Metal treatment – Process of modifying or maintaining internal physical...
Patent
1989-01-03
1992-07-28
Dean, R.
Metal treatment
Process of modifying or maintaining internal physical...
148542, 148550, 148552, C22F 100
Patent
active
051338114
DESCRIPTION:
BRIEF SUMMARY
This invention relates to the manufacture of improved thixotropic materials and to an improved method and apparatus for casting and forging thixotropic material.
The formation of metal slurries comprised of degenerate dendritic or nodular discrete solid particles within a liquid matrix and which exhibit thixotropic properties is well known, for example from U.S. Pat. Nos. 3,948,650 and 3,954,455, and UK Patent 1400624. All these patents refer to the production of such slurries by means of the vigorous agitation of the melt during solidification. However, physical agitation of a melt becomes more and more difficult as the melt approaches solidification. An improved process is disclosed in European published Applications 0090253 and 0139168 where a process for the preparation of a metal composition for forming in a partially solid, partially liquid condition is disclosed. The process comprises hot working the metal composition between the recrystallization temperature and the solidus temperature and introducing a critical level of strain either concurrently with or as a separate step subsequent to hot working. Upon completion of the hot working and any required cold working, the metal composition is reheated to a temperature above the solidus and below the liquidus.
Whilst such a process is an improvement over the earlier arrangements it still requires several process steps and an object of the present invention is to provide a simplified process which achieves a composition suitable for forming in a partially solid, partially liquid, or thixotropic condition.
According to the present invention there is provided a method of producing a thixotropic material comprising the steps of deforming a fully solidified metal or metal alloy material below its temperature of recrystallisation, heating the deformed material to cause recrystallisation of the microstructure of the material, and raising the temperature of the material above its solidus temperature whereby the recrystallised structure partially melts to provide discrete particles in a liquid matrix which behaves thixotropically.
The discrete solid particles in the liquid matrix will rapidly spheroidise under surface tension forces to produce a dispersion of near round solid particles.
The deformation and recrystallisation steps are carried out sequentially with cold or warm working being followed by heating to effect recrystallisation. Suitably the working is extrusion or rolling. In this specification by `warm working` we mean working conducted at a temperature between room temperature and the temperature of recrystallisation for the material being worked.
The preferred starting-material in the method of production is a fully solidified alloy which may or may not have initially a dendritic microstructure. The starting material may be deformed by some suitable means such as by extrusion, rolling, tensile extension or compression. The deformation may be performed at low temperatures but to such an extent that, on raising the temperature, recrystallisation of the structure occurs.
The subsequent step of raising the temperature allows partial melting of the alloy. This melting will start normally in the lowest melting point regions which were the last to solidify in the original casting and comprise regions at the grain boundaries and between dendrite arms where microsegregation has occurred. In most cases high angle grain boundaries introduced by the recrystallisation process will also melt causing each grain to separate as a discrete solid particle within the matrix liquid. Even where the boundaries are not completely wetted by the liquid phase (i.e. melted), a groove will be established down the grain boundary at the liquid/solid interface such that the surface tension forces are locally balanced. With fine enough microstructures these grooves may be deep enough to cause fragmentation of the solid into small discrete particles surrounded by matrix liquid.
The recrystallisation and melting steps of the present invention can occur successively in the same heati
REFERENCES:
patent: 3948650 (1976-04-01), Flemings
patent: 3954455 (1976-05-01), Flemings
patent: 4295896 (1991-10-01), Flemings et al.
Boyed Luis G. E.
Kirkwood David H.
Sellars Christopher M.
Dean R.
University of Sheffield
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