Metal founding – Process – Disposition of a gaseous or projected particulate molten...
Patent
1991-01-22
1992-09-01
Wyszomerski, George
Metal founding
Process
Disposition of a gaseous or projected particulate molten...
75338, 118 501, 118 72, 427367, 427 38, 419 69, B05C 504, C23C 412, B22F 908
Patent
active
051431394
DESCRIPTION:
BRIEF SUMMARY
This invention relates to a method of spray deposition, to spray deposits formed by the method and to apparatus for carrying out the method.
In the production of spray-deposited, shaped preforms, liquid metal or metal alloy is sprayed onto an appropriate collector. The process is essentially a rapid solidification technique for the direct conversion of liquid metal into a deposit by means of an integrated gas-atomising/spray depositing operation. A controlled stream of molten metal is poured into a gas-atomising device where it is impacted by high velocity jets of gas, usually Nitrogen or Argon. The resulting spray of metal particles is directed onto the collector where the hot particles re-coalesce to form a highly dense deposit. The collector may be fixed to a control mechanism which is programmed to perform a sequence of movements within the spray, so that the desired deposit shape can be generated. Such deposits, after removal from the collector, can then be further processed, normally by hot-working, to form semi-finished or finished products.
The above methods are described in our European Patent Publications Nos. 200349; 198613; 225080; 244454, and 225732. It will be noted from these prior disclosures that for a high density spray deposit to be formed it is essential that the deposition conditions are so controlled that the atomised droplets are deposited onto a semi-solid/semi-liquid layer which is maintained at the surface of the spray deposit throughout the deposition operation. However, it is very difficult or often impossible to achieve this with the initially deposited layers of metal which are deposited onto the collector and not onto previously deposited metal. Consequently, the initially deposited metal can be chilled by heat conduction to the collector surface with the result that a semi-solid/semi-liquid surface is not immediately formed for subsequently arriving droplets to be deposited into. This results in poor bonding between the atomised droplets and also in individually deposited droplets often retaining their identity in the deposit resulting in porosity in the initially deposited layers of metal. When the collector is traversed through the spray this effect is further aggravated by the initially deposited metal being formed from the outer edges of the atomised spray where deposition rates are lower than in the centre regions of the spray. However, as the deposit increases in thickness, by careful control of the deposition conditions, the semi-solid/semi-liquid surface, into which the atomised droplets are deposited, is quickly generated and maintained resulting in high density, non-particulate microstructures, as described in our prior patents. The porosity which forms at the collector/deposit interface is nearly always interconnected with the result that oxygen from the atmosphere can penetrate into the pores during cooling of the deposit or during subsequent processing in an air atmosphere. For example, in the case of a stainless steel tube preform produced by traversing a thin walled, mild steel tubular collector through an atomised spray of stainless steel the interconnected porosity at the interface can be 10-20% of the deposit thickness. Consequently, current practice is to machine both the mild steel collector and the porous layer of stainless steel away from the tube before it can be used or further processed. This problem can be alleviated to a certain extent by preheating the collector but this is extremely difficult as the relatively cold atomising gas flowing over the surface of the preheated collector cools the surface of the collector prior to its passage under the spray and therefore reduces much of the benefit. Furthermore, to minimise porosity completely very high preheat temperatures are necessary, ideally at least to the solidus temperature of the metal being deposited and this can result in severe distortion of the collector and is often not practicable.
The removal of the collector by machining (particularly in the case of tubular deposits) and of part of the
REFERENCES:
patent: 3833983 (1974-09-01), Baker et al.
patent: 4333775 (1982-06-01), Mahrus
patent: 4697631 (1987-10-01), Bungeroth et al.
patent: 4926924 (1990-05-01), Brooks et al.
Chesney Peter F.
Leatham Alan G.
Pratt Charles R.
Osprey Metals Limited
Wyszomerski George
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