Study On The Process And Principle Of Semi-solid Powder Rolling For Preparation Of 7050 Aluminum Strip

A new type of metal strip near-net-shape manufacturing--- Semi-solid Powder Rolling(SSPR) was proposed, which combines the advantages of semi-solid rolling and powder rolling, providing a new energy and materials saving, short process for preparation of the high-performance strips. Up to now, there is no relavant research report on semi-solid powder rolling. Based on the characteristics of semi-solid powder rolling and the theories of semi-solid rolling and spray rolling, this study mainly focuses on the relationships among the rolling processing parameters and characteristics, the mechanical properties and microstructures as well as on the microstructure evolution, combination mechanisms, the densification mechanism and forming mechanism, etc. It can provide technical supports and theoretical basis for this technology on the development and implementation for the continuous and stable production.Semi-solid powder rolling can be divided into three parts:(1) preparation of semi-solid powders;(2) rolling of semi-solid powders;(3) post-treatment process of the strips. Taking AA7050 aluminum alloy for an example, the effects of heating temperature, holding time and particle size on the quality of strips were studied. The second part is semi-solid powder rolling, which involves the microstructure evolution, combination mechanisms of powders, densification and deformation mechanisms, oxidation process and oxides of strip, and the cooperation mechanism among solidification, deformation and densification. Finally, three post-treatments of semi-solid powder rolled strips were studied, which consists of sintering, "pre-sintering- cold deformation- sintering" and hot rolling. The results show that heating temperature and particle size are the two important parameters that affect the quality of the strip. Strips were obtained by using powders with the mean particle size of 75, 104, 147, 270 μm, respectively. A higher relative density(> 90%) and a micro hardness(~ 170 HV) were achieved. The coarser the powders, the less the liquid was required in order to prepare the desired strip with a relative density higher than 90%. But the microhardness is lower and the grains are coarser compared with the strips prepared by fine powders under the same conditions. As for powders with the mean particle size of 75 μm, the optimum heating temperature is 625-640 °C, and for powders with the mean particle size of 270 μm, the optimum heating temperature is 585-625 °C. Strips prepared by fine powders have a low tensile strength(~ 400 MPa) with a high elongation(~ 13%). The fracture mechanism is a ductile fracture. Strips prepared by coarse powders have a high tensile strength(~500 Mpa) with a low elongation(~10%), the fracture mechanism is mixed fracture. By analyzing the the rolling force with different mean paritlce sizes and calculating the internal pressure of pores, the main factors affecting the relative density of the strip were obtained: firstly, the force and deformation degree of fine powders are less than those of coarse powders. Secondly, fine powders have more average interfaces impeding the flowing and filling of liquid in the pores. Thirdly, premature solidification of the liquid phase mainly results in low relative density, and a fine powder solidifies much faster than that a coarse powder.In order to study the microstructure evolution during semi-solid powder rolling, the rollers stopped abruptly, and the process can be divided into four sections: feeding zone, drag-in zone, densification zone, the strip zone. When the liquid fraction is less than 10%, the liquid phase is isolated within the particles. When the liquid fraction is higher than 10-20%, the liquid phase forms networks between the particles. According to the distribution of liquid phase, the metallurgical bonding among semi-solid powders occurs through three combination mechanisms: the first one is hot rolling densification, it still occurs no matter how much the liquid is. The second is the recrystallization. It only occurs when the liquid fraction is less than 10% with the particle orientation satisfying a special relationship. The third one is the flowing and filling of liquid, it occurs when the liquid fraction is higher than 10-20%. By analyzing the influence of oxygen on the strip, it shows that good bonding condition between oxides and the matrix and the uniform distribution of oxide particles play a positive role on the mechanical properties. Otherwise, it will play a detrimental effect. Hot rolling is not only an effective post-treatment technology but also an effective method of strengthening the bonding quality between the oxides and the matrix. Using the densification process of porous strip during hot rolling for reference, the densification process during semi-solid powder rolling can be divided into three stages based on the changes in the relative density. Additionally, the relative densities in stageⅠand stage Ⅲ are higher than that in hot rolling. Densification mechanisms are the rearrangement and restacking of powder particles, flowing and filling of liquid, deformation. On the basis of the model for the deformation and solidification behavior of droplet impinging onto a hard substrate, the relationship between the deformation strain and relative density was derived. This formula can be applied not only to the conditions of liquid fraction less than 20%, but also liquid fraction higher than 20%. In order to analyze the progress among solidifiction, deformation and densification, the factor of progress remarkablility( F) was proposed. The physical meanings of these factors are the progress remarkability of solidification, deformation and deformation at every step during the rolling procedure, respectively. F can be classified into three factors, one is Fsolidif and the other two are Fdefor and Fdensi. It was found that, solidification mainly occurs in the feeding zone and at the beginning of the drag-in zone, while deformation and densification mainly occur in the densification zone. Due to the prepared strip containing 1-10% porosity, a post-treatment process is needed to improve the propertiy of the strip. In this work, three post-treatment processes was studied, through the "pre-sintering- cold deformation-sintering", the relative density of strips reaches up to 92.6% with the highest microhardness of 300 HV. The relative density of strip can reach almost 99% with the microhardness of 176 HV after hot rolling. The relative density of strips drops after sintering, because the densification effect caused by solid phase sintering is less than the increment in porosity caused by the relative density reduction effect.