The Researches On Deformation And Fracture Behaviors Of Porous Materials Prepared By Spray Deposition During Compression And Rolling

As an advanced material preparation technology, Spray Deposition has been widely used to produce alloys and metal matrix composites. However, the spray deposited preforms usually contain more than 10vol.% porosity that is a great disadvantage for the mechanical properties. In order to prepare fully dense products of high mechanical properties, the porous preforms must be further densified and deformed plastically. The densification and plastic deformation behaviors and fracture behaviors of spray deposited porous preforms were investigated in this dissertation. The involved deformation processes are compression and rolling. Two novel rolling methods named as "Frame-Confined Rolling" and "Ceramic Rolling" were applied. The experimental results show that these two methods can greatly improve the workability of the spray deposited preforms without forming cracks during rolling.The main conclusion are as follows:(1) The deformation and fracture behaviors of spray deposited preforms during compression were investigated.Comparing with the fully dense materials, the deformation behaviors of porous preform at room temperature are characterized by a lower circumferential strain at the same axial strain, which can be contributed to the micro-pores in the matrix. The tensile stress in the circumferential direction of the spray deposited preform resulting from the bulge surface is smaller than that of the fully dense material. Therefore, a relatively stable deformation can be maintained during compression of spray deposited preforms. Rapid densification was observed until the axial strain was up to approximately 50% when the relative density was about 98%. However, slow and small increase of the relative density occurred after then. The evolution of pores during compression is characterized by deformation, collapse, elongation and disappearance.The deformation behaviors of porous preform during hot compression are characterized by a strain hardening phenomenon at all strains. The critical strain for the occurrence of strain softening in porous alloys is much higher than that in the fully dense alloys. The influences of deformation temperature and the strain rate on the flow stress of spray deposited FVS0812 alloy are the same as those on the fully dense as-extruded materials. The flow stress increases with the strain rate and decreases with the temperature. The circumferential strain increases with thetemperature at a relatively low strain rate.The contact friction force between the surface of the die and the sample shows great effect on the fracture behaviors of porous spray deposited performs. In addition, the amount of deformation to fracture increases with the deformation temperature.(2) The deformation and fracture behaviors of spray deposited preforms during rolling were investigated.Compared with the fully dense materials, the deformation behaviors of porous preforms during rolling are characterized by a smaller elongation and spreading with the same thickness reduction. The thickness strain is characterized by a decrease from the surface to the center across the sample thickness, even though the thickness reduction in one pass is up to 40%. The plastic Poisson ratio of a spray deposited preform with the relative density of 84.4% during rolling is about 0.25.The porous preform is liable to cracking or tearing when the thickness reduction is up to a critical value, or at a certain relative density, other than at the initial stage during conventional rolling, due to the porosity and a lack of sound metallurgical bond between the deposited layers and the particles.The hydrostatic stress shows great effect on the densification deformation of spray deposited preforms during rolling. The rolling workability can be improved by an intensified hydrostatic stress.(3 )A pack-rolling method was developed basing on the effect of lateral strain on the fracture behaviors. The effects of packing materials on the densification, the deformation and fracture behaviors of spray deposited preforms during pack-rolling were researched.A "aluminium-stainless steel- double layer pack" rolling process was developed which has been used to produce FVS0812 aluminum alloy sheet with excellent mechanical properties from porous spray deposited preforms.(4) Two novel rolling methods named as "Frame-Confined Rolling" and "Ceramic Rolling" are applied, which have been verified to be effective methods in improving the workability of the spray deposited preforms without forming surface and edge cracks during rolling.The frame die imposes a more severe hydrostatic stress field on the spray deposited preforms during rolling and keeps a more homogenous temperature distribution compared with the conventional rolling, which will be of great benefit to improve the rolling workability of the porous spray deposited preforms. At the same time, the steel-frame die provides limitation on the length and width strain...