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AEA technology - PM developments across the board
A E A T e c h n o l o g y - PM developments across the board Anna Henly (Metal Powder Report)
HarweU, one of the UK Atomic Energy Authority's development laboratories, is home to many interesting projects in powder metallurgy (PM). Its researchers have been responsible for a number of innovations in P M over the last 2 0 years. Metal Powder Report visited Harwell to find out about current developments in the field. II
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EA Technology, the trading name of the UK Atomic Energy Authority, is Europe's largest contract research organization with over 10 000 staff and a turnover of over £400 million. Its major industrial market is Europe where it has had 1300 contracts with non-UK customers in the European Community over the past three years, and is currently the prime contractor for 76 research and development projects for the European Commission. The company is also involved in worldwide research, it is the UK's largest contractor to the US Strategic Defense Initiative and presently has more than 20 major contracts with the Far East. Metal Powder Report recently visited Harwell in Oxfordshire, the AEA's largest development laboratory employing some 4000 people. Dr Andy Feest, head of the Advanced Engineering Materials Department, outlined AEA Technology's powder metallurgy (PM) work, most of which is carried out at Harwell. He says that the company has implemented a total system approach to materials development involving design, performance prediction, process optimization, manufacture, testing, reliability and n o n - d e s t r u c t i v e testing (NDT). It draws on in-house centres of excellence such as the National NDT Centre and National Tribology Centre. It has been involved in PM since the
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0026.-0657/91/$3.50 ©, Elsevier Science Publishers Ltd
1960s, initially being involved with uranium fuels (UO2) and later b r o a d e n i n g its research to include carbides for use as fast reactor fuels. Originally set up to support the UK nuclear industry, the AEA's laboratories have seized the opportunity to sell their skills and technologies to other industries which can benefit from the specialized expertise gained under the harsh conditions of the nuclear power environment. Its diversification to encompass non-nuclear activities has meant the adaption of many technologies for different uses. For example, protective atmosphere processing technologies originally developed in the carbide fuels field were subsequently used as a basis for work in the late 1960s on ultrafine PM. The initial developments were in the fields of refractory metals, iron and tool steels, and exploited the lower sintering temperatures and improved mechanical properties, particularly toughness, arising from refined microstructures. Another offshoot of nuclear technology is the centrifugal shot casting (CSC) process for the preparation of high purity alloy powders. CSC evolved from technology for producing spherical carbide fuel particles of controlled diameter with the aim of achieving optimum packing. It has led to a family of related processes used in the preparation of refractory or reactive alloys. Progress made at Harwell in the 1960s included ceramic/plastic forming developments such as ceramic injection moulding. This technology is now being adapted to the metal injection moulding field. A similar success story is the powder infiltration technology developed for fibre reinforced glasses which has recently been applied to fibre reinforced metals. The AEA says that the growth of the PM industry has swelled the number of outlets for its multidisciplinary capabilities. Recent developments include: -- process monitoring in metal atomization; --plasma spraying of coatings, monoliths and composites; --co-spray processing of particulate rein-
Atomization process control
FIGURE 1: M o n i t o r i n g atomization on metal atomizer.
forced m e t a l m a t r i x c o m p o s i t e s (MMC); - - p o w d e r p r e p a r a t i o n (eg. for s p r a y i n g or for c o m p o s i t e fillers); --non-destructive t e s t i n g of PM MMC; - - C o n f o r m p r o c e s s i n g of PM alloys a n d MMC; a n d - s t r u c t u r e / p r o p e r t y r e l a t i o n s h i p s in PM MMC. Many of t h e r e c e n t d e v e l o p m e n t s have been c a r r i e d o u t on b e h a l f of c o n s o r t i a of c o m p a n i e s often with p a r t i a l s u p p o r t from t h e UK D e p a r t m e n t of Trade a n d I n d u s t r y (DTI) or t h e C o m m i s s i o n of t h e E u r o p e a n C o m m u n i t i e s (CEC), w h e r e a s o t h e r s have been c o n d u c t e d as confidential p r o j e c t s for single clients. F e e s t says t h a t e x p l o i t a t i o n of PM d e v e l o p m e n t s h a s g e n e r a l l y been d o n e by t h e i n d u s t r i a l p a r t n e r s c o l l a b o r a t i n g in t h e p r o j e c t s b u t A E A Technology is also e n g a g e d in m a n u f a c t u r i n g in a s m a l l n u m b e r of specific m a r k e t s , for e x a m p l e ThO2 a r t e f a c t s for t h e c h e m i c a l industry, a n d t h o r i u m a r t e f a c t s for t h e electrical a n d electronic industries.
Current PM projects at Harwell ]
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A n u m b e r of p r o j e c t s on PM r e l a t e d issues are c u r r e n t l y u n d e r w a y at Harwell. These include: - - a DTI LINK p r o g r a m m e on TiA1 m a t r i c e s for c o m p o s i t e s - - r a p i d l y s o l i d i f i e d p o w d e r c o m p a c t i o n is one of t h e c o m p e t ing p r o c e s s i n g r o u t e s being evaluated; - - m o d e l l i n g of e x t r u s i o n of m e t a l p o w d e r billets - - ongoing i n - h o u s e work; --application of Conform p r o c e s s i n g to PM products; -non-destructive t e s t i n g of MMC; a n d - - a t o m i z a t i o n control.
A E A Technology is working on, m a i n l y gas, a t o m i z a t i o n techniques, u s e d to p r o d u c e p o w d e r s of a d v a n c e d alloys a n d also for p a r t i c l e reinforced m e t a l m a t r i x c o m p o sites. P r e s e n t i n d u s t r i a l p r a c t i c e involves a s s e s s i n g the size d i s t r i b u t i o n of a p o w d e r after p r o d u c t i o n - - which can be s o m e hours, or even a day, later. The p o w d e r s a r e then separated into grades with tight d i s t r i b u t i o n s by sieving a n d m a t e r i a l n o t m e e t i n g r e q u i r e m e n t s is reprocessed. Dr Negus, r e s p o n s i b l e for t h e a t o m i z a tion p r o c e s s control p r o g r a m m e at Harwell, t o l d Metal P o w d e r Report t h a t t h e s e p r o b l e m s can be solved by on-line control of t h e m e a n d r o p l e t size a n d d i s t r i b u t i o n width. The a b i l i t y to switch to n e w o r different m a t e r i a l s w i t h o u t w a s t e is also an i m p o r t a n t target. Fast, reliable, on-line m e a s u r e m e n t in d e n s e p a r t i c l e s t r e a m s with good s a m p l i n g s t a t i s t i c s is difficult, says Negus. Optical t e c h n i q u e s have been d e v e l o p e d for studying v a r i o u s a s p e c t s of a t o m i z a t i o n such as core b r e a k - u p , d r o p l e t velocity, d r o p l e t s i z e , d r o p l e t c o n c e n t r a t i o n a n d flux. The AEA's b a c k g r o u n d in b o t h t h e n u c l e a r and n o n - n u c l e a r i n d u s t r i e s h a s m a d e it p a r t i c u larly w e l l - e q u i p p e d to h a n d l e such a project. It h a s m o r e t h a n t e n y e a r s ' e x p e r i e n c e in developing non-invasive t e c h n i q u e s for use in hostile e n v i r o n m e n t s ( i n c l u d i n g electronics for signal p r o c e s s i n g a n d c o n t r o l ) . L a b o r a t o r y i n s t r u m e n t s have been used in a v a r i e t y of i n d u s t r i a l s i t u a t i o n s b u t t h e recent thrust has been towards small, r o b u s t i n s t r u m e n t s for on-line m o n i t o r i n g by t h e use of s o l i d - s t a t e lasers a n d d e t e c t o r s o r by t h e use of fibre optics. A previous p r o g r a m m e on a t o m i z a t i o n involved a s t u d y of d e n s e w a t e r sprays a n d i m p i n g i n g w a t e r j e t s at BNFL Metals a n d also setting u p an a t o m i z e r at Harwell in c o n j u n c t i o n with O s p r e y Metals of Neath, UK. A s s e s s m e n t of d i a g n o s t i c t e c h n i q u e s for iso-kinetic s a m p l i n g for size a n d velocity m e a s u r e m e n t s on a l a b o r a t o r y a t o m i z e r was c a r r i e d out, a n d a w h i t e - l i g h t p a r t i c l e m o n i t o r was d e v e l o p e d a n d t e s t e d in an i n d u s t r i a l plant. Negus told Metal Powder Report t h a t a new i n d u s t r y c o - s p o n s o r e d two y e a r A E A p r o g r a m m e with funding to t h e t u n e of £230 000 has been a p p r o v e d by t h e DTI a n d will s t a r t early in 1991 at Harwell. The objective is on-line control of m e a n p o w d e r size a n d on-line m o n i t o r i n g of d i s t r i b u t i o n width, he says. The m i c r o p r o c e s s o r system could allow r a p i d e x p a n s i o n to the control of o t h e r p a r a m e t e r s . The system will be d e m o n s t r a t e d on a m e t a l a t o m i z e r a t Harwell with 2 k g / m i n u t e t h r o u g h p u t . MPR March 1991 41
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FIGURE 2: The Conform process can extrude infinite lengths of materials from powder materials,
Conform process
FIGURE 3: The Conform production evaluation line at Risley, UK.
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M P R M a r c h 1991
Developed by the AEA at its Springfield Laboratory, the Conform process is an extrusion technology with the ability to extrude infinite lengths of material. It involves a rotating extrusion wheel which is grooved and carries the feedstock - - both particulate feed and solid rod - - under the grip segment. In this region the feedstock is plastically deformed to fill the groove. The material then meets the abutment, which forces it to pass through the die to become the desired product. The AEA claims there are over 90 production machines operating worldwide with solid bar feedstock. According to
delegates at a seminar on the process held recently at the AEA's Risley laboratory site, Conform using powders, although still in development stages is currently capable of producing advanced materials and MMCs and is ripe for commercial uses. In the future the AEA hopes to use Conform to produce bearings alloys and electrical contacts. The process has m a n y a d v a n t a g e s , claims Jon Heywood of the Risley site, since it is basically a single stage process. Powder is simply consolidated and extruded thus avoiding sintering, mechanical alloying, spray deposition, canning or hot isostatic pressing. He believes that products produced using the Conform process will be c h e a p e r as a result of more efficient materials usage and a less labour intensive process. Considerable work is being carried out to investigate the variety of feedstock powders, reinforcements and other additions. An in-depth feature on Conform will appear in the April issue of Metal Powder Report.
Mechanically alloyed powder materials Work is also being carried out on mechanically alloyed powder materials (MA) at Risley by Andy Jones. MA materials are produced by high energy attrition of matrix powders and reinforcement particles. In particular, under the Cost 501, a European collaborative research programme, researchers are trying to develop and optimize high temperature MA Fe-Cr-A1 alloy which has properties such as biaxial creep strength and high temperature oxidation resistance. It is proposed to use the material in heat exchangers. Work is also being carried out to investigate the effect of powder processing on materials performance, in particular the effect of entrained defects on grain structures and biaxial creep performance. High nitrogen austenitic steels with considerably improved wear, corrosion and high temperature properties are also being developed. Manufacturing routes under investigation include; (1). MA TiN Particulate TiN is added to stainless alloy powders, mechanically alloyed and extruded at high temperature where the variable is the volume fraction of TiN. (2). NIDON CrxN is added to stainless steel powder blend, mechanically alloyed, and extruded at high temperature (CrxN donates N to Ti) where the variables are the volume fraction
of TiN a n d dissolved N. (3). NIDONA S t a i n l e s s a l l o y p o w d e r s a r e n i t r i d e d in a m m o n i a to form a CrxN crust on t h e p o w d e r surface, a n d e x t r u d e d a t h i g h temperature. (4). Injection The d o n o r can be injected as p a r t of a cos p r a y i n g process, a n d t h e p r o d u c t m a y be h o t i s o s t a t i c a l l y p r e s s e d (HIPed) or e x t r u d e d a t high t e m p e r a t u r e w h e r e t h e v a r i a b l e s a r e t h e v o l u m e fraction of TiN a n d dissolved N. I m p r o v e d p r o p e r t i e s in m a t e r i a l s from t h e m a j o r i t y of t h e s e p r o c e s s r o u t e s come from a c o m b i n a t i o n of s t r e n g t h t h r o u g h h a r d e n i n g with d i s p e r s e d t i t a n i u m n i t r i d e a n d alloying w i t h nitrogen. The p r o j e c t s t e m s from e a r l i e r w o r k to develop TiN s t r e n g t h e n e d 20Cr/25Ni a u s t e n i t i c steels as thin w a l l e d t u b e for n u c l e a r fuel cladding. This achieved high t e m p e r a t u r e s t r e n g t h t h r o u g h gas n i t r i d i n g of t h i n walled tubes. M a n u f a c t u r i n g r o u t e s for t h i c k s e c t i o n s have been d e v e l o p e d using PM techniques. The TiN p r o v i d e s very effective h a r d e n i n g and also imparts desirable tribological properties. Hyper-stoichiometric additions of N l e a d to beneficial corrosion p r o p e r t i e s . A p p l i c a t i o n s for s u c h m a t e r i a l s i n c l u d e impellers, valves, p u m p p a r t s a n d general l o a d e d r u b b i n g c o n t a c t s p a r t i c u l a r l y for t h e food p r o c e s s i n g a n d c h e m i c a l i n d u s t r y w h e r e r e q u i r e m e n t s are for low a n d high t e m p e r a t u r e corrosion a n d a b r a s i o n resistance. F u r t h e r a p p l i c a t i o n s could i n c l u d e cryogenics a n d s p a c e w h e r e t h e high p r o o f strength, s t a b l e a u s t e n i t i c s t r u c t u r e a n d tribological p r o p e r t i e s a r e i m p o r t a n t .
Non-destructive testing of MMCS A basic c o n s t r a i n t on MMC use is t h a t p e r f o r m a n c e c o n t r o l l i n g f a c t o r s such as fibre v o l u m e - f r a c t i o n , m i c r o p o r o s i t y a n d fibreorientation are within specification, says Dr G r a h a m C u r t i s of t h e NDT Centre. Hence q u a l i t y control e m p l o y i n g NDT is of t h e u t m o s t i m p o r t a n c e , he adds. S e v e r a l p r o g r a m m e s i n v o l v i n g MMC m a n u f a c t u r e r s a n d u s e r s are u n d e r w a y at t h e N a t i o n a l NDT Centre a t Harwell a i m e d a t p r o v i d i n g this q u a l i t y control.
(1) Defining the basis of a stiffness quality monitor This p r o g r a m m e h a s a l r e a d y e s t a b l i s h e d t h a t p a r t i c u l a t e SiC filler a n d m i c r o p o r o s i t y levels in 2014 a l u m i n i u m alloy could in fact be n o n d e s t r u c t i v e l y d e t e r m i n e d via m e a s u r e m e n t s of u l t r a s o n i c wave velocity a n d a t t e n u a t i o n . Analysis of t h e s e factors can
FIGURE 4: Instrumental 30 tonne vacuum hot press for consolidation of metal matrix composites (MMC) powder compacts
lead to t h e d e t e r m i n a t i o n of a stiffness quality monitor.
(2) Manufacturing nonuniformities monitoring This p r o g r a m m e aims to d e t e c t significant defects in MMCs which can e s c a p e notice, in-service, for s o m e time. There is concern over p o s s i b l e m a n u f a c t u r i n g non-uniformities of MMCs which m a y vary according to t h e m a n u f a c t u r i n g r o u t e taken. The Centre can demonstrate the most appropriate nondestructive methods available using s p e c i m e n s of 2014/SIC or 8090/SIC prod u c e d by v a c u u m h o t p r e s s i n g a n d rolling, which c o n t a i n an e x a m p l e of one of two or t h r e e types of ' m a n u f a c t u r i n g non-uniformities'.
(3) Filler and porosity monitoring This p r o g r a m m e is d e s i g n e d to investigate SiC filler v o l u m e - f r a c t i o n a n d m i c r o p o r o s i t y levels in 2014 a l u m i n i u m alloy. It is t h e first s t e p in a d a p t i n g t h e b a s i c c o n c e p t for production quality control. A software p a c k a g e with d a t a of wave velociW a n d a t t e n u a t i o n c a l i b r a t e d to filler a n d porosiW levels is a l r e a d y well developed.
Novel ceramics by sol-gel processing Harwell claims to be able to m e e t i n c r e a s e d d e m a n d for a high level of p u r i t y a n d t e c h n i c a l q u a l i t y c o n t r o l in fields such as coatings, fibres, abrasives, c o n d u c t i n g cera m i c s a n d c a t a l y s t s using solgel p r o c e s s i n g t e c h n i q u e s it h a s developed. Free flowing s p h e r i c a l p o w d e r s can be difficult to p r o d u c e using t r a d i t i o n a l cer-
MPR March 1991 43
FIGURE 5: Experimental ceramic coating provides a thermal barrier in a heavy-duty diesel engine.
amic processing methods, says Dr David Segal of the Materials Chemistry Department. In sol-gel processing, colloidal dispersions (sols) of hydrous oxides are prepared and blended to preferred compositions before being dehydrated to gels and finally calcined. According to Segal, a wide range of oxides can be produced by this route in the form of spheres. The porosity and density of the product can be determined by the way in which the colloidal particles are dispersed in the sol. The process is said to have the following advantages: --mixing of components at the colloidal level leads to product formation at low temperature and therefore energy economy; --since the sol-gel transition is reversible there is minimal waste; use of liquid feeds permits flexibility and dust-free operation; --products can be made to a controlled shape; and - - p l a s m a s p r a y c o a t i n g s can lead to reduced wear. Powders with yttrium barium cuprate and strontium titanate compositions have been made by aqueous sol-gel processes. Plasma-spraying has been used to fabricate a strontium titanate coating ona steel substrate and to deposit yttrium barium cuprate powder on to the coated metal. Coatings were subsequently rendered superconducting by thermal treatment in an oxygen atmosphere and have been characterized by X-ray diffraction and electrical resistance measurements. -
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Powder injection moulding Powder injection moulding (PIM) is a powerful technique for near-net-shape production of complex shapes in materials which are available as relatively fine powders. The process involves mixing the 44 MPR March 1991
metal or ceramic powder with a binder to form a uniform feedstock, and injecting the feedstock into an appropriately oversized mould. This compact is then treated to remove the binder before sintering to final shape. The company has established a technology base in several areas relevant to powder injection moulding including; --powder/binder mixing, forming and debinding; --flow modelling in complex geometries; and --process monitoring by acoustic techniques. Partial support is being sought from the DTI u n d e r the RAMP initiative for a collaborative programme due to last two years, with the goal of transferring existing technologies from within AEA Technology to the PM industry. Ceramic components have been produced by plastic forming techniques for over 20 years by AEA Technology. This involves powder/binder mixing, forming, debinding and sintering. To date, work has been carried out on a broad range of techniques, including rolling, calendering, corrugating, extrusion, band-casting and injection moulding of a variety of ceramic materials. Binder systems have been developed for this range of plastic forming techniques based on mixed waxes and thermoplastic/ plasticizer/lubricant systems. These have been optimized for ease of mixing, effective forming characteristics and ease of debinding. The focus of these binder systems will be a powder system of interest to the collaborative consortium such as carbonyl iron powder. Areas of interest in b i n d e r / p o w d e r mixing include the ability of existing binders to mix with a metal powder and any adaptation required to give effective mixing, in addition to rheological properties of binder/powder systems. Injection moulding studies will cover mouldings to a generic shape (four parallel bars) under a range of injection moulding conditions. Finally, an assessment of economic routes to debind moulded forms, will be carried out. It is based initially on in-house practice and takes into account requirements for uniform shrinkage and minimized binder residues. To achieve these research goals, sintered forms will be produced. Modelling of the injection of a powder/ binder mix into a mould allows rapid optimization of mould and gate geometries. In order to construct such a model, all the significant processes have to be taken into account (ie. non-Newtonian flow, flow/thermal coupling, solidification) to the required geometrical complexity,
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FIGURE 6: The centrifugal shot casting process is particulalry suitable for producing powders from metals and alloys which suffer from the effects of oxidation on final product properties such as titanium and its alloys.
which is generally 3D. So far t h i s a r e a of w o r k h a s b e e n successful a n d a r a n g e of c o m p u t a t i o n a l fluids d y n a m i c s p r o g r a m m e s have been developed. However, optimization of injection m o u l d i n g dies r e q u i r e s a m e a n s of m o n i t o r ing o r ' f i n g e r p r i n t i n g ' of t h e i n j e c t i o n m o u l d i n g event in o r d e r to c o r r e l a t e t h e p r e d i c t i o n of a m o u l d filling c o m p u t e r p r o g r a m w i t h t h e a c t u a l s e q u e n c e of events d u r i n g m o u l d filling. Techniques for acoustic m o n i t o r i n g of m e t a l processing, a n d also f r a c t u r e events have been d e v e l o p e d by A E A Technology a n d have a l r e a d y been a p p l i e d to p o w d e r m i x i n g a n d m e t a l e x t r u s i o n . E x t e n s i o n of t h e s e t e c h n i q u e s to p a r t i c u late flow a n d m o u l d filling events will be assessed.
Centrifugal shot casting Centrifugal s h o t c a s t i n g is a m e t h o d of p r o d u c i n g m e t a l a n d r e f r a c t o r y powders, originally d e v e l o p e d for r e f r a c t o r y m a t e r i a l s u s e d as n u c l e a r r e a c t o r fuels, says Dr Peter Sutcliffe. It h a s been a d a p t e d to p r o d u c e p o w d e r s for a w i d e r a n g e of i n d u s t r i a l interests. The a d v a n t a g e s of t h e p r o c e s s are t h a t it is c a p a b l e of; - - p r o c e s s i n g a wide r a n g e of m a t e r i a l s of m e l t i n g p o i n t u p to 3500°C u n d e r a controlled atmosphere; --it can p r o d u c e a high p u r i t y p r o d u c t of low oxygen content; - - v e r s a t i l e p a r t i c l e s h a p e s from s p h e r e s to flakes a r e possible;
is a high t h r o u g h p u t r a t e a n d good p o t e n t i a l for scale up a n d d e v e l o p m e n t to c o n t i n u o u s process; - - t h e p r o c e s s h a s m i n i m a l use of e x p e n s i v e i n e r t gases such as argon; a n d - - r e s i d e n c e t i m e in m e l t p r o v i d e s a homogenization of r a d i a l s e g r e g a t i o n in b a r feed stock or t h e p e r f o r m a n c e of simult a n e o u s c h e m i c a l reactions. Centrifugal shot casting involves a s t a t i o n a r y e l e c t r o d e a n d a r o t a t i n g watercooled crucible, which u s u a l l y c o n t a i n s a skull of t h e m a t e r i a l to be p r o c e s s e d . Heating is a c c o m p l i s h e d by an electric arc struck between the electrode and the crucible. U n d e r the a c t i o n of centrifugal force t h e m e l t moves up t h e side wall to t h e lip of t h e crucible where it b r e a k s up a n d is ejected as droplets. The whole p r o c e s s of melting, atomization and solidification t a k e s p l a c e w i t h i n a leak t i g h t enclosure under a controlled atmosphere. The m e t h o d of delivery of m a t e r i a l to t h e crucible is d e t e r m i n e d by t h e e l e c t r i c a l c o n d u c t i v i t y of t h e feed. F o r m e t a l l i c m a t e r i a l s , t h e furnace is o p e r a t e d in t h e m a n n e r of a c o n s u m a b l e a r c m e l t i n g fllrnace, m a t e r i a l being m e l t e d off the end of an e l e c t r o d e of t h e r e q u i r e d c o m p o s i t i o n a n d falling as m o l t e n d r o p l e t s into the crucible. The facts, t h a t t h e e l e c t r o d e is of the m a t e r i a l being p r o c e s s e d , t h a t the solidified skull p r o t e c t s the melt within it fl'om t h e crucible m a t e r i a l a n d t h a t the p r o c e s s is c a r r i e d o u t in a high purit3', u s u a l l y inert a t m o s p h e r e , result in m i n i m a l c o n t a m i n a --there
MPR March 1991 45
FIGURE 8: Vacuum plasma spray chamber showing robot, tools and indexing turntable.
tion. F o r n o n - m e t a l l i c r e ~ a c t o r y or ceramic m a t e r i a l s , a n a n n u l a r n o n - c o n s u m a b l e elect r o d e is u s e d a n d t h e m a t e r i a l to be s p h e r o i d i z e d is fed in a s u i t a b l e form down t h e c e n t r e of t h e electrode, t h r o u g h t h e arc p l a s m a , into t h e crucible. A wide s p e c t r u m of m e t a l a n d alloy p o w d e r s have a l r e a d y b e e n p r e p a r e d by t h i s m e t h o d from p u r e iron, t h r o u g h low alloyed steels, s t a i n l e s s steels, high s t r e n g t h steels, tool steels, high t e m p e r a t u r e steels t h r o u g h nickel b a s e d superalloys, c o b a l t alloys a n d t i t a n i u m alloys to t h e r e f r a c t o r y m e t a l s such as t a n t a l u m a n d niobium. The t e c h n i q u e is said to be p a r t i c u l a r l y s u i t a b l e for t h e p r e p a r a t i o n of p o w d e r s of t h o s e m e t a l s a n d alloys w h i c h e i t h e r suffer from t h e effects of o x i d a t i o n on final p r o d u c t p r o p e r t i e s e.g. t i t a n i u m a n d its alloys, or are s u b j e c t to gross s e g r e g a t i o n e.g. tool steels, or are difficult to form by h o t working b e c a u s e of t h e p r e c i p i t a t i o n h a r d e n i n g effects e.g. a d v a n c e d superalloys. In general t h e n a t u r e of t h e p r o d u c t can be v a r i e d by c o n t r o l of t h e o p e r a t i n g c o n d i t i o n s from e s s e n t i a l l y s p h e r o i d a l mat e r i a l of high p a c k i n g d e n s i t y to flake w h i c h h a s significant g r e e n - s t r e n g t h in t h e asp r e s s e d condition. The s p h e r o i d a l p a r t i c l e size d i s t r i b u t i o n a p p r o x i m a t e s to a l o g a r i t h m i c G a u s s i a n
46 MPR March 1991
distribution with a mean diameter that can be c o n t r o l l e d in t h e r a n g e 150-1000 microns and a geometric standard deviation of 0.3-0.4. The chief p a r a m e t e r s c o n t r o l l i n g t h e m e a n d i a m e t e r a r e t h e crucible r o t a t i o n a l s p e e d a n d to a l e s s e r e x t e n t t h e crucible d i a m e t e r a n d p r o p e r t i e s of t h e m o l t e n s t a t e of t h e m a t e r i a l . The c o n t r o l l e d e n v i r o n m e n t u n d e r w h i c h t h e p r o c e s s is c a r r i e d o u t l e a d s to a high q u a l i t y m e t a l l i c p o w d e r of s m a l l g r a i n size and with an oxygen content essentially m a i n t a i n e d to t h e level of t h e s t a r t i n g material.
Future projects I
Work will c o n t i n u e a t t h e A E A ' s l a b o r a t o r i e s in t h e a r e a of PM research. In a d d i t i o n to t h e p r o j e c t s u n d e r w a y t h e r e a r e several o t h e r initiatives w h i c h have b e e n p r o p o s e d i n c l u d i n g t h e w o r k on p o w d e r i n j e c t i o n m o u l d i n g . T h e r e will also be E u r o p e a n c o l l a b o r a t i v e p r o g r a m m e s on r a p i d solidific a t i o n t e c h n i q u e s a p p l i e d to recycling of a l u m i n i u m alloys a n d on fibre r e i n f o r c e d a l u m i n i d e c o m p o s i t e s in c o n n e c t i o n w i t h vacuum plasma spraying techniques.
HarweU, one of the UK Atomic Energy Authority's development laboratories, is home to many interesting projects in powder metallurgy (PM). Its researchers have been responsible for a number of innovations in P M over the last 2 0 years. Metal Powder Report visited Harwell to find out about current developments in the field. II
II
I
A
EA Technology, the trading name of the UK Atomic Energy Authority, is Europe's largest contract research organization with over 10 000 staff and a turnover of over £400 million. Its major industrial market is Europe where it has had 1300 contracts with non-UK customers in the European Community over the past three years, and is currently the prime contractor for 76 research and development projects for the European Commission. The company is also involved in worldwide research, it is the UK's largest contractor to the US Strategic Defense Initiative and presently has more than 20 major contracts with the Far East. Metal Powder Report recently visited Harwell in Oxfordshire, the AEA's largest development laboratory employing some 4000 people. Dr Andy Feest, head of the Advanced Engineering Materials Department, outlined AEA Technology's powder metallurgy (PM) work, most of which is carried out at Harwell. He says that the company has implemented a total system approach to materials development involving design, performance prediction, process optimization, manufacture, testing, reliability and n o n - d e s t r u c t i v e testing (NDT). It draws on in-house centres of excellence such as the National NDT Centre and National Tribology Centre. It has been involved in PM since the
40 MPR March 1991
0026.-0657/91/$3.50 ©, Elsevier Science Publishers Ltd
1960s, initially being involved with uranium fuels (UO2) and later b r o a d e n i n g its research to include carbides for use as fast reactor fuels. Originally set up to support the UK nuclear industry, the AEA's laboratories have seized the opportunity to sell their skills and technologies to other industries which can benefit from the specialized expertise gained under the harsh conditions of the nuclear power environment. Its diversification to encompass non-nuclear activities has meant the adaption of many technologies for different uses. For example, protective atmosphere processing technologies originally developed in the carbide fuels field were subsequently used as a basis for work in the late 1960s on ultrafine PM. The initial developments were in the fields of refractory metals, iron and tool steels, and exploited the lower sintering temperatures and improved mechanical properties, particularly toughness, arising from refined microstructures. Another offshoot of nuclear technology is the centrifugal shot casting (CSC) process for the preparation of high purity alloy powders. CSC evolved from technology for producing spherical carbide fuel particles of controlled diameter with the aim of achieving optimum packing. It has led to a family of related processes used in the preparation of refractory or reactive alloys. Progress made at Harwell in the 1960s included ceramic/plastic forming developments such as ceramic injection moulding. This technology is now being adapted to the metal injection moulding field. A similar success story is the powder infiltration technology developed for fibre reinforced glasses which has recently been applied to fibre reinforced metals. The AEA says that the growth of the PM industry has swelled the number of outlets for its multidisciplinary capabilities. Recent developments include: -- process monitoring in metal atomization; --plasma spraying of coatings, monoliths and composites; --co-spray processing of particulate rein-
Atomization process control
FIGURE 1: M o n i t o r i n g atomization on metal atomizer.
forced m e t a l m a t r i x c o m p o s i t e s (MMC); - - p o w d e r p r e p a r a t i o n (eg. for s p r a y i n g or for c o m p o s i t e fillers); --non-destructive t e s t i n g of PM MMC; - - C o n f o r m p r o c e s s i n g of PM alloys a n d MMC; a n d - s t r u c t u r e / p r o p e r t y r e l a t i o n s h i p s in PM MMC. Many of t h e r e c e n t d e v e l o p m e n t s have been c a r r i e d o u t on b e h a l f of c o n s o r t i a of c o m p a n i e s often with p a r t i a l s u p p o r t from t h e UK D e p a r t m e n t of Trade a n d I n d u s t r y (DTI) or t h e C o m m i s s i o n of t h e E u r o p e a n C o m m u n i t i e s (CEC), w h e r e a s o t h e r s have been c o n d u c t e d as confidential p r o j e c t s for single clients. F e e s t says t h a t e x p l o i t a t i o n of PM d e v e l o p m e n t s h a s g e n e r a l l y been d o n e by t h e i n d u s t r i a l p a r t n e r s c o l l a b o r a t i n g in t h e p r o j e c t s b u t A E A Technology is also e n g a g e d in m a n u f a c t u r i n g in a s m a l l n u m b e r of specific m a r k e t s , for e x a m p l e ThO2 a r t e f a c t s for t h e c h e m i c a l industry, a n d t h o r i u m a r t e f a c t s for t h e electrical a n d electronic industries.
Current PM projects at Harwell ]
I
A n u m b e r of p r o j e c t s on PM r e l a t e d issues are c u r r e n t l y u n d e r w a y at Harwell. These include: - - a DTI LINK p r o g r a m m e on TiA1 m a t r i c e s for c o m p o s i t e s - - r a p i d l y s o l i d i f i e d p o w d e r c o m p a c t i o n is one of t h e c o m p e t ing p r o c e s s i n g r o u t e s being evaluated; - - m o d e l l i n g of e x t r u s i o n of m e t a l p o w d e r billets - - ongoing i n - h o u s e work; --application of Conform p r o c e s s i n g to PM products; -non-destructive t e s t i n g of MMC; a n d - - a t o m i z a t i o n control.
A E A Technology is working on, m a i n l y gas, a t o m i z a t i o n techniques, u s e d to p r o d u c e p o w d e r s of a d v a n c e d alloys a n d also for p a r t i c l e reinforced m e t a l m a t r i x c o m p o sites. P r e s e n t i n d u s t r i a l p r a c t i c e involves a s s e s s i n g the size d i s t r i b u t i o n of a p o w d e r after p r o d u c t i o n - - which can be s o m e hours, or even a day, later. The p o w d e r s a r e then separated into grades with tight d i s t r i b u t i o n s by sieving a n d m a t e r i a l n o t m e e t i n g r e q u i r e m e n t s is reprocessed. Dr Negus, r e s p o n s i b l e for t h e a t o m i z a tion p r o c e s s control p r o g r a m m e at Harwell, t o l d Metal P o w d e r Report t h a t t h e s e p r o b l e m s can be solved by on-line control of t h e m e a n d r o p l e t size a n d d i s t r i b u t i o n width. The a b i l i t y to switch to n e w o r different m a t e r i a l s w i t h o u t w a s t e is also an i m p o r t a n t target. Fast, reliable, on-line m e a s u r e m e n t in d e n s e p a r t i c l e s t r e a m s with good s a m p l i n g s t a t i s t i c s is difficult, says Negus. Optical t e c h n i q u e s have been d e v e l o p e d for studying v a r i o u s a s p e c t s of a t o m i z a t i o n such as core b r e a k - u p , d r o p l e t velocity, d r o p l e t s i z e , d r o p l e t c o n c e n t r a t i o n a n d flux. The AEA's b a c k g r o u n d in b o t h t h e n u c l e a r and n o n - n u c l e a r i n d u s t r i e s h a s m a d e it p a r t i c u larly w e l l - e q u i p p e d to h a n d l e such a project. It h a s m o r e t h a n t e n y e a r s ' e x p e r i e n c e in developing non-invasive t e c h n i q u e s for use in hostile e n v i r o n m e n t s ( i n c l u d i n g electronics for signal p r o c e s s i n g a n d c o n t r o l ) . L a b o r a t o r y i n s t r u m e n t s have been used in a v a r i e t y of i n d u s t r i a l s i t u a t i o n s b u t t h e recent thrust has been towards small, r o b u s t i n s t r u m e n t s for on-line m o n i t o r i n g by t h e use of s o l i d - s t a t e lasers a n d d e t e c t o r s o r by t h e use of fibre optics. A previous p r o g r a m m e on a t o m i z a t i o n involved a s t u d y of d e n s e w a t e r sprays a n d i m p i n g i n g w a t e r j e t s at BNFL Metals a n d also setting u p an a t o m i z e r at Harwell in c o n j u n c t i o n with O s p r e y Metals of Neath, UK. A s s e s s m e n t of d i a g n o s t i c t e c h n i q u e s for iso-kinetic s a m p l i n g for size a n d velocity m e a s u r e m e n t s on a l a b o r a t o r y a t o m i z e r was c a r r i e d out, a n d a w h i t e - l i g h t p a r t i c l e m o n i t o r was d e v e l o p e d a n d t e s t e d in an i n d u s t r i a l plant. Negus told Metal Powder Report t h a t a new i n d u s t r y c o - s p o n s o r e d two y e a r A E A p r o g r a m m e with funding to t h e t u n e of £230 000 has been a p p r o v e d by t h e DTI a n d will s t a r t early in 1991 at Harwell. The objective is on-line control of m e a n p o w d e r size a n d on-line m o n i t o r i n g of d i s t r i b u t i o n width, he says. The m i c r o p r o c e s s o r system could allow r a p i d e x p a n s i o n to the control of o t h e r p a r a m e t e r s . The system will be d e m o n s t r a t e d on a m e t a l a t o m i z e r a t Harwell with 2 k g / m i n u t e t h r o u g h p u t . MPR March 1991 41
= - - - . - - - ~ ~'e~E~-,,] =.1[= [ ~ Ir:~ ~ ~ ::~±'~ t e j _i : : j m m m l
PA~]flCUt A l L
J t
..........
'
PRODUCI
FIGURE 2: The Conform process can extrude infinite lengths of materials from powder materials,
Conform process
FIGURE 3: The Conform production evaluation line at Risley, UK.
42
M P R M a r c h 1991
Developed by the AEA at its Springfield Laboratory, the Conform process is an extrusion technology with the ability to extrude infinite lengths of material. It involves a rotating extrusion wheel which is grooved and carries the feedstock - - both particulate feed and solid rod - - under the grip segment. In this region the feedstock is plastically deformed to fill the groove. The material then meets the abutment, which forces it to pass through the die to become the desired product. The AEA claims there are over 90 production machines operating worldwide with solid bar feedstock. According to
delegates at a seminar on the process held recently at the AEA's Risley laboratory site, Conform using powders, although still in development stages is currently capable of producing advanced materials and MMCs and is ripe for commercial uses. In the future the AEA hopes to use Conform to produce bearings alloys and electrical contacts. The process has m a n y a d v a n t a g e s , claims Jon Heywood of the Risley site, since it is basically a single stage process. Powder is simply consolidated and extruded thus avoiding sintering, mechanical alloying, spray deposition, canning or hot isostatic pressing. He believes that products produced using the Conform process will be c h e a p e r as a result of more efficient materials usage and a less labour intensive process. Considerable work is being carried out to investigate the variety of feedstock powders, reinforcements and other additions. An in-depth feature on Conform will appear in the April issue of Metal Powder Report.
Mechanically alloyed powder materials Work is also being carried out on mechanically alloyed powder materials (MA) at Risley by Andy Jones. MA materials are produced by high energy attrition of matrix powders and reinforcement particles. In particular, under the Cost 501, a European collaborative research programme, researchers are trying to develop and optimize high temperature MA Fe-Cr-A1 alloy which has properties such as biaxial creep strength and high temperature oxidation resistance. It is proposed to use the material in heat exchangers. Work is also being carried out to investigate the effect of powder processing on materials performance, in particular the effect of entrained defects on grain structures and biaxial creep performance. High nitrogen austenitic steels with considerably improved wear, corrosion and high temperature properties are also being developed. Manufacturing routes under investigation include; (1). MA TiN Particulate TiN is added to stainless alloy powders, mechanically alloyed and extruded at high temperature where the variable is the volume fraction of TiN. (2). NIDON CrxN is added to stainless steel powder blend, mechanically alloyed, and extruded at high temperature (CrxN donates N to Ti) where the variables are the volume fraction
of TiN a n d dissolved N. (3). NIDONA S t a i n l e s s a l l o y p o w d e r s a r e n i t r i d e d in a m m o n i a to form a CrxN crust on t h e p o w d e r surface, a n d e x t r u d e d a t h i g h temperature. (4). Injection The d o n o r can be injected as p a r t of a cos p r a y i n g process, a n d t h e p r o d u c t m a y be h o t i s o s t a t i c a l l y p r e s s e d (HIPed) or e x t r u d e d a t high t e m p e r a t u r e w h e r e t h e v a r i a b l e s a r e t h e v o l u m e fraction of TiN a n d dissolved N. I m p r o v e d p r o p e r t i e s in m a t e r i a l s from t h e m a j o r i t y of t h e s e p r o c e s s r o u t e s come from a c o m b i n a t i o n of s t r e n g t h t h r o u g h h a r d e n i n g with d i s p e r s e d t i t a n i u m n i t r i d e a n d alloying w i t h nitrogen. The p r o j e c t s t e m s from e a r l i e r w o r k to develop TiN s t r e n g t h e n e d 20Cr/25Ni a u s t e n i t i c steels as thin w a l l e d t u b e for n u c l e a r fuel cladding. This achieved high t e m p e r a t u r e s t r e n g t h t h r o u g h gas n i t r i d i n g of t h i n walled tubes. M a n u f a c t u r i n g r o u t e s for t h i c k s e c t i o n s have been d e v e l o p e d using PM techniques. The TiN p r o v i d e s very effective h a r d e n i n g and also imparts desirable tribological properties. Hyper-stoichiometric additions of N l e a d to beneficial corrosion p r o p e r t i e s . A p p l i c a t i o n s for s u c h m a t e r i a l s i n c l u d e impellers, valves, p u m p p a r t s a n d general l o a d e d r u b b i n g c o n t a c t s p a r t i c u l a r l y for t h e food p r o c e s s i n g a n d c h e m i c a l i n d u s t r y w h e r e r e q u i r e m e n t s are for low a n d high t e m p e r a t u r e corrosion a n d a b r a s i o n resistance. F u r t h e r a p p l i c a t i o n s could i n c l u d e cryogenics a n d s p a c e w h e r e t h e high p r o o f strength, s t a b l e a u s t e n i t i c s t r u c t u r e a n d tribological p r o p e r t i e s a r e i m p o r t a n t .
Non-destructive testing of MMCS A basic c o n s t r a i n t on MMC use is t h a t p e r f o r m a n c e c o n t r o l l i n g f a c t o r s such as fibre v o l u m e - f r a c t i o n , m i c r o p o r o s i t y a n d fibreorientation are within specification, says Dr G r a h a m C u r t i s of t h e NDT Centre. Hence q u a l i t y control e m p l o y i n g NDT is of t h e u t m o s t i m p o r t a n c e , he adds. S e v e r a l p r o g r a m m e s i n v o l v i n g MMC m a n u f a c t u r e r s a n d u s e r s are u n d e r w a y at t h e N a t i o n a l NDT Centre a t Harwell a i m e d a t p r o v i d i n g this q u a l i t y control.
(1) Defining the basis of a stiffness quality monitor This p r o g r a m m e h a s a l r e a d y e s t a b l i s h e d t h a t p a r t i c u l a t e SiC filler a n d m i c r o p o r o s i t y levels in 2014 a l u m i n i u m alloy could in fact be n o n d e s t r u c t i v e l y d e t e r m i n e d via m e a s u r e m e n t s of u l t r a s o n i c wave velocity a n d a t t e n u a t i o n . Analysis of t h e s e factors can
FIGURE 4: Instrumental 30 tonne vacuum hot press for consolidation of metal matrix composites (MMC) powder compacts
lead to t h e d e t e r m i n a t i o n of a stiffness quality monitor.
(2) Manufacturing nonuniformities monitoring This p r o g r a m m e aims to d e t e c t significant defects in MMCs which can e s c a p e notice, in-service, for s o m e time. There is concern over p o s s i b l e m a n u f a c t u r i n g non-uniformities of MMCs which m a y vary according to t h e m a n u f a c t u r i n g r o u t e taken. The Centre can demonstrate the most appropriate nondestructive methods available using s p e c i m e n s of 2014/SIC or 8090/SIC prod u c e d by v a c u u m h o t p r e s s i n g a n d rolling, which c o n t a i n an e x a m p l e of one of two or t h r e e types of ' m a n u f a c t u r i n g non-uniformities'.
(3) Filler and porosity monitoring This p r o g r a m m e is d e s i g n e d to investigate SiC filler v o l u m e - f r a c t i o n a n d m i c r o p o r o s i t y levels in 2014 a l u m i n i u m alloy. It is t h e first s t e p in a d a p t i n g t h e b a s i c c o n c e p t for production quality control. A software p a c k a g e with d a t a of wave velociW a n d a t t e n u a t i o n c a l i b r a t e d to filler a n d porosiW levels is a l r e a d y well developed.
Novel ceramics by sol-gel processing Harwell claims to be able to m e e t i n c r e a s e d d e m a n d for a high level of p u r i t y a n d t e c h n i c a l q u a l i t y c o n t r o l in fields such as coatings, fibres, abrasives, c o n d u c t i n g cera m i c s a n d c a t a l y s t s using solgel p r o c e s s i n g t e c h n i q u e s it h a s developed. Free flowing s p h e r i c a l p o w d e r s can be difficult to p r o d u c e using t r a d i t i o n a l cer-
MPR March 1991 43
FIGURE 5: Experimental ceramic coating provides a thermal barrier in a heavy-duty diesel engine.
amic processing methods, says Dr David Segal of the Materials Chemistry Department. In sol-gel processing, colloidal dispersions (sols) of hydrous oxides are prepared and blended to preferred compositions before being dehydrated to gels and finally calcined. According to Segal, a wide range of oxides can be produced by this route in the form of spheres. The porosity and density of the product can be determined by the way in which the colloidal particles are dispersed in the sol. The process is said to have the following advantages: --mixing of components at the colloidal level leads to product formation at low temperature and therefore energy economy; --since the sol-gel transition is reversible there is minimal waste; use of liquid feeds permits flexibility and dust-free operation; --products can be made to a controlled shape; and - - p l a s m a s p r a y c o a t i n g s can lead to reduced wear. Powders with yttrium barium cuprate and strontium titanate compositions have been made by aqueous sol-gel processes. Plasma-spraying has been used to fabricate a strontium titanate coating ona steel substrate and to deposit yttrium barium cuprate powder on to the coated metal. Coatings were subsequently rendered superconducting by thermal treatment in an oxygen atmosphere and have been characterized by X-ray diffraction and electrical resistance measurements. -
-
Powder injection moulding Powder injection moulding (PIM) is a powerful technique for near-net-shape production of complex shapes in materials which are available as relatively fine powders. The process involves mixing the 44 MPR March 1991
metal or ceramic powder with a binder to form a uniform feedstock, and injecting the feedstock into an appropriately oversized mould. This compact is then treated to remove the binder before sintering to final shape. The company has established a technology base in several areas relevant to powder injection moulding including; --powder/binder mixing, forming and debinding; --flow modelling in complex geometries; and --process monitoring by acoustic techniques. Partial support is being sought from the DTI u n d e r the RAMP initiative for a collaborative programme due to last two years, with the goal of transferring existing technologies from within AEA Technology to the PM industry. Ceramic components have been produced by plastic forming techniques for over 20 years by AEA Technology. This involves powder/binder mixing, forming, debinding and sintering. To date, work has been carried out on a broad range of techniques, including rolling, calendering, corrugating, extrusion, band-casting and injection moulding of a variety of ceramic materials. Binder systems have been developed for this range of plastic forming techniques based on mixed waxes and thermoplastic/ plasticizer/lubricant systems. These have been optimized for ease of mixing, effective forming characteristics and ease of debinding. The focus of these binder systems will be a powder system of interest to the collaborative consortium such as carbonyl iron powder. Areas of interest in b i n d e r / p o w d e r mixing include the ability of existing binders to mix with a metal powder and any adaptation required to give effective mixing, in addition to rheological properties of binder/powder systems. Injection moulding studies will cover mouldings to a generic shape (four parallel bars) under a range of injection moulding conditions. Finally, an assessment of economic routes to debind moulded forms, will be carried out. It is based initially on in-house practice and takes into account requirements for uniform shrinkage and minimized binder residues. To achieve these research goals, sintered forms will be produced. Modelling of the injection of a powder/ binder mix into a mould allows rapid optimization of mould and gate geometries. In order to construct such a model, all the significant processes have to be taken into account (ie. non-Newtonian flow, flow/thermal coupling, solidification) to the required geometrical complexity,
^ss~
~RCOOkEO ,HI~E TROOE
FIGURE 6: The centrifugal shot casting process is particulalry suitable for producing powders from metals and alloys which suffer from the effects of oxidation on final product properties such as titanium and its alloys.
which is generally 3D. So far t h i s a r e a of w o r k h a s b e e n successful a n d a r a n g e of c o m p u t a t i o n a l fluids d y n a m i c s p r o g r a m m e s have been developed. However, optimization of injection m o u l d i n g dies r e q u i r e s a m e a n s of m o n i t o r ing o r ' f i n g e r p r i n t i n g ' of t h e i n j e c t i o n m o u l d i n g event in o r d e r to c o r r e l a t e t h e p r e d i c t i o n of a m o u l d filling c o m p u t e r p r o g r a m w i t h t h e a c t u a l s e q u e n c e of events d u r i n g m o u l d filling. Techniques for acoustic m o n i t o r i n g of m e t a l processing, a n d also f r a c t u r e events have been d e v e l o p e d by A E A Technology a n d have a l r e a d y been a p p l i e d to p o w d e r m i x i n g a n d m e t a l e x t r u s i o n . E x t e n s i o n of t h e s e t e c h n i q u e s to p a r t i c u late flow a n d m o u l d filling events will be assessed.
Centrifugal shot casting Centrifugal s h o t c a s t i n g is a m e t h o d of p r o d u c i n g m e t a l a n d r e f r a c t o r y powders, originally d e v e l o p e d for r e f r a c t o r y m a t e r i a l s u s e d as n u c l e a r r e a c t o r fuels, says Dr Peter Sutcliffe. It h a s been a d a p t e d to p r o d u c e p o w d e r s for a w i d e r a n g e of i n d u s t r i a l interests. The a d v a n t a g e s of t h e p r o c e s s are t h a t it is c a p a b l e of; - - p r o c e s s i n g a wide r a n g e of m a t e r i a l s of m e l t i n g p o i n t u p to 3500°C u n d e r a controlled atmosphere; --it can p r o d u c e a high p u r i t y p r o d u c t of low oxygen content; - - v e r s a t i l e p a r t i c l e s h a p e s from s p h e r e s to flakes a r e possible;
is a high t h r o u g h p u t r a t e a n d good p o t e n t i a l for scale up a n d d e v e l o p m e n t to c o n t i n u o u s process; - - t h e p r o c e s s h a s m i n i m a l use of e x p e n s i v e i n e r t gases such as argon; a n d - - r e s i d e n c e t i m e in m e l t p r o v i d e s a homogenization of r a d i a l s e g r e g a t i o n in b a r feed stock or t h e p e r f o r m a n c e of simult a n e o u s c h e m i c a l reactions. Centrifugal shot casting involves a s t a t i o n a r y e l e c t r o d e a n d a r o t a t i n g watercooled crucible, which u s u a l l y c o n t a i n s a skull of t h e m a t e r i a l to be p r o c e s s e d . Heating is a c c o m p l i s h e d by an electric arc struck between the electrode and the crucible. U n d e r the a c t i o n of centrifugal force t h e m e l t moves up t h e side wall to t h e lip of t h e crucible where it b r e a k s up a n d is ejected as droplets. The whole p r o c e s s of melting, atomization and solidification t a k e s p l a c e w i t h i n a leak t i g h t enclosure under a controlled atmosphere. The m e t h o d of delivery of m a t e r i a l to t h e crucible is d e t e r m i n e d by t h e e l e c t r i c a l c o n d u c t i v i t y of t h e feed. F o r m e t a l l i c m a t e r i a l s , t h e furnace is o p e r a t e d in t h e m a n n e r of a c o n s u m a b l e a r c m e l t i n g fllrnace, m a t e r i a l being m e l t e d off the end of an e l e c t r o d e of t h e r e q u i r e d c o m p o s i t i o n a n d falling as m o l t e n d r o p l e t s into the crucible. The facts, t h a t t h e e l e c t r o d e is of the m a t e r i a l being p r o c e s s e d , t h a t the solidified skull p r o t e c t s the melt within it fl'om t h e crucible m a t e r i a l a n d t h a t the p r o c e s s is c a r r i e d o u t in a high purit3', u s u a l l y inert a t m o s p h e r e , result in m i n i m a l c o n t a m i n a --there
MPR March 1991 45
FIGURE 8: Vacuum plasma spray chamber showing robot, tools and indexing turntable.
tion. F o r n o n - m e t a l l i c r e ~ a c t o r y or ceramic m a t e r i a l s , a n a n n u l a r n o n - c o n s u m a b l e elect r o d e is u s e d a n d t h e m a t e r i a l to be s p h e r o i d i z e d is fed in a s u i t a b l e form down t h e c e n t r e of t h e electrode, t h r o u g h t h e arc p l a s m a , into t h e crucible. A wide s p e c t r u m of m e t a l a n d alloy p o w d e r s have a l r e a d y b e e n p r e p a r e d by t h i s m e t h o d from p u r e iron, t h r o u g h low alloyed steels, s t a i n l e s s steels, high s t r e n g t h steels, tool steels, high t e m p e r a t u r e steels t h r o u g h nickel b a s e d superalloys, c o b a l t alloys a n d t i t a n i u m alloys to t h e r e f r a c t o r y m e t a l s such as t a n t a l u m a n d niobium. The t e c h n i q u e is said to be p a r t i c u l a r l y s u i t a b l e for t h e p r e p a r a t i o n of p o w d e r s of t h o s e m e t a l s a n d alloys w h i c h e i t h e r suffer from t h e effects of o x i d a t i o n on final p r o d u c t p r o p e r t i e s e.g. t i t a n i u m a n d its alloys, or are s u b j e c t to gross s e g r e g a t i o n e.g. tool steels, or are difficult to form by h o t working b e c a u s e of t h e p r e c i p i t a t i o n h a r d e n i n g effects e.g. a d v a n c e d superalloys. In general t h e n a t u r e of t h e p r o d u c t can be v a r i e d by c o n t r o l of t h e o p e r a t i n g c o n d i t i o n s from e s s e n t i a l l y s p h e r o i d a l mat e r i a l of high p a c k i n g d e n s i t y to flake w h i c h h a s significant g r e e n - s t r e n g t h in t h e asp r e s s e d condition. The s p h e r o i d a l p a r t i c l e size d i s t r i b u t i o n a p p r o x i m a t e s to a l o g a r i t h m i c G a u s s i a n
46 MPR March 1991
distribution with a mean diameter that can be c o n t r o l l e d in t h e r a n g e 150-1000 microns and a geometric standard deviation of 0.3-0.4. The chief p a r a m e t e r s c o n t r o l l i n g t h e m e a n d i a m e t e r a r e t h e crucible r o t a t i o n a l s p e e d a n d to a l e s s e r e x t e n t t h e crucible d i a m e t e r a n d p r o p e r t i e s of t h e m o l t e n s t a t e of t h e m a t e r i a l . The c o n t r o l l e d e n v i r o n m e n t u n d e r w h i c h t h e p r o c e s s is c a r r i e d o u t l e a d s to a high q u a l i t y m e t a l l i c p o w d e r of s m a l l g r a i n size and with an oxygen content essentially m a i n t a i n e d to t h e level of t h e s t a r t i n g material.
Future projects I
Work will c o n t i n u e a t t h e A E A ' s l a b o r a t o r i e s in t h e a r e a of PM research. In a d d i t i o n to t h e p r o j e c t s u n d e r w a y t h e r e a r e several o t h e r initiatives w h i c h have b e e n p r o p o s e d i n c l u d i n g t h e w o r k on p o w d e r i n j e c t i o n m o u l d i n g . T h e r e will also be E u r o p e a n c o l l a b o r a t i v e p r o g r a m m e s on r a p i d solidific a t i o n t e c h n i q u e s a p p l i e d to recycling of a l u m i n i u m alloys a n d on fibre r e i n f o r c e d a l u m i n i d e c o m p o s i t e s in c o n n e c t i o n w i t h vacuum plasma spraying techniques.