Study On The Process And Principles Of Semi-solid Powder Rolling For Preparation Of B₄C/AA2024 Composite Strips

Semi-solid powder rolling（SSPR） combines the advantages of powder rolling and semi-solid rolling, providing a near net shape forming, short process for preparation of metal and MMC strips. The process of MMC strips by SSPR includes the uniform mixing of reinforced particles and matrix powders on a certain proportion, the preparation of semi-solid powders, the rolling of semi-solid powders and the post-treatment process of composite strips. In this work, B₄C/AA2024 composite strips were prepared by SSPR. The effects of processing parameters on the microstructures and mechanical properties were studied. The microstructure evolution of matrix and the effects of B₄ C particles on the matrix microstructure evolution, the bonding mechanisms between Al and Al as well as B₄ C and Al, the densification process, the relationship between deformation and densification, the oxidation mechanisms of AA2024 powders and the effects of oxides on the strip properties were analyzed in detail. The objective of this work is to provide theoretical guidanceand technological support for further development and application of this technology.The results show that temperature and rolling force are two important parameters that affect the quality of strips. However, the way of influence is different. Increasing temperature reduces the deformation stress and crushing strength through increasing the liquid fraction. The metallurgical bonding between powder particles were reinforced by the flowing and filling of liquid. The rolling force acts on the powders directly to make them deform and break. The metallurgical bonding between powder particles was completed through rolling compaction and diffusion of atoms. When the liquid fraction is lower than 20%, the liquid phases are isolated within the powders. When the liquid fraction is higher than 20%, many isolated molten pools converge together to form a fluid network, then extend to the powder boundaries.The shortest time for powder heating with the furnace ranges from 55 to 60 min, and with 50 min for heating at the target temperature.The best liquid fraction to prepare a 10%B₄C/AA2024 composite strip ranges from 38% to 60%. The relative densities of 10%B₄C/AA2024 composite strips range from 0.75 to 0.95. The maximum Brinell-hardness of 10%B₄C/AA2024 composite strips is 150 HB with the tensile strength of 250 MPa. The maximum tensile strength of 10%B₄C/AA2024 composite strips reach to 500 Mpaafter hot rolling with a thickness reduction of 0.2, and Brinell-hardness of 200 HB.The materials in deformed zonewere obtained by a method of stopping rolling suddenly. The process of semi-solid powder rolling can be divided into four sections: feeding region, drag-in region, densification region, strip region.The powders tend to deform with a liquid fraction lower than 20%, and prefer to break with a liquid fraction higher than 20%. In densification region, the transformation from powders to a dense strip basically completes.The mian bonding mechanism between Al and Al in SSPR-ed strips is diffusion of atoms. Diffusion of atoms occurs during the whole process which is the most important bonding mechanism. Recrystallization phenomenon is observed in AA2024 strips when the liquid fraction is lower than 20%（fl< 20%）. Flowing and filling of liquid can improve the bonding mechanism of Al-Al whenthe liquid fraction is higher than 20%（fl> 20%）.The main bonding mechanism between B₄ C and Al is rolling compaction. When liquid fraction is higher than 20%, interfacial reaction between B₄ C and Al plays an important role in the improvement of metallurgical bonding. The reaction products are Al3 BC and AlB2.The outward diffusion of Al and Mg element and the inward diffusion of O element are the main oxidation mechanisms for 2024 aluminium alloy powders during the heating.The oxidation reaction of AA2024 powders main occur during the rolling process, and the oxidation mechanism is also diffusion-controlled oxidation. According to the calculating formula of the densification of oxidation film, the PBR value of Al2O3/Al composites is about 1.29. It means that the oxidation film is dense and integral to protect AA2024 powders.The whole densification process can be divided into three stages: In the first stage, the rearrangement and restacking of powders are the main densification mechanism. In second stage, deformation, break up, rearrangement, metallurgical bonding are the main densification mechanism. The flowing and filling of liquid and interface reaction are helpful to densification when liquid fraction is higher than 20%. In third stage, densification mechanisms are similar to hot-rolling densification of porous strips.