semisolid proccesing kirkwood d.h

8/10/2019 Semisolid Proccesing Kirkwood D.H.

1/17

Semisolid metal processing

D. H. Kirkwood

Semisolid metal processing is a relatively new

method for forming alloys in the semisolid

condition to near net shaped products. It relies

on the behaviour of semisolid slurries in which

the solid exists in the form of spheroidal

particles: if left undisturbed such slurries are

stiff and may be handled, but flow like liquids

on being sheared. Slugs of semisolid alloy may

therefore be injected into a die (thixocasting) or

shaped between closed dies (thixoforging) to

produce components near-to-shape with good

surface finish, free from porosity, and

possessing fine uniform microstructures, whichmay be heat treated to give superior mechanical

properties. The rheology of non-dendritic alloy

slurries and the recent ideas which have beenproposed to account for their pseudoplastic and

thixotropic behaviour are considered. This is

followed by a description of the technologies

available for producing non-dendritic structures

and the processes for subsequently shaping the

semisolid alloy. Finally, some indication is

given of the mechanical properties of

components produced by the semisolid

processing route. IMR/263

1994 The Institute of Materials and ASM International.The author is in the Department of Engineering

Materials, University of Sheffield, Sheffield S1 4DU, UK.

Introduction

The beginnings of interest in the mechanical prop-erties and rheology of semisolid alloys which eventu-ally led to semisolid metal processing can be traced

back to the work of Flemings and his co-workers atMIT in the early 1970s. This work was originallydirected at the problem of hot tearing in alloy castings,but it was quickly realised that a new technology fornear net shaping of complex forms had been discov-ered which resulted in a flow of patents from thatsource. In these days of narrowly targeted researchwhen investigators are required to define goals andidentify de1iverables,it is perhaps timely to rememberthat important discoveries are often made accident-ally, and the ability to recognise unexpected events,the vision to see potential value, and the freedom tofollow up clues are all vital factors in making import-ant breakthroughs in science and technology.

It is 20 years since the original discovery by Spenceret al? at MIT of the unusual rheological propertiesof vigorously stirred tin-lead slurries and the commer-cialisation of this discovery has apparently been slowin spite of a great deal of effort and money havingbeen invested in the development of the technology,as well as in the scientific understanding of semisolid

processing. However, the occurrence of two recentinternational conferences devoted to the topic, thefirst at Sophia Antipolis in 19902 and the second at

MIT in 1992,3 testifies to a new awareness of thepotential advantages in this novel technology, shownboth in Europe and Japan as well as in the USA, andthis wider interest may at last lead to an accelerationin its adoption by industry during the 1990s. A thirdinternational conference was held in Japan in June1994.

This review is an attempt to cover both the scientificbackground and the technological aspects of theprocess as far as they are understood at present. Forthe former the author has drawn heavily on the recentreview by Professor Flemings," and on the article by

Kenney et a1.5

for the practical aspects of the tech-nology. Both areas have been supplemented by theproceedings of the two international conferencesreferred to abovc.v"

Terminology

The process of stirring alloys during solidification toproduce non-dendritic solid within a slurry and theninjecting this semisolid slurry directly into dies as inliquid metal die casting, was originally called rheo-casting. An alternative process whereby the slurry isfirst cast as a billet, cut into appropriately sized slugs,and reheated back to the semisolid condition beforeinjecting into the die, was termed thixocasting. Theequivalent process in which the semisolid slug isshaped between closed dies was called thixoforging.All these variants for shaping alloys are now collec-tively referred to as semisolid metal forming processes(SSM), but all these terms are used in this review.The term 'semisolid metal processing' is used here tocover both operations of shaping and of productionof the raw billet.

Rheology of alloy slurriesThe modelling of slurry flow into die cavities duringthixoforming requires a fundamental understandingof their rheological behaviour and a knowledge ofthe basic parameters which control the process.Concentric cylinder viscometers or rheometers areusually employed for this purpose since the sheargeometry is simple and the shear within the gap canbe made to be acceptably uniform. This condition isparticularly important for non-newtonian fluids. Twotypes of cylindrical viscometer are possible: the Searletype in which the inner cylinder rotates and the outer

remains stationary, and the Couette which has arotating outer cylinder which inhibits the onset ofturbulent flow.

Alloy slurries of tin-lead were the first to beinvestigated, since their low working temperature(I'J200C) allows convenient experimental conditionsand more accurate data to be obtained. Also thedensity difference between solid and liquid in thisalloy system is small, reducing the possible effect ofparticle segregation.

International Materials Reviews 1994 Vol. 39 No.5 173


2/17

174 Kirkwood Semisolid metal processing

7r----------------..1.6

61.4

6

1.2

5N

1.0EN

Z 'E~c ri

(J)

(/) z

0.8~ ~4I- U i(/) 0

0: U

~ e n

0.6~sl-

(/)~3a:

~a. .

0.4


3/17

3 Structure of Sn-15 wt-%Pb sample sheared

continuously at 750s-1andcooled at 033K min-1

to afraction solid of 055 (Ref.6) x 50

viscosity behaviour of many non-newtonian fluids.Joly and Mehrabian attempted to fit their results onSn-Pb slurries to this equation, and the values of nare recorded in Table 1 for 045 fraction solid as wellas the value of 1]determined at y = 200 S-1.

The generally accepted explanation of the pseudo-plastic flow observed in alloy slurries is that structurein the form of agglomerates or clusters of primary

( a ) c { ?

( b ) ~Structure evolution in rheocasting

( C l~

increasing shear rate

increasing time

decreasing cooling rate

~( d l~

( e lOa initial dendritic fragment; b dendritic growth; c rosette;

d ripened rosette; e spheroid

4 Schematic illustration of evolution of structure

during solidification with vigorous agitation

(after Ref.4)

Kirkwood Semisolid metal processing 175

- . . . :a . >

0: :

I

N

(J)

I 00E N L!) '

'; ".0(J) '; "

c o 0 (J)U ' < : t 0'0,

I V

' < : t

0 . ; : > - - /\~0 ~ 00

. ; : > - - (Y) C ? (Y)a . > ,,(0 I " " -

0 6 6 6 6..c II I " " - L!) 0a: lO : : " c 6 6 6 6 ~ I + I I +

" -~o

.S ; ~c a a . >

. s : : . EQ) 0

. D . u(J)

(ij : >C o >

's ,o

(5Q ,)

. s : : .

a:

E

a . > E6o a.~ a . >

< . 9 " 0

~ Eo E

oL!)

l " " -

IoN

(y)0'>

o(0

> -~co.0

Q..

L!)

~ 'IMO6L!)

II I~C < ; ) .;:> - -

oL!)

l " " -

IooN

' < : t 0(0 L!)I""- N

I IL!) 0

. co: N

o. ~

II. ; : > - -

I(J)

o 0.?- 0'>

" : I ~. ;: > - - ; :> - -

o.;:> - -

*

a . >Co

Z

in(j)

6

: ( Y)

L!)

L!)

oo0'>

Io(0

a . >Co

Z

~~r oa . >

..c(J)a . >

Eco(J)

1 0"0

a . >

: 5(J)

coa . >

E"0

Cco

"0

a . >

r o..c a.a. a . >co Q .

E .~II

, ;:> - -

i~coa . >

..c(J)

Co

'~

~coa.

~a.

International Materials Reviews 1994 Vol. 39 No.5

CO NC O I " " -

66

COCO'


4/17

176 Kirkwood Semisolid metal processing

8 A Y o = 1155-1

7

w Y o = 230 5-1

o Y o = 350 5-1

6 initialshear rateY o

N

'Ee n

z~5

e noo~4>I-

Zwa:

~3c,

semisolid proccesing kirkwood d.h

Documents