects also detected, Lazy S typed and kiss bond defects observed after examming the cross section of weldment, formation of these kinds of defects is related to the insufficient flow and mixing of composite materials. However, the yielded weldment by GT35 stirrer with a good configuration and no macroscopic cracks detected, and no welding defects were detected using X-ray radiography method. The grain size in nugget zone is about 1-2μm identified as typically recrystallized structure. Joint strength of 10vol%B4CP/Al matrix composite by using H13 stirrer is 511MPa (more than 90% compared to the parent composites) and elongation of 9% very closed to parent composites. Joint strength of 10vol%B4Cp/Al matrix composite by using H13 stirrer is 300MPa, however, this value increased to 575MPa by using GT35 typed stirrer, the joint by GT35 pin fractured at location between TMAZ and HAZ in the weldment due to the Al20Mn3Cu2 coarsening effect by thermal cycle. B4Cp/2024Al matrix composites and 2024 can be dissimilar FS welded using GT35 pin using pin rotation of 800rpm and traveling speed of 75mm/min. The results show that sound weld bead formed after FSW and no macro defects detected. Vortice typed structure formed in the nugget zone related to the special flow of different materials. When using B4Cp/2024Al matrix composites as advancing side and 2024 aluminum alloy as retreating side, the tensile strength of FSW joint is 278MPa, and fracture located at 2024 side.