Mushy / Semi-Solid Metal Forming Technology - Present and Future

M. Kiuchi' ( I ) , R. Kopp2 (1) 'Kiuchi Laboratory, Teikyou Heisei University, Tokyo, Japan

21nstitute of Metal Forming, RWTH Aachen University, Aachen, Germany

Abstract Mushy, semi-solid and / or thixo processing of metals (alloys) is becoming popular as a new potential manufacturing technology for parts and components in automobile, electronic and machine industries. Internal structures and mechanical properties of those metals that include solid and liquid fractions are quite different from those of hot or molten metals. Diversified possibilities are known today to process those metals based on die casting, hot metal forming or polymer injection technologies, each of which has its own specific advantages and disadvantages. Up to now thixocasting and thixomolding have been used in industrial applications for light metal alloys. The potentials of those processes are wider by far however. They include the processing of specially designed alloys and composites, the combination of forming and joining processes as well as reduction of production costs and energy consumption.

Keywords: Semi-Solid, Forming, Modelling, Rheology

1 INTRODUCTION Before starting the discussion, the terminology used in this paper is briefly explained. The word 'metal' is used in order to describe a metal concerned and its alloy. When a solid metal is heated up to a temperature higher than its solidus line, it starts to melt and a liquid component appears in it. This state is called 'mushy state' and the metal is called 'mushy metal'. When a molten metal is cooled down to a temperature lower than its liquidus line, some solidified grains appear in it. This state is described as 'semi-solid state' and the metal is called 'semi-solid metal'. In addition, a word 'thixo' which means 'thixotropy' is used to express both of above-mentioned states. It is mainly used relating to mushy / semi-solid casting and forming [ I ,2,3,4]. A metal becomes soft and its flow stress decreases as its temperature rises. This characteristic is widely utilized in various hot metal-forming processes. But even though the flow stress of a hot metal is much lower than that of a cold one, a force necessary to deform a work-piece in a hot process is usually large and a pressure acting to tool surfaces is rather high. Due to this, it is not easy to get a product with required shape and dimensional accuracy. In addition there are problems to be solved concerning surface quality and tool life. The question arises as to what does happen when the metal is heated up to a temperature higher than its solidus line. A metal melts completely when it is heated up to a temperature higher than its liquidus line. The molten metal is able to fill up a cavity concerned when it is poured into a mold. The molten metal solidifies as its temperature drops and a solid product with same shape and dimension with the mold is obtained. The molten metal's good characteristic of adjusting its shape to that of the mold is widely utilized in various casting processes. In casting, appropriate flow control of molten metal in mold's cavity, prevention of internal defects such as porosity, improvement of geometrical accuracy and elevation of surface quality are still important tasks of engineering. Poor mechanical properties of products caused by dendrites

growing in them are serious problems to be solved as before. Here the question arises again of what does happen when the metal is cooled down to a temperature slightly lower than its liquidus line.

In the end of 1960's, an innovative casting process was proposed, that was named as 'Rheocasting' [5]. In this process a molten metal is simultaneously stirred as well as cooled. Through stirring every dendrite growing on a crucible's surface is crushed into fine solid grains. They disperse in the molten metal. Thus a semi-solid metal consisting of liquid and solid components is made. The obtained semi-solid metal is squeezed into a given mold, cooled, solidified and shaped into a solid product.

Since then the characterization of 'Rheocasting' and its application were widely investigated [6,7,8,9]. Desirable stirring speed and cooling rate to obtain fine solid grains and suitable viscosity of semi-solid metals, their effects on mechanical properties and internal structures of cast products were studied by many researchers. Depending upon those results, 'Rheocasting' was extended to several types of semi-solid die-casting and / or thixo die-molding which will be explained later in this paper [10,11].

On the other hand, since the beginning ofthe 197O's, several mushy forming processes such as mushy extrusion, mushy forging and mushy rolling were investigated. Each process was characterized from mechanical as well as metallurgical viewpoints and the possibility of utilization was discussed. The mushy processes aim to treat not only cast alloys but also wrought alloys. This is an important objective of each mushy process essentially different from semi-solid one [12,13,14].

In recent years, various shaping and forming processes of mushy, semi-solid and / or thixo metals are becoming more and more popular. Those processes are conducted widely for manufacturing mechanical parts and structural components of passenger cars as well as electric and electronic appliances.

For instance, frames, pads, shoes, discs, wheels and other mechanical parts for braking and steering systems, pressure accumulation cylinders and valves for fuel supply systems, frame components and joints for car bodies, piston heads and connecting rods for combustion engines are manufactured in aluminum alloys by several types of semi- solid die-casting and / or thixo die-molding processes [15,16,17]. Castings of portable computers and mobile telephones, wheels and driving gears of sport utility bicycles, structure frames of electric hand tools machines, components of furniture and housing are made of magnesium alloys by utilizing a kind of thixo injection molding technology [18]. However, the customer's requirements on quality of products are becoming more and more severe. Higher dimensional accuracy, better surface quality, uniform mechanical property, superior strength, thinner wall thickness, smaller weight, geometrical complexity and functional flexibility are required. In addition, environmental consciousness as well as excellent productivity and drastic cost reduction are demanded. In order to respond to such requirements, extensive technological improvements of processes, machines, dies and tools are needed. To promote such improvements, a better understanding of characteristics of mushy / semi- solid metals is indispensable. In addition the detailed discussion on each process should be conducted [19,20,21]. In the first half of this paper, the physical and theoretical discussion of flow and deformation of mushy / semi-solid metals will be introduced. In the second half, the present status and future possibilities of mushy / semi-solid processing will be explained. The comprehensive theory of flow and deformation of the mushy / semi-solid metal is effective to get technological knowledge for design of tools and processes. The detailed discussion on actual processes and products are useful to promote the innovation of mushy / semi-solid technology. 2 METALLOGRAPHY AND MECHANICAL

A mushy metal and a semi-solid metal look similar to each other because both metals include solid and liquid components. Due to such similarity, only the word 'semi- solid' is widely used to express both metals. However, a metal starting to melt has peculiar mechanical properties largely different from those of a metal starting to solidify.

PROPERTY OF MUSHY / SEMISOLID METAL

Figure 1: Solid component and liquid component in mushy / semi-solid metal.

The difference is too great to express by a common word. The difference between mushy metal and semi-solid metal becomes small when they reach the same temperature range. There both metals include a similar proportion of the solid or liquid component, as expected naturally, they have fundamentally similar mechanical properties.

Generally, the mushy metal includes a high rate of solid component and deforms like a lump of clay or a sherbet. The semi-solid metal includes a high rate of l iquid component and flows like a non-Newtonian fluid. Arising from this distinction, in this paper, the words 'mushy' and 'semi-solid' are used accordingly following to the situation concerned.

The liquid component in a mushy metal is generated through partial melting of solid grains. The partial melting takes place on grain boundaries and the liquid component is usually present there. The remaining solid grains are separated from each other by the liquid component (Figure

The solid component in a semi-solid metal is generated through partial solidification of a molten metal. When the molten metal is cooled, the solidification starts on the wall of the crucible and the dendrite grows on it. If the mechanical or electro-magnetic stirring is applied to this partially solidified metal, the dendrite is broken into solid grains (or particles) and they disperse in the remaining molten metal.

The mushy / semi-solid metal has mechanical properties quite different from those of the solid or molten metal. The differences are summarized as follows.

1. Due to the liquid component existing on the grain boundaries, the bonding force between solid grains is very weak and sometimes almost zero. Arising from this, the deformation and flow of the mushy metal take place under very low working forces.

2. When the 'solid fraction' (or 'fraction solid') f5, which is defined as the weight in per cent of the solid component, is in the range roughly from 60% to 95%, the mushy metal deforms or flows like a lump of clay or sherbet. When f5 is approximately less than 60%, the mushy metal flows like as slurry even under the gravity force. When f5 is higher than 95%, the mushy metal deforms like a solid metal.

1).

Figure 2: Mushy stirring and mixing.

3. When f5 is lower than 90%, the mushy metal can be stirred and mixed with other materials such as ceramic particles, ceramic fibers and graphite fibers (Figure 2). Thus various mushy mixtures of metal and other materials can be made [22,23,24,25].

4. If the mushy metal is stirred and cooled simultaneously, the solid grains are separated by the stirring force and fix their shapes independently without connecting with others. Thus the mushy metal is made into particles and I or powder.

5. Two pieces of mushy metals can be joined together by ut i l iz ing a peculiar characteristic of their l iquid components. When their interfaces are pressed to each other, the liquid components included in both sides penetrate and diffuse interface and solidify together (Figure 3). By this mutual penetration of the liquid components and the solidification as one body, the joining between the mushy metals is accomplished.

Figure 4: Rotation, slipping and movement of solid grains in mushy metal.

Figure 3: Joining of mushy metals

The semi-solid metal is a viscous fluid. Its viscosity is higher than that of the molten metal due to the existence of solid grains dispersed within it. The viscosity of the semi-solid metal is influenced by number and size of dispersing solid grains, which are determined by the cooling rate and the shear rate associated with the stirring [26]. The size of solid grains becomes smaller as the shear rate of the stirring increases. On the other hand, the grains grow during the st irr ing. Therefore the temperature control and the speed control are important to get the required solid grain size and appropriate viscosity of the semi-solid metal [27]. It is very important for the semi-solid metal to obtain a low viscosity and a high solid fraction. The low viscosity is necessary to get good fluidity that is indispensable for filling up the die cavity. The high solid fraction is effective to prevent various defects, a obtain fine internal structure and get excellent quality of cast products.

Generally, the macroscopic deformation of the solid metal consists of not only microscopic deformation and rotation of individual grains but also relative slip among them. In the solid metal, the mechanical restraints among grains interact with each other and restrict their deformation. They are not able to deform, rotate and displace freely. If the mechanical interactions and restraints are released due to the melting of the grain boundaries, the rotation and relative slip of the solid grains can take place quite freely and consequently the macroscopic deformation of the mushy metal can occur under a small external working force (Figure 4). As the partial melting of the grain boundaries advances, the mechanism of deformation of the mushy metal changes drastically from that of the solid metal. The restraints between the solid grains are removed rapidly

and the mechanical strength of the multi-grain structure diminishes rapidly. When the volume of the liquid component increases adequately enough for the solid grains to rotate, slip and displace independently, the mushy metal starts to flow like a slurry. When the mushy metal deforms, the liquid component tends to flow through gaps among solid grains. It flows separately from the solid grains. This is similar to a flow of a viscous liquid through a labyrinth. As the solid fraction f5 decreases, the liquid component flows more and more freely and smoothly through the inter-grain gaps. In special cases, the liquid component flows out of the mushy metal through the gaps or channels that lead from its inner portion to its free surface (Figure 5). However, when f5 is higher than the above level, the liquid component trends to be trapped at the closed gaps among the solid grains. In this state, the restraint among the solid grains is not so much reduced, therefore, the flow stress of the mushy metal does not drop so remarkably. It should be noticed that, even though the solid grains restrict each other, the connecting force between them weakens to a very low level due to the presence of the liquid component on the grain boundaries. This means that the mushy metal, even if its f5 is high, usually has very low elongation.

Figure 5: Flow of liquid component in mushy metal.

3 Al alloys and Cu alloys were subjected to the compression test in the mushy state and their stress-strain curves were measured [4]. The method to measure the flow stress of mushy metal is as following (Figure 6) [13].

FLOW STRESS OF MUSHY METAL

Figure 6: Installation of a sub-press unit and a specimen [I 31.

A cylindrical specimen with 16mm diameter and 18-22mm height is set in a sub-press unit which consists of a flat top punch, a flat bottom die and a heavy wall container. The sub-press unit is used not only to upset the specimen but also to protect the specimen from cooling. The assembly of the specimen and the sub-press unit is heated in an electric induction furnace up to a scheduled temperature which corresponds to the required solid fraction and is kept at the temperature for a while to ensure the uniformity of the temperature distribution in the mushy specimen. Then the sub-press unit containing the specimen is transferred to a cam-plastometer and subjected to the compression test under a constant strain-rate. The solid fraction of the specimen is, in some cases, calculated from its temperature by using its phase diagram. But when the tested specimen includes many chemical compositions, its phase diagram fai ls to show the relationship between the solid fraction and the temperature. In such a case, a substitute method should be developed and used in order to determine the value of solid fraction of the mushy specimen. Some examples of the measured stress-strain curves of the mushy specimens, whose solid fractions are either known from their phase diagrams or are estimated by a newly developed calibration curve, are shown in Figure 7. It follows from the figure that the flow stress of the tested mushy metals drops remarkably as the solid fraction decreases from 100% to, for instance, 80%. Similar stress- strain relationships, especially the drop of the flow stress, are observed with respect to every mushy metal. Figure 8 shows the relationship between flow stress o at 4% compressive strain and temperature T more directly, concerning Al and Cu alloys. Here, it is clearly observed again that the flow stress of the mushy metal drops rapidly when its temperature exceeds the solidus line. The values of measured flow stress are rearranged from a view point of the relationship between normalized flow stress on and solid fraction f5. The result is shown in Figure 9. Here, on is defined as the ratio of the flow stress at the tested temperature to the flow stress at the solidus line. An important fact emerges from the figure, namely that the

35

30 - m g 25

b'. 20

v

v) v)

r2 1 5

10

5

- AI-5.7%CU - - - - AI-0.93%Si

0 0.1 0.2 0.3 0.4 0.5 Strain E

Figure 7: Measured stress-strain curves of mushy metals [I 31

700 800 901 Temperature T ("C ) L L

Figure 8: Flow stress-temperature relationships[l4].

relationship between on and f5 can be expressed by one characteristic curve concerning all of the tested mushy metals. This means that the rate of reduction of the flow stress is dominated by the solid fraction [14]. The chemical composition of the mushy metal gives only a slight influence on the rate of decrease in its flow stress. This is quite understandable because the difference between the flow stress of solid component and the viscosity of liquid component in the same mushy metal is always much greater than the difference between the flow stresses of solid components present in different mushy metals. Furthermore it is also greater than the difference between the viscosities of liquid components present in different mushy metals. Thus, the effect of the amount of the liquid component, which corresponds to the solid fraction f5, is dominant in the flow and deformation of the mushy metal. The size and shape of the solid grains naturally has an influence on the flow stress of the mushy metal. When the grains remaining in the mushy metal are extraordinarily large or long, the drop of the flow stress caused by the partial melting of the grain boundaries delays comparing with that of the mushy metal with round and small solid grains. The relaxation of the restraint among large and long

grains advances more slowly than the relaxation among round and small solid grains when partial melting takes place. The rate of decrease in the flow stress of mushy metals including large and long solid grains is lower than that of mushy metals including round and small solid grains. The influence of the strain rate on the flow stress of the mushy metal is generally observed. It should be considered as well when the forming and shaping of the mushy metal is designed and conducted. One important characteristic can be pointed out as follows. The effects of strain rate on the deformation of mushy metals is not stronger than its effects on the hot solid metal.

Figure 9: Relationship between normalized flow stress and solid fraction [14].

4 VISCOSITY OF SEMISOLID METAL The primary objective of every casting process is to fill up the cavity of mold with the molten metal and to cope with such a task, it is absolutely necessary to know the value of viscosity. Generally, the viscosity of the molten metal is measured by the rotational co-axial double cylinders method (Figure 10). In this method, the coaxial double cylinders are installed and the gap between outer and inner cylinders is filled by an appropriate amount of the molten metal concerned. Then the inner cylinder is rotated under the required angle velocity and the driving torque G is measured. The (apparent) viscosity q is calculated by the following equation [28].

(r: - r f ) . G 4x. rf . r: . w . L

q =

G = driving torque r, = radius of inner cylinde r2 = radius of outer cylinde w = angle velocity of rotati' L = contact length betweei inner cylinder and molten metal

Figure 10: Co-axial rotating double cylinder method [28].

By this method the viscosity of the semi-solid metal has been investigated from various view points. Effects of the solid fraction, shear rate, gap between two cylinders, chemical composition and solid grain size on the viscosity have been widely measured [28,29]. Figure 11 show some examples of the measured relationships between viscosity and solid fraction of the semi-solid metal. It follows from these results that the viscosity of the semi-solid metal rises up very rapidly after its solid fraction exceeds a critical value. It seems that the flow mechanism of the semi-solid metal shifts from that a fluid to that of a slurry. The critical value becomes higher as the shear rate increases. However, the upper limit seems to occur in the range from 45% to 50%.

3.0

0 20 40 60 Solid Fraction (%)

Figure 11: Relationship between apparent viscosity and solid fraction [29].

5 FORMULATO PREDICT VISCOSITY OF SEMI-SOLID METAL

The measured values of the viscosity of the semi-solid metal were summarized into several mathematical formulas by Hirai [29], Modigell [30] and others with the aim of predicting the viscosity conveniently. Each formula includes the process parameters such as chemical composition, solid fraction, solid grain size, temperature and shear rate. The resulting formula proposed by M. Hirai is as follows:

r 1 a

q = q L ' I+

Here,

a=cx.p,.C y

fs,cr = 0.72-p.C'l3 . ? - ' I 3 (upper limit of fJ

cx = 2.03~ 10' x (X/ l OO)'I3

2'(1/fs -l/fs,cr)

113, . -4/3

L

p = 1 9 . o ~ ( x ~ 1 0 0 ) ~ ~ ~

C = solidifying rate dfJdt (l/s) ? = shear rate (s-I) X = primary alloying component (mass%) pm = density of molten metal at liquidus line (kgm3) qL = apparent viscosity of liquid component (Pas)

4.01 /I

A

g 2.0 0 5 s ! ln

Estimated non-dimensional flow stress from viscosity

Allovs

AMg-Sn

X S b-Te QZn-Bi

@AI-Sn

UAI-CU

,

0 1 .o 2.0 3.0 4.0 Calculated Viscosity 7 (mPa - S)

Figure 12: Correspondence between measured viscosity and viscosity predicted by M. Hirai [29].

This formula covers a wide range of the semi-solid state and provides useful knowledge on viscosity for the design of semi-solid processes (Figure 12). 6 FLOW STRESS OF METAL IN WHOLE MUSHY /

In the whole mushy / semi-solid temperature range, that is the range from the solidus line to the liquidus line, the flow stress of the metal changes drastically. Figure 13 shows the overall change in the flow stress of the mushy and / or semi-solid metal relating to the change in the solid fraction from 100% to 0% [31]. When the solid fraction is roughly lower than 40%, the semi- solid metal flows like a non-Newtonian fluid. In this solid fraction range, owing to the fluidity of the semi-solid metal, die-casting and injection molding are widely conducted for manufacturing various parts such as components of cars and electronic devices. In the range where the solid fraction is higher than 60%, the mushy metal deforms like a lump of clay. The excellent deformability and the low flow stress are utilized to conduct die forging, die molding, rolling and extrusion. When the solid fraction increases from 40% to 60%, the flow stress changes markedly. The flow of the semi-solid metal shifts to the deformation of the mushy metal. In other words, the fundamental feature changes from fluidity to deformability. The values of flow stress of the semi-solid metal in the low solid fraction range shown in Figure 13 were converted from the viscosity measured by the rotating double cylinders method. The shear flow stress z of the semi-solid metal was derived from the measured torque G by using the next equation

SEMISOLID RANGE

(3)

From the calculated shear flow stress z, the normal flow stress o was assumed. The relationship between normalized flow stress on and solid fraction f5 of the semi-solid metal in the low solid fraction range can be expressed as follows.

0, = A . f:, (n=6.0-12.0, fs I 40%) (4)

The value of n depends upon the shear rate, however, the

Non dimensional flow stress from compression test

I x

I x

I x

I x

I x

I x

0

10

10 -1

10'

1 8

-5 10

0 20 40 60 80 100 Solid fraction fs (%)

Figure 13: Flow stress in whole mushykemi-solid range

Z

[31 I.

effect of shear rate is not so serious, but slight

The relationship between on and f5 of the mushy metal in high solid fraction range can be expressed as follows.

0, = B . fsm (m = 4.0-6.0, fs 1 70%) ( 5 )

The value of m depends upon the grain size, grain shape and strain rate as well. In the case shown in Figure 13, the value of m is roughly 6.0.

The relationship between the normalized flow stress on and the solid fraction f5 in the intermediate range of f5 is not clarified yet. More detailed investigations into the shift of the deformation mechanism and more extended measurements of the flow stress and the viscosity are required.

7 YIELD CRITERION AND CONSTITUTIVE EQUATION OF MUSHY METAL

Figure 14 shows the measured relationships between the flow stress of mushy metal at 4% compressive strain and its temperature [31]. As the billet's temperature exceeds the solidus line, its flow stress drops remarkably. However, the feature of dropping of the flow stress of the mushy billet is not always similar to others. The flow stress of one mushy billet drops very rapidly, but the flow stress of another mushy billet drops slowly [32]. The solid component of the mushy metal consists of the formation of solid grains. The formation of solid grains is expressed as 'solid skeleton'. Even though the billet is heated up to the mushy state, as long as the solid grains restrict each other and the solid skeleton is stiff enough, its flow stress does not drop so rapidly. When the billet's temperature rises much higher and its solid fraction decreases, the solid skeleton is apt to collapse and the flow stress shows a notable drop. Through the investigation into the dropping feature of the flow stress, it was found that the solid skeleton, which is the formation of solid grains, of the mushy metal has an

Temperature T ( "C) Figure 14: Drop of flow stress of mushy metal [31].

Unified Skeleton Formation of Solid Grains

fl = a . (1 - f S ) y , fo = fs" (6)

o< f3< (9 .%/2 ) , f2 = [1 - f3 . (2 /9 .%) r1

f5 = solid fraction of mushy metal ( > 60% )

(apparent average stress working to mushy metal) (effective stress working to solid skeleton) fo =

o, = normal yield stress of mushy metal

om = mean normal stress ( = hydrostatic pressure) J,,J, = invariants of stress tensor

a,m,n = material's constants f,,f,,f, = effect coefficients

S-Shea r

Figure 15: Unified solid skeleton and formation of solid grains.

especially low shear strength. This is a most important characteristic of the flow stress of the mushy metal. Hill's theory of plasticity indicates that the shear strength of the solid metal is about a half of its normal strength. This theory can be used effectively for the analysis of deformation of the solid skeleton of sintered porous metal, because it has the metallurgically unified solid skeleton, but not for the formation of solid grains (Figure 15). However, as for the solid skeleton of the mushy metal, this relationship between the normal strength and the shear strength can not be accepted. This is due to the fact that the solid skeleton of the mushy metal is composed of the independent solid grains. They contact with each other, but are not metallurgically connected. Due to this, the shear strength of the solid skeleton of the mushy metal is lower than the shear strength of the solid skeleton of the sintered metal. This is the reason why the yield criteria of the sintered porous metal and the constitutive equations derived from them can not give satisfactory analytical results concerning the flow and deformation of the mushy metal. The mushy metal needs a different criterion which can deal with the peculiar drop of the shear strength of its solid skeleton. i.e. the very low shear strength of its solid component. As the result of careful investigation into the remarkable drop of the shear strength of the solid skeleton and the consistent consideration on the conformity with Hill's mathematical theory of plasticity, M. Kiuchi et. al. proposed the following formula of the yield criterion of the mushy metal [31].

Figure 16: Yield curve based on Equation 6 [31]

Figure 16 shows the profile of the yield curve based upon Equation 6. As shown in the figure, the yielding takes place under very low stress when the mushy metal is subjected to a pure shear deformation. Here, the value of ym,/ym,, shown in the figure depends on the value of the effect coefficient f,.

From Equation 6, the constitutive equations can be derived as follows.

8 To get process stability and defined properties of the products certain restrictions for process variables and material suitability have to be kept in mind. Concerning the process variables during forming of metals in the mushy / semi-solid state a two-way-approach is suggested to avoid the forming of any defects (such as porosity, overlapping, freezing, shrinkage holes and segregation) during part production. The forming of porosity and overlapping might occur if two flow fronts merge. This may be the case if undercuts and holes have to be produced. In this case the die design has

PROCESS WINDOW I LIMIT DIAGRAMS

to be adapted (overflow areas). On the other hand a turbulent die filling behavior of the metal (jetting) will course undesirable forming of voids. So called T-form die filling experiment as well as its simulation have been carried out to investigate this problem [33]. Figure 17 is the schematic diagram based on the results of T-form die filling experiments with material Snl5%Pb. This flow contour diagram correlates the dimensionless Bingham Bi and Reynolds number Re, which have been modified to fit for non-Newtonian media. The dimensionless figures are defined as following:

(9)

with K as fraction of solid particles agglomerated, k ,n the parameters of flow curve (see Equation 13), v the punch velocity, p the density, D the diameter of the inflow section and z,, the initial shear yield stress.

Figure 17: Flow contour diagram with dimensionless Bingham and Reynolds number modified for non-

Newtonian flow behavior.

pressure p

Figure 18: Limit diagram (pvT-process window).

With help of the video camera the experiment was visualised to evaluate the flow behaviour during the die filling for each test condition. Finally the plotting field could be divided in to three sections respective to flow behaviour: laminar, transient and turbulent as in Figure 17. The progression of the borderlines for different flow behavior are independent of specific alloys and can thus be deployed for various semi- solid metals with thixotropic behavior. Next to these restrictions for the material's flow behavior defects like segregation, freezing and forming of shrinkage holes have to be avoided. The three dominant factors during and after die filling are the metal's velocity v and temperature T and the pressure p during solidification (PI-P,). PI, P,, P, is the forming history of one point during the forming process (Figure 18). The variables are depending on the material

and can be sketched as a limit diagram Next to restrictions for forming variables the suitability of the alloys and their microstructure for mushy / semi-solid processing has to be ensured as well. The question whether a certain microstructure is suitable for thixoforming is vital for the process layout. To answer this question the following four criteria have been suggested [34,35]: Criterion 1: With lower amounts of solid phase the viscosity

of the slurry does decrease in general. In the literature amounts of 0.4 - 0.6 of fraction solid f5 are considered to be proper values for thixoforming. Next to the fraction solid criterion the morphology and the contiguity of the solid phase are vital for the suitability of the material. The morphology can be described by the two criteria shape factor and grain size.

Criterion 2: The shape factor F is defined by [34]:

with C the mean circumference and A the mean area of the grains. In the case of F = 1, all grains are completely globular, otherwise the shape factor is F 2 1. If the surface of the grains is not globular enough, the grains sticktogether when sheared and no thixotropic behavior can be achieved. To assure process stability the shape factor should not exceed the value of 2.

Criterion 3: The viscosity of the semi-solid material does decrease with smaller grains. Although there is no absolute upper value for the grain size, as a rule of thumb the grain size should not exceed one twentieth of the smallest wall thickness to be produced [36].

Criterion 4: The contiguity of the solid phase C" is decisive for the state of coalescence. The contiguity is defined as [37]:

with S? the connecting surface between two solid particles and Sf' the surface between the solid and the liquid phase. In the case of C" = 0 all solid particles are completely surrounded by liquid phase. The total amount of coalesced solid phase is stated by the volume contiguity of solid phase V," , which can be calculated:

vc" = fs .cs

If the volume contiguity exceeds the value of 0.3 no thixotropic behavior is achieved, the material behaves mainly like a solid body. If the volume contiguity is less than 0.1 the skeleton of the solid phase is not strong enough to prevent a billet deformation prior to the forming step resulting in poor handling properties of the material.

Questions concerning how the process window can be enlarged by alloy development are covered in section 12.5 'Alloy development'.

9 NUMERICAL SIMULATION OF FLOW AND DEFORMATION

To improve process stability the numerical simulation of the flow behavior is the tool which has to be developed. Currently various commercial and non-commercial FEM and FVM programs are used to predict material behavior during the forming process. In this paper only a brief overview about issues and results of numerical simulation

can be given for single and two-phase simulation. The feedstock manufacturing and reheating steps are not dealt with here. The rotating double cylinder method (Figure 10) enables sudden changes of shear rate as well as constant shear rates lasting for a long period. This method gives insight to the viscosity reaction following a change of shear rate. If, for example, the shear rate increases suddenly up to a constant level, the viscosity jumps up before it decreases

h ur 5 F z c 0 Il l

5

10

1

x VlSCOSlty (fs=45%) 250

200 3 . 7 v

I I I 0.1 ' '0 0 480 960 1440 1920

Time t (s)

Figure 19: Viscosity data of shear rate experiments (Snl5Y0Pb) [30].

slowly until a steady state is reached (Figure 19). These results of shear rate experiments have lead to a single phase material law, which describes the shear rate and

time dependent viscosity trend as a Herschel-Bulkley type of fluid [30]. Here, z,, is the iqitial shear yield stress, it depends on the fraction solid, is the shear rate, k, m are material parameter,

K = C. (Ke - K) (14)

they depend on the fraction solid, and K is the structural parameter with Here, c i s the rate constant of approaching the steady state value and K~ is the fraction solid particles agglomerated at the steady state, which only depends on the current shear rate. The structural parameter ranges from zero to one. It describes the degree of agglomeration of the solid particles in the semi-solid material [30]. As also presented in Equation 2, in this material law the viscosity depends on the shear rate and on the solid fraction. But apart than Equation 2 this approach also considers the time dependant decrease of viscosity. Using this material law with numerical simulation load curves of isothermal mushy compression tests have been calculated and compared with the corresponding measured load curves (Figure 20). The separation of liquid and solid phase caused by deformation of the mushy alloy is an issue of great interest, because an inhomogeneous distribution of liquid and solid phase leads to insufficient mechanical properties in some areas of the structural part. The aim is to prevent phase segregation in thixoforming. Therefore there is a need to predict segregation effects in a thixoforming process by numerical simulation, which implies a valid two-phase material model. An approach for two phase modeling of semi-solid alloys

has been developed by [30]. The liquid phase is regarded as a Newtonian fluid with a constant viscosity, whereas the solid phase is taken into account as a compressible phase with a flow behavior as described in Equation 1. In the momentum conservation equation of the solid phase a coefficient related to the Poisson's ratio v accounts for the compressibility. Other than the model presented in section 6, this approach is based on fluid mechanics, which is why there is no yield criterion. The interaction of solid

k

rl/ f/ . @/ -us)= -2. vp

and liquid phase is described by the Darwlay" Here, f, is the fraction liquid (f, = 1 - fJ, is the difference between the velocity of the liquid phase and the velocity of the solid phase, kp is the permeability coefficient and q, is the viscosity of the liquid phase. Figure 21 shows results of a two phase simulation using the above described model. The calculated distribution of solid phase is an important issue of the two-phase simulation.

-I s)

73 m 0 -I

0 0.1 0.2 0.3 0.4 0.5 Time t (s)

Figure 20: Load data of isothermal compression test.

Figure 21: Forming process of a structural part -two phase simulation results [33].

10 MUSHY / SEMI-SOLID FORMING AND MOLDING Several routes are known at different stages of research, development and use in industrial production. The single routes differ especially with respect to the level of process integration, thermal history of the process and the raw material used. Concerning the thermal history of the process there are three variants known as displayed in Figure 22.

The so-called mushy processes have been used first in industrial production. Usually MHD (magneto-hydro- dynamic) - stirr ing is employed. Al l the displayed conventional processes are aimed at producing a solid feedstock. This semi-product has to be reheated (usually by inductive heating) to the semi-solid state and can then be thixoformed. The level of process integration is relatively low as feedstock and component fabrication are clearly separated. All kinds of raw materials known for conventional

The semi-solid or rheo-processes are focused on in present research and development work, as cost savings are expected compared to the conventional processing. These savings are mainly due to a reduction of process steps, namely the production of a solid intermediate product. The material is cast into an ingot at a temperature slightly above the liquidus-line. The target temperature in the semi-solid state is adjusted and the material directly transferred to the forming equipment. The level of process integration is remarkably higher compared to conventional processes, as feedstock and component fabrication are coupled. This also means that a separation of the two processes (place and t ime) is not possible. The same raw material requirements apply for rheo-processes as for conventional processes. It has to be mentioned, that rheo-processes seem to be favorable for inhouse-recycling considerations.

Mushy Semi-solid or Thixo- Processes Rheo-processes molding

T t ?-

Time Time Time

a: DC (MHD or a: Casting a: Granulate grainrefiring) b: Adjustment b: Screwing

al: SIMA c: Forming c: Injection b: Reheating d: Component d: Component c: Forming d: Component

Figure 22: Current process routes in semi-solid production.

The thixomolding process has emerged from polymer injection molding. Solid granulate is filled into a heating and extrusion device, brought into the semi-solid state and thixoformed, thus the material never gets into the fully liquid state. The process integration is highest, as feedstock and component fabrication are fused in the same equipment. This means restriction to the forming operation (only injection molding is possible) and raw material requirements (granulate has to be used).

In this section conventional processes (direct casting, SIMA) and thixomolding are dealt with as they are currently used in industrial production. More recent developments as rheo- processes are covered in section 12 'Extension'. As all component fabrication processes reviewed here take place in the semi-solid state both casting as well as hot metal forming technologies and injection molding may be considered. Currently die casting and molding are used industrially. All semi-solid processes similar to metal forming processes are in the research or development stage. An overview about different concepts of currently known forming technologies (with the exception of molding) is displayed in Figure 23. As forging and extrusion have already been investigated for some time, they are covered in this section. The newly investigated processes of thixorolling is par of a later section. 10.1 Feed-Stock Manufacturing and Reheating In the fol lowing the conventional process route of thixoforming is described in more detail from feedstock material production, heating to the semi-solid state and forming.

Figure 23: Different thixoforming technologies

Direct Casting Following the first investigations on 'rheocasting' suitable feedstock with globular microstructure can be achieved by stirring a melt during solidification in the semi-solid state leading to a fragmentation of the growing dendrites and spheroidisation. This stirring movement can either be realized by mechanical or electromagnetic impetus. Most commonly in industr ial appl icat ion the magneto- hydrodynamic (MHD) stirring is used, as mechanical stirring leads to problems with alloy impurity and wear. This cast material can be reheated directly without any further treatment and will display a globular microstructure in the semi-solid state. One typical feature of MHD-stirred material is entrapped liquid phase in the solid grains, which results from the breakage of the dendrites and the subsequent growth and building of new contact phases (Figure 24).

AISi7MgO.6 (A357) [38]

The MHD-stirring is usually used for aluminum cast alloys. Other alloys used in mushy / semi-solid forming like aluminum wrought alloys, magnesium alloys and steels are commonly continuously cast without MHD-stirring and have to be treated subsequently. One way, which has been tested so far for steel is the liquid core reduction process, where the cast slab is rolled with semi-liquid core. Thus a mechanical stirring in the core region is achieved [33].

Thermomechanical Treatment Route - SlMA (Strain Induced Melt Activated) The first application for thermomechanical treatment of material for thixoforming has been used for aluminum wrought alloys and has been under patent of ITT [39]. The process is called SlMA (Strain Induced Melt Activated) and consists of extrusion of cast material and subsequent cold forming. This process has been successfully applied to various wrought alloys (especially A lMgSi l and AlZnMgCul.5) with modifications such as neglecting of the extrusion step, warm forming, use of swaging or profile rolling process [33,40,41].

Next to aluminum alloys the SlMA process has been successfully tested to produce magnesium with globular microstructure as well. Cast alloys, like MgAI9Zn1 (AZ91) or MgA15Mn (AM50) or wrought alloys, like MgA18Zn (AZ80) or MgZn6Zr (ZK60), can be processed [42]. For steel processing the thermo-mechanical feedstock production route is the only currently used method. In contrast to the SIMA-process for aluminum and magnesium steel is used as conventional commercial thermomechanically rolled material. By this method steel meets the microstructure requirements in the semi-solid state for thixo processing [43,44,45,46].

Reheating of Billet Made from Feed Stock

The reheating of the feed stock is usually done inductively. As the cycle time of the heating process is higher than of the forming process, in industrial applications multiple heating stations are used. The required the number of station varies depending on cycle times and the number of forming machines. The heating step is vital for quality issues, as only homogeneous distribution of solid and liquid fraction ensures process stability during thixoforming.

Magnesium billets are reheated more quickly in comparison to aluminum billet, as the latent heat is lower. Furthermore magnesium feedstock usually is processed by thermo- mechanical treatment. Thus no casting skin can be observed on the surface, which leads to lower mechanical stability. Only shorter heating times can ensure stability of the billet for handling. Typically, fraction liquid ratios of at least 45% can be used, due to the extremely fine grain structure. At forming temperature the billets are still stable in shape, but to avoid self-ignition during the induction heating the coil should be flooded with inert gas [41]. The induction heating of steels has to take into consideration the special electromagnetic and high temperature behavior of these materials. These are the change of electromagnetic behavior from ferro magnetism to paramagnetism, high radiation losses at high temperatures combined with heat distribution problems (cooler surface) and high temperature oxidation combined with the lack of building of protective oxide surface layer to inhibit further oxidation. Special reheating strategies are chosen to obtain a suitable material for thixoforming [43,45,46].

10.2 Forming Technologies In this part the conventional process routes for semi-solid forming are described and compared to the conventional process routes of die casting and metal forming (forging, extrusion). The specific advantages of the semi-solid processing are clarified and references concerning machine and tool design as well as product properties are given. Mushy/ Semi-Solid Die Casting

Industrially relevant research work in the semi-solid field started with the die casting processes. Up to date it is the most common process industrially used in the semi-solid state. Mainly aluminum cast alloys are employed, the standard is AIMg7Si (A356 and A357), but research and development is also carried out with other aluminum cast and wrought alloys as well as AI-MMC and magnesium alloys. Steels do not play a significant role for semi-solid casting process yet, although some research workis known [451. Compared to conventional die casting the following advantages of semi-solid forming are known:

1. Thick / thin walled structures can be produced 2. Cast and wrought alloys can be used

Max. dynamic injection force [kN]

Max. injection force intensified

[kNI

OL-320 OL-320 OL-700 OL-700 DC Thixo DC Thixo

177 224 277 444

145 95 669 745

1 400 1 650 1 600 1 835 I Plunger stroke

I I

Plunger diameter 1 50-90 1 1 1 I 55-80 70-120 80-105

I I

Table 1: Standard Diecasting (DC) and Thixocasting (Thixo) machine specifications (Idra) [48].

(CH), Frech (D) and ldra (I). Some typical specifications of thixocasting machines (Idra) are displayed in Table 1. Mushy / Semi-Solid Forging (Thixoforging)

In comparison to semi-solid die casting, thixoforging and related process like forward and lateral extrusion are not yet industrially applied. The differences in process layout are explained in Figure 25. Compared to the conventional hot forming in the solid state, there are process related advantages for mushy / semi- solid technologies:

1. 2. 3.

4.

5.

6.

Lower forming loads are necessary Small scale forming equipment can be used Geometric complexity can be widely enlarged (undercuts, holes, branching, sharp edges and radii as well as thin walled structures) and thus a net or near net shape part produced The range of alloys can be expanded to include brittle and high strength materials Productivity gains can be realized by using multi-forming operations Less or no flashes and burrs are needed

Concerning the properties of mushy / semi-solid forged parts i t has to be kept in mind that no hot forming microstructure is reached, which accounts for the excellent mechanical properties in forged parts. The goal for mushy / semi-sol id processed parts has to be to obtain a

homogeneous defect-free globular microstructure. Certain areas of the thixoforged part can be subsequently formed in the hot or cold solid state to locally improve the mechanical properties. Compared to a casting process, forging of mushy /semi- solid metals leads to a very favorable situation for producing defect-free products. Investigations on pressure distribution and forming of segregation for

forging compared to lateral extrusion (which is similar to casting) have led to the result that for forging the pressure after die filling can be maintained until complete solidification occurs in the whole volume. Whereas for lateral extrusion / casting processes the pressure drops directly after die filling caused by the solidification process [49]. In addition phase segregation is more likely to occur if too high pressure gradients are existing during die filling. For forging processes the pressure gradient is usually less than extrusion.

Figure 25: Process description of (A) forging and (B) lateral extrusion [49].

Apart from these advantages of forging processes in the mushy / semi-solid state problems of process and tool layout are evident, as upper and lower die are not closed before die filling. This means that variations in feedstock material volume may cause variat ions in product dimensions. It has to be assured that excess material is directed into parts that either have to be finished by subsequent metal cutting processes or are uncritical for dimension variations. In addition mushy / semi-solid material can flow between the moving die halves, leading to problems of die jamming connected with production breakdown. These problems have to be kept in mind when issues of process and die design are dealt with. Some examples for aluminum and steel applications for forging as well as lateral extrusion are presented in Figure 26 and Figure 27. All equipment running in research and development of thixoforging are pilot machines. Some hints for machine layout can be given:

Figure 27: Aluminum parts a) [51], b) [52]

1. Hydraulic forging machines should be used to maintain pressure during solidification.

2. Press velocities should be around 0.8 m/s [51]. 3. Forces should assure pressures of around 100 MPa

within the mushy / semi-sol id material during solidification to avoid shrinkage holes [51].

4. Multiple action presses are to be favored for flexibility reasons.

Split dies and ejectors have to be integrated (undercuts, cores) plus the necessary hydraulic pistons. mushy memi-solidExfrusion$Thixoextrusion)

The process of bar, profile and tube extrusion in the semi- solid state is different from the other mentioned processes as a semi-continuous process has to be established. Vital for the extrusion process is the complete solidification of the material within the extrusion channel under a certain pressure high enough to avoid the formation of shrinkage holes. This means that temperature and pressure are the two main process parameters relevant for the product quality. Up to now no industrial relevant part has been produced by using thixoextrusion. All work known so far is in a research and development stage. The main advantage of thixoextrusion is based on the low forming loads necessary. In early works the loads have been quantified to be one fifth of those required for conventional extrusion processes [53]. The high forming loads of hot extrusion lead to various restrictions, such as: 1. geometrical restrictions, especially for thin-walled,

hollow profiles and profile circumferences 2. speed restrictions as to high ram speeds lead to

undesirable heating of the product during forming and connected defects like hot cracks, remelting of the surface etc.

This background gives the direction for potential thixoextrusion products:

1. enlargement of billet and product circumference as well as very high reduction ratios

2. enlargement of geometrical flexibility towards thin-walled structures, sharp edges and radii

3. processing of materials with low formability or high flow resistance such as steels, magnesium alloys, metal matrix composites (MMC)

4. productivity gains Next to these process advantages the different material behavior results in different product properties. The well- known press effect (grain elongation in press direction, extrusion texture) leads to anisotropic mechanical properties. For subsequent processing of extrudates (like e.g. internal high pressure forming of hollow structures) the anisotropy may lead to problems, as mechanical properties in radial direction are poor. The reduction of the cross section during the process may cause separation of liquid phase in longitudinal direction. Furthermore solid particles tend to migrate to the center of the flow capillary. Both effects can result in segregation both in radial and longitudinal direction [54].

Mushy m e m i-Solid [Molding UThixom olding @I)

Thixomolding combines the benefits of high-pressure die casting and injection molding and is at present used to process magnesium alloys. The production cycle starts with the dosing of cold granules by a volumetric feeder on a screw. During the rotation of the screw the feedstock material is sheared, reheated by electric heating devices up to the forming temperature of about 580°C and stored in an accumulation chamber. After reaching the shot volume

in the accumulation chamber the thixotropic magnesium emulsion is injected through a nozzle into the mold. In the course of the process no fully liquid material has to be handled, due to the one-step process, which is fed with cold granules. Depending on screw- and injection speed it is possible to vary the fraction solid in the injected magnesium emulsion between 5-30%, which is very low for semi-solid processing. Thus differences in the microstructure are emerging (Figure 28). The lower process temperature (in comparison with die casting) and the volume fraction of solid lead to components with low shrinkage porosity, no hot cracks and advantages in geometry precision.

Area

USA + America

Figure 28: Microstructure of a thixomolded MgAI9Znl (AZ91) alloy [55].

The concepts of the machines, used for Thixomolding, are based on plastic molding technologies and are modified in respect to the use and manufacturing of magnesium instead of plastic. Therefore parts like the screw, the cylinder and even the whole injection unit are re-sized and redesigned to fulfill the increased specifications, like pressure, during transport, shearing and heating of Mg-granules [56]. The tool design for Thixomolding is a mixture of high strength tool steels, known from die casting, combined with an ingate and die venting design known from plastic injection molding. Due to higher amount of solid particles (a-phase), in comparison to die casting, especially the ductility of thixomolded parts is higher as shown in Table 2. The Thixomolding process is under license from Thixomat, USA. Machines are available from USAand Asian machine tool manufacturers with clamp forces between 75-1650 t.

HPDC Thixo

750 35

Material I MgA15Mn I Thixomolded I 138.5 I 268.7 I ;;#; 1 (AM 50A)

MgA15Mn Die Cast 112.1 232.2 (AM 50A)

YS UTS Elong. [MpaI [MPaI ["/I

Process

I ~~~~~~ I Thixomolded I 147.5 I 278.2 I 18.8 I I ' I

MgAI6Mn I (AMGOB) I Die Cast I 114.5 I 238.8 I 11.6 I I I

Table 2: Tensile Properties of Magnesium [57]

11 APPLICATIONS Thixocasting / thixomolding is currently the process which is by far the most important from the point of view of industrial application of all mushy or semi-solid process technologies. At present the diffusion of the mushy / semi- solid forming technology is still quite low compared to e.g. high pressure die casting (HPDC) (Table 3) [58]. Taking aluminum alloys as the main example, the tonnage of feedstock material production for thixoforming appli- cations from 76 - 152 mm billets and the distribution of

I I

I Europe I 1500 I 24 I I I

I Japan I 470 I 26 I I I

Table 3: Distribution of thixoforming and high pressure die casting (HPDC) cold chamber machines [58].

specific alloys are shown in Table 4 [58]. This aluminum material is mainly used for automotive application.

I Alloy I Tons peryear I I AISi7Mg0.3 (A356) 1 2400 I I AISi7MgO.6 (A357) 1 12800 I I Others 1 800 I I Total 1 16000 I

Table 4: Production of 76 - 152 mm feedstock bars (thixo-material) [58].

Concerning the thixomolding process of magnesium alloys 145 machines are running in pilot series or in production in eight countries to produce parts like housings for electronic purposes [57,59]. Production figures for other semi-solid forming technologies and alloys (copper, steel) are not known at present.

In the following section some prominent examples for automotive application of aluminum thixocast material are given. Front door pillars

Figure 30: Audi A3 Four door version, pillars A and B (Thixalloy Components, Ltd., Subsidiary of SAG,

Lend Austria) [60].

Numbers of parts 8 parts used in front and rear side door

Annual volume 500,000 parts / year

Alloy: AISi7Mg0.3 (A356) material development will be given. Heat treatment: As cast 12.1 Rheocasti ng Properties: Yield strength: > 120 and 220 Mpa

Elonaation: > 10% The rheocasting process is a process which has already been investigated early in the 1960's. In the course of

Characteristic

I

Wall thickness: Part weight: 0.43 and 0.2kg

Car Steering Parts

1.8 - 19mm, average: 3mm

HPDC + Thixo- Rheo- HPDC vacuum forming casting

Figure 29: Mercedes S-class W220, control arm (courtesy: ALCAN Singen GmbH).

Number of parts:

Annual volume:

Alloy: AISi7Mg0.3 (A356) Heat treatment: as cast

Surface treatment:

Engine Park

2 (two sets LH+RH each)

100.000 cars I year (i.e. 200.000 parts I year)

Lifeti me: 1998 - 2005

Blasting with aluminum grain

Investment cost

Figure 31: Fiat Stilo, engine bracket (Stampal S.p.A.) [60,61].

XXXX XXXX XxxXX XxxXX

Alloy: AISi7MgO.6 (A357) Heat treatment: A5 Requirements: The size of the thixoformed parts is restricted mainly by the size of feed stock producible and by the max. press force of the forming machine. Up to now the largest thixo-product is a component for multilink suspension of Alfa Romeo which is made from AISi7MgO.6 (A357) and weigh around 7.5kg [59]. The surface quality of thixoformed parts corresponds to that of high pressure die casting. For some instances the roughness of 2-28pm (Rp-value) have been measured for the products from Al-alloys [62]. Further application for the automotive sector as well as in other fields exist as mentioned in the introduction, but cannot be displayed here in detail. For a more complete coverage see [61].

12 EXTENSION In this extension part new process ideas with high potential for future applications as well as guidelines for alloy and

Net shape + 20% weight reduction

X Raw material cost

process development this idea has recently been redeveloped as a production process at first by the Japanese company UBE and is called 'New Rheocasting'. The process route is displayed in Figure 32. A slightly overheated melt (cool melt) just above the solidus line is poured into avessel and cooled down to the semi-solid state. During pouring solidification starts on the vessel's surface the emerging and growing solid particles are stirred automatically, thus giving a globular microstructure. For temperature adjustment and easy removal of the slug the material is inductively heated before transferring the material to the forming device. The New Rheocasting process has been compared to the conventional high pressure die casting process (HPDC) and thixocasting and may be summarized as follows (Table 5). For complete summary see [62].

X X X X X x x

Figure 32: New Rheocasting [48]

Overall cost

Heat treatment

X X xxx xx No Yes Yes Yes

Anodising

Immediate material

recyclability Structural

(porosity) quality

No No xx xx

Yes Yes No Yes

X xx X X X X X X X X

I

I

X=low, XXXXX=high

Table 5: Comparison HPDC - Thixocasting - New Rheocasting [62].

Employing this process route globular microstructure has already been achieved for magnesium and aluminum alloys. Compared to cast material which is MHD-stirred there is less eutectic enclosed in the solid grains (Figure 33). First feasibility studies for component production for aluminum and magnesium have been carried out already. One example is the engine bracket displayed in Figure 31

Figure 33: Microstructure of Rheocast material a) aluminum (AISi7Mg0.3), b) magnesium (MgAISZnl).

which has been produced by thixocasting as well i

rheocasting [62].

12.2 Processing of steels The processing of steels in the mushy / semi-solid has not yet been employed in industrial scale, nevertheless there is research work going on for casting, forging, extrusion and rolling processes as well.

The impetus to use steels is justified by the outstanding mechanical properties, high-temperature applicability as well as resistance to corrosive media, which can be varied in a wide range according to the demand depending on chemical composition and thermo-mechanical treatment. Nevertheless limitations do occur for both processing in the liquid as well as in the solid state. Mainly due to the high melting point the feasibility of die-casting technologies is restricted (wear problems of the dies). The high yield stress in the solid state leads to restrictions in the part's geometry and to expensive process chains when processed in hot or cold forming state. One promising approach to obtain higher geometrical flexibility combined with cost cutting potentials while still taking advantage of the special properties of steels is the processing in the mushy or semi- solid state.

The feasibility of the thixoforming of steels has been proved already for different steel grades such as carbon steel, high- speed steel, cold working steel as well as austenitic steels [43,44,45,46].

The main current focus of research and development are the design, material and coatings for tools like dies and moulds. The high temperatures lead to high thermal shock and wear on the tool's surface. This means that special requirements have to be met for die materials (either high temperature metallic alloys like TZM, ceramics or hybrid solutions like hipped ceramic coatings). For die casting machines problems of thermal expansion and distortion in the machine (especially punch) do occur since the initial temperature of the die and the punch has to be elevated compared to light metal alloys processing to avoid freezing of the semi-solid material.

12.3 Mushy / Semi-Solid Rolling The rolling of metal in the mushy or semi-solid state is a rather new process, which may have however high potential for technological application concerning the processing of high-strength alloys (especially steels) combined with process shortening potentials compared to conventional hot rolling. There are different process variants. One process is based on conventional hot rolling (mushy-rolling) the other may be compared to a strip casting process (semi-solid rol li n 9).

Compared to strip casting the globular microstructure of

a) 1: water-cooled roller, 2: EM-stirring, 3: pouring gate, 4:melt, 5: bung, 6: semi-solid slurry,

7: stirring crucible [63] b) 1: inductive heating,

2: flow direction of liquid phase, 3: water cooling, 4: mushy state, 5: solid state [64]

Figure 34: a) semi-solid rolling and b) mushy rolling

the produced strip and reduced porosity in the center of the strip after casting can lead to advantages for semi-solid processing. 12.4 Mushy / Semi-Solid Joining One special approach to take advantage of the material's high flowability in the mushy / semi-solid state is to add functional features to a formed part. At present there are three possible ways known to obtain such an additional functionality. Either special contours, like a screw thread [65], can be thixoformed or the functional components can be added to a material, which is heated to the semi-solid state by joining, e.g. steel pins into an aluminum bulk [66]. These studies have so far been restricted to aluminum alloys and brass [65,66,67]. The third way to add additional functionality is the combination of forming and joining in one process step. One example for a steel - steel combination is presented in Figure 35. This modification consisted of integrating the functional part (stainless steel screw) to the die's end plate.

Figure 35: Steel -steel joining in mushy state a) cross section of joined surface, b) joined part.

12.5 Alloy Development One major task in future development is to adjust the alloys to the specific demands of semi-solid processing to enhance process stability. Currently standard wrought or cast alloys are typically used with only slight modifications. Several approaches are known to enhance the suitability of these alloys giving hints at the direction to go for alloy optimization. The goal of alloys and material development has to be to meet the criteria mentioned in section 8 'process window'.

S'["/K-l] I I

HS6-5-2 AISi7Mg AlMgSil MgAISZnl

0.44 0.83 4.0 0.87

I L' [Jcrn3K-l] 1 16.28 1 11.5 1 47 1 5.9 I I I

IL'/S'["/-'crn3]1 37.0 1 13.86 1 11.75 1 6.78 I I I

Table@: Bensitivity Ub Bnergy EnputDrEbssEorEdifferent materials.

In mable @ Dalues [for E*, a*Cand a*/S*Dre E o m pared [for differentmaterials. [MagnesiumBlloysbrExampleEdo Wave less WeatmontentDhanmluminum mlloysmndmreDhusEess easy UofiandleDorQxocessGontrol. LTheCaim [bfCalloy development Was [fo b e [fo mievelopmaterials with Eavorable thermo-chemical @ropert ies[ forUnushyf lBemi-sol id processing WhileEalising [all propertiesDecessary Ebrbrther use. Thenorm aactorms WvellmsmhemontiguityDolumemightbe adjusted [by Caddi ng Uare [Earth me ta l s Uo Om prove Uhe wetability[bfUheBolidQrains[byUeduction mfUheOiquid phase'sBurface Uension. LThus Bpheroidisation Cand [flow behaviorEksOmprovedQl91. TheQrain Qrowth h [meBemi-solidBtateEan bemeducedby addingDlementsWvithWighermeltingpointsmanmhematrix material. En this Dasethe movementrafthe phase Boundaries (particle Burface) man Be Buppressed Ql91. Uhe Buitability mf these Capproaches [have CaI ready [been proved On Uest Mie fillings. 13 CONCLUSION REMARKS In Eonclusion EhreeBspectsBremiescribed, Wvhichmay b a d the may [for Un ushy flBemi-solid processi ng Uo [become Ca key DechnologyaorOnnovationOnmetalnorming. 13.1 New Products with High Functionality 0 n e Om aj o r Mut u re Opote nti a I Oof Om us hy fl B e m i -s 01 id technologiesbysh[meEombinationmfbrmingmdQining processes, [mus mreati ngmewkvels Df htegrated parts [and minimizing mssem bli ng mosts. Eollowing Dhis mpproach mot onlyB3ew~eometricflexibilityDomparedtDDonventional pure sol id m eta1 [form i n g Uec h no1 ogi es E a n [be Cac h i eved, [but completelymewmlassesCmfpartsmanbeproducedmsingm one Btepprocess Dloute. 13.2 Low Cost & High Productivity Manufacturing One major mbstacle [fbr [mixo-processed parts h production technology E o m petition Carises [from EostGomparisons. ThereBreDariouspaths Which Dan BeEbllowedKoravercome

thisGurrentBi tuat ion Dpar tDrom UheBcale-oriented considerations Qlarger production [batches [Enables Oower costs). LTwo@rocess [brientedmpproaches Cshould [be mentionedmere. Processhtegration BndBhortening Will UeadEbDonsiderable costEutsOn Berni-sol idUechnologies, Uhusunaking thixoformingmoremompetitive. DneDital Btepmay BeDhe shif tnrom m ushy Drocessi ng Uo Csemi-solid [bruheo- tech no1 og i es, Cavoi di ng U he [Energy Eons u m i n g On du ctive heatingmndpresumably DnhancingprocessBtability. Cost cutsKupm3QO%BreEeported@8]. UMoreoverElexibleBlloying adjustmentman beEeached@lug-on-demand), which might be navorable nor medi urn-to-small Batch Bizes. TheOow[forming[IloadsUequiredCLlsingUnushyflBemi-solid technologiesGomparedUo6onventional [bul k[forming processesGanUesultOn multi-forging[brUnuIti-extrusion processesUnanufacturingm-partsper[formingBequence. Thus mhe productivity man behcreasedmany mimes. Furthermostmuttingpotential man BeDxploitedBy Dleducing the meed [fbr Quality [festi ngprocedures m ke mon-destructive componentEksts. UhisQoal man mnly Eeachedby Enhancing process Btability CandUhe Otnowledge Cand Unonitoringmf process mindows. LTheMevelopment[bfCsuch Drocess windows Uequiresmore [basicOnvestigation [bfmaterial behavior, Qjatheringmfmaterial MataDndOmprovementmf simulation Uools 13.3 Energy & Material Saving and Emission Reducing Next Uo Uhe Gost [Effectsmf Drocess Bhortening Uhe environmental EffectsBhouldBeDonsidered[asWell. Process shortening means Energy Eaving. mhis does [apply Especially for Dhe Bemi-solid Cmrmheo-processes. Apart from processmriented Energy Bavingpotentials [mere aremther[I6sues[matmeedUbbemonsideredEoncerning[me use BfDhixo-materials. Especially mhen mom pared Do Bul k for mi ng parts aforged B r [Extruded) Uhe mom ponent Mesign has Uo [be E o m pletely Ueadjusted Uo [fit Uo Uhe Qjeometric possi bilitiesrafEhemushyflBemi-solidEbrrningEkchnologies. FollowingUhisCapproachOargeUnaterial BavingsEan [be expected, [Ifbadmptimized parts man B e produced mithout the Og eo m et ri c Orest ri cti o n s Oof 0s 01 id Ofo r m i n g . OT h e implementation DfEhese[advantageswiII meedBmompletely new may [fo mpproach process mndmomponent mesign 14 ACKNOWLEDGEMENTS The Cauthors moul d Oi ke Uo Qjive Uhan ks Uo Uhe [following persons [and mompanies whomave montri buted Ub[mis paper (CI RP m e m bers menoted B y [t):

S. [IDkanoaRheo-Technology. atd)

C. tYoshida@3hinko[Wesearch E o . , [ILtd)

S. [Woyama UAhresty [W&D Eorp.)

K. DchikawaaAIST)

K. WliwaaAIST)

M . Ei hiomi a Osa ka [41 niversity )

R. Ehivpuri*aOSU)

G. Ehiarmetta, P. ffiiordano~StampalE3.p.A.)

E. [IDoege[tal FUM, BlannovermJniversity)

M. UvlodigellaIVT, [WWTH[AachenmJniversity)

K. Eiegert [t al FU , Buttgart mJ niversity )

W. BerzlaAlcanEiingenffimbH)

D.WpelianaWPI)

K. Etein hoff aTU Delft)

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