Research On Porous Defects In Spray-deposited 7000 Series Aluminum Alloy Preforms And Its Evolution During Post Treatment

Spray forming is a kind of rapid solidification technology, which can be used to manufacture highly alloyed structural materials. These highly alloyed materials are hard to be prepared through conventional casting. In this paper, porous defects in spray-deposited preforms and evolution of pores in the post processing were investigated. The main work was aimed at the influence of spray forming process parameters to prepare 7000 aluminum alloy on porous defects. The evolution of pores during post treatment was studied. Porous defects in the spray-deposited preform were classified. The relationship between elimination of porous defects and dense technique has been investigated.The distribution and formation mechanism of porous defects in the deposited preform were studied. According to morphology and formation mechanism, porous defects in spray-deposited 7000 aluminum alloy preforms are classified as gas pores, interstices, interconnected pores and mixed pores. The influence of gas pressure, drawing velocity, spray distance, offset and gas-metal ratio on porosity in the preforms was discussed. Drawing velocity and offset have an effect on shaping and yield of the preform. Porous distribution and density were associated with gas pressure, spray distance and gas-metal ratio. The reasonable process parameters used in this paper were reviewed as drawing velocity of 20mm/min, offset of 45mm, gas pressure of 0.7MPa, spray distance between 430mm and 470mm. The yield of spray-deposited preforms was beyond 60% and density was greater than 90%.Spray-deposited 7000 aluminum preforms were treated with pack HIPing, homogenizing, hot extruding, solid solution and aging. The influence of heat treatment on the morphology of pores in the preforms was studied. The results reviewed that the interstices, interconnected pores and mixed pores in the preforms can be removed through diffusion after HIPing treatment at 450℃/120MPa for 8h. Gas pores are difficult to be eliminated duing to the barrier of internal gas. The volume of gas pores was greatly compressed, even hard to be distinguished in micro scale. HIPing treatment can be used to help to remove the interstices, interconnected pores and mixed pores in the preform, so oxidation during post treatment will be reduced or weakened. During homogenization, the compressed gas pores after HIPing treatment expands for heat treatment. The number and volume of reformed pores are improved with longer time and higher temperature in the homogenization process. The gas pores in the preform can be broken through subsequent hot extrusion porecess, and distribute along the direction of extrusion. A part of closed gas pores are destroyed, and the interface is metallurgically bonded for the gas discharge. The other part of gas pores are destroyed, the interface of gas pores remains closed, and the volume of the gas pores shrinks because of the hydrostatic pressure during extrusion. In this paper, pores in the preform are invisible in micro scale through hot extrusion with extrusion rate of more than 21. The succedent solid solution treatment makes spherical gas pores with size within 20μm reappear in the preform, and density of the preform reaches 99.4%. The preform density achieves 99.7% with extrusion rate 50 after solid solution, however grains in the preform grow up obviously, which can reduce mechanical properties of the alloys.The fractures of spray-deposited 7000 aluminum alloy preforms and the billets after thermal processing were studied. The morphology evolution of porous defects during thermal processing and the influence of gas in the preform on porous defects were researched. The results show that nitrogen element in the preform locates as gas and metal nitrides. Nitrogen in the preform does not escape from surface after HIPing, homogenizing, hot extruding, solid solution and aging treatment. Thermal expansion of nitrogen in the preforms is responsible for the formation of gas pores during heat treatment. Metal nitrides do not reduce properties of the alloys for good wettability with the matrix. The interstices, interconnected pores and mixed pores can be completely eliminated by conventional thermal deformation processing, but it is difficult to remove gas pores.