Processing of Al-based MMCs by Indirect Squeeze Infiltration of

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					                                                                                           Juni 1999



    Processing of Al-based MMCs by Indirect Squeeze Infiltration of Ceramic
        Preforms on a Shot-Control High Pressure Die Casting Machine

                            S. Long, O. Beffort, G.Moret and Ph.Thevoz




                                             Abstract

       The feasibility study on the production of Al-based metal matrix composites via indirect

squeeze pressurised liquid metal infiltration of ceramic preforms on a shot-control commercial die

casting machine and the related processing optimisation are briefly summarised. It has been

demonstrated that under optimised processing conditions high quality composite castings can be

repeatably produced.

       The criteria for the selection of optimal processing parameters are the same as those

previously established for direct squeeze casting. That is, preform preheat and melt superheat should

be adjusted to preclude premature melt solidification before and during infiltration; infiltration

speed should be controlled to avoid permanent deformation of the ceramic preform; the maximum

pressure available on the die caster should be deployed to minimise non-infiltration defects; the

gating system should be designed to ensure feeding of the solidification shrinkage in the composite

casting.
                                         1. Introduction

       Metal matrix composites (MMCs) of metallic alloys reinforced with ceramic particles and

fibres are designed to offer high specific mechanical properties and tailorable physical properties

that are highly desirable in various applications but not available in the parent monolithic materials.

Among the MMC fabrication technologies developed, liquid metal infiltration of particulate and/or

fibrous preforms by various casting techniques features a high potential of producing net-shape

composite structures. Among the casting techniques adaptable for the fabrication of Al-based

composites, the high pressure die casting technique keeps attracting research and commercial

interest due to its high productivity, high degree of automation, high surface quality and geometrical

flexibility. However, the repeatable production of composite structures with a high quality

microstructure and desirable properties presents a great challenge to composite engineering.

       High pressure die casting has been widely used to produce monolithic Al- and Mg-based

alloy castings with high surface quality and complex geometries at high production rates, but also at

the expense of inferior microstructure soundness and mechanical properties due to the nature of

encouraging spontaneous solidification in process design. Thus, this technique is usually suitable for

monolithic castings with a near eutectic composition, thin and uniform thickness and moderate

requirement on mechanical properties. Although infiltration trials by high pressure die casting have

been reported previously [Jar.92. Kau.94, Kan.95], it remains to be exploited how to repeatably

produce composite castings with desired microstructures and satisfactory performance.

       In the present study, the feasibility of the fabrication of Al-based MMC castings by indirect

squeeze pressurised infiltration of ceramic preforms on a Bühler shot-control (SC) die casting

machine was comprehensively examined and the optimal processing conditions were identified. The

obtained composite microstructures were characterised and their mechanical properties were

compared with those of identical composites produced by direct squeeze casting and with those

predicted by composite mechanics.


                                                  2
                       2. Description of the Processing Feasibility Study

       Die casting possesses the high pressure required to achieve liquid metal infiltration of

ceramic preforms. However, as pointed-out previously [Lon.96, Lon.97a], the pre-requirements for

high quality composite castings via pressurised infiltration techniques are as follows in accordance

with the processing sequence:

       1. No melt solidification before the beginning of infiltration

       2. No melt solidification during preform infiltration

       3. No preform deformation during processing

       4. Pressurised feeding of melt solidification shrinkage

       To obtain high quality composites by indirect squeeze pressurised infiltration on a

commercial high pressure die casting machine, the optimal processing conditions must be identified

to meet the above pre-requirements via experimental infiltration of ceramic preforms, infiltration

hydrodynamic analysis, and numerical optimisation of the gating system to:

  1. Determine the optimal thermal parameters, namely: melt superheat and preform preheat

  2. Identify the suitable range of the hydrodynamic parameters, i.e., infiltration speed and

      infiltration pressure, with consideration of melt chemistry and the geometrical features of the

      ceramic phases in the preforms

  3. Obtain a desired solidification sequence and eliminate matrix shrinkage voids.



2-1. Component Materials of Composite Castings

       In the first step, an age-hardenable AlCu4MgAg alloy [Bef.95] and Saffil chopped fibre

preforms of different fibre volume fractions (10-30 vol.%) were used for the feasibility study and

processing optimisation. The Saffil preforms with the dimension 120x80x15 mm3 were infiltrated

via the 120x80 mm2 surface, as illustrated in Fig. 2. In a second step, under optimised processing

conditions, ceramic preforms of continuous Altex fibres and SiC particles with volume fractions of


                                                  3
50% and 65%, respectively, were infiltrated with pure Al or the AlCu4MgAg alloy for the

characterisation of the microstructure and the mechanical performance. The details of the

reinforcements and matrix alloys are summarised in Table 1.


Table 1                 Component Materials and Their Mechanical Properties

 Reinforcement           Reinforcement        Tenisle      Ceramic E     Matrix Alloy      Tensile
     Name                  Geometry           Strength      Modulus        (wt.%)          Strength
 γ- Al2O3-based         Continuous Fibres,   1800(MPa) 210 (GPa)           Al 99.99        40(MPa)
  Altex Fibres              d=15 µm
 δ-Al2O3-based    Chopped Fibre, d=3 µm, 2100(MPa) 300 (GPa)            Al-4Cu-1Mg-       450 (MPa)
  Saffil Fibres    aspect ratio= 50-100                                    0.4Ag             (T6)
  SiC Particles        Norton F500S; Sharp       n.a.         n.a.      Al-4Cu-1Mg-       450 (MPa)
                        shape, d =12 µm                                    0.4Ag             (T6)


2-2. Facility Set-up

       The Bühler 53D SC-Series die casting machine, as shown in Fig. 1, was used for

experimental investigations. The geometry of the casting with the gating system and the positions of

the thermocouples for temperature measurements is shown in Fig. 2. The dimensions of the die

cavity were 120x80x25 mm3.




Fig. 1 The SC die casting machine used for indirect squeeze pressurised liquid metal infiltration for
                          MMC casting production in Bühler AG, Uzwil.




                                                 4
                                                                     Preform
                     T4
                                                                        Die Cavity

                     T3
                                                                        Gating
                                                                        System
                     T2
                                                                        Injection Chamber
                     T1

          4 Thermocouples


                                                                                     Plunger
                                                                     Melt
                                   Melt



                                                    Die Wall




 Fig. 2 Schematic of the composite casting with the gating system and the four thermocouples for
     measurement of melt temperature change during die filling. (The thermocouple positions:
        T1--20 mm beneath the die cavity in the gating system, T2, T3 and T4 are 40 mm,
                     80 mm and 120 mm above T1, respectively) [Lon.99a].


2-3. Infiltration Processing

       Before infiltration, the preforms and the melt of the matrix alloy were preheated to the pre-

selected temperatures, and the die and injection chamber were preheated to 280 °C.




                                              (a)                                              (b)

 Fig. 3 Processing procedure of indirect squeeze infiltration of the Saffil preforms. a) the inserted
 preform in the die cavity, b) the ejected composite casting from the die cavity after solidification.




                                                    5
                1500                                                                                               200
                              1. Die Filling
                                                3. Solidification Pressurisation        4. Casting Ejection

                1200                  2. Preform Infiltration                                                      160



                                                            Plunger Pressure
                    900                                                                                            120
          P (bar)                      Plunger Speed
                                                                                                                          V
         & D (mm)
                                                                                                                        (mm/s)

                    600                                                                                            80
                                               Ejector Pressure

                                                Plunger Displacement
                    300                                                                                            40




                      0                                                                                            0
                          0     5          10          15         20        25     30     35       40         45

                                                                   Time (s)

      Fig. 4 A typical indirect squeeze casting cycle on Bühler SC-series caster, where a high
      die filling speed of 140 mm/s, but a slow infiltration speed of 25 mm/s were used. Note
    the pressure change during infiltration similar to that of direct squeeze infiltration [lon.99a].


       During composite casting, the following actions sequentially take place: placing the preform

into the die chamber as shown in Fig. 3a, pouring the melt into the injection chamber (the so-called

shot sleeve), moving the plunger to drive the melt to fill the die cavity and to infiltrate the preform

at the pre-selected plunger speed. Once the maximum pressure is achieved, this value is maintained

until the complete solidification of the casting. Post solidification, the casting is ejected from the

die, as shown in Fig. 3b, followed by the preparation for the next infiltration cycle.

       The thermal and hydrodynamic processing parameters were continuously recorded

throughout the processing cycles. The record of a typical casting cycle is given in Fig. 4.



2-4.Investigation Scope

       To study the processing feasibility and to identify the optimal processing conditions, the

main processing variables, namely, melt superheat Tm, preform preheat Tf, maximum pressure Pmax,

infiltration speed in terms of plunger speed Vp, were varied within the ranges as specified in Table


                                                                       6
2. Throughout the whole experimental investigations, the plunger speed for die filling and the die

temperature are    80 mm/s and 280 °C, respectively. The effect of processing conditions on

infiltration quality was examined by optical microscopy.


Table 2 The Investigated Processing Parameter Ranges for the AlCu4MgAg /Saffil MMCs

       Tm (C°)           Tf (C°)         Pmax (MPa)         Vp (mm/s)            Vf (%)

       700-850          400-800            10-100             20-90              10-30




                  3. Processing Feasibility and Processing Optimisation

3-1 Optimisation of Thermal Parameters

3-1-1 Melt Superheat

        Due to the large temperature difference between the superheated melt and the die

temperature, the melt flow is expected to be subjected to an intensive heat loss. Fig 5 shows the

temperature loss of a AlCu4MgAg melt superheated to different temperatures during pouring and

die filling.

        As the curves show, an extensive temperature loss of about 100 °C appears during the period

from the beginning of melt pouring until the melt reaches the first thermocouple, which takes ∼7-10

seconds. When the melt successively passes the next three thermocouples in the die cavity, the

extent of the melt temperature decreases successively due to the shortened time duration and the

reduced temperature difference between the melt flow and the die cavity. The temperature loss

ranges from a few degrees up to more than 30 °C according to the degree of the melt superheat

employed.




                                                 7
                         850
                                                                                 Tm = 750 °C
                                                                                 Tm = 800 °C
                         800                                                     Tm = 830 °C



                         750
               T (°C)                                         T1

                         700                                         T2
                                                                            T3     T4

                         650                  Melt Liquidus



                         600
                               0         50         100        150        200     250      300
                        (Melt Pouring)
                                              Melt Flow Distance (mm)

            Fig. 5 Temperature changes of an AlCu4MgAg melt during die filling in the
              indirect squeeze casting process for different melt superheats [Lon.99a].


       It has been understood that the employment of low melt superheat (Tm) will induce

undesirable premature melt solidification during die filling and preform infiltration [Lon.97a], but

the employment of an excessively high melt superheat is likely to raise difficulties in preparation of

a high quality melt in foundry practice and adversely prolongs the melt-ceramic contact time. The

temperature curves in Fig. 5 show that, with a melt superheat of ∼800 °C, the AlCu4MgAg melt

temperature remains a few degrees above the liquidus of pure Al (660 °C) after the die filling. These

few degrees superheat are sufficient to prevent premature melt solidification but not excessively

high to prolong melt-reinforcement contact time to a large extent. Noting that Al-based alloys

generally have a lower liquidus temperature than pure Al, a melt superheat of 800 °C is

recommended as the optimal temperature for the production of Al-based composites under similar

processing conditions.




                                                          8
   3-1-2. Preform Preheat

            After die filling, the melt contacts the preform and starts to penetrate into the interspaces

   between the ceramic phases in the preform. Due to the microscopic scale of the interspaces, the heat

   exchange between the melt flow and the ceramic phase is intensive if a temperature difference exists

   between them. Therefore, the preform preheat constitutes one of the most important thermal

   parameters for pressurised infiltration free of premature melt solidification during infiltration.


Tf=400 °C                                                  Tf=800 °C




                                                     (a)                                                         (a)

Tf=400 °C
Tf=400°C                                                   Tf=800 °C
                                                           Tf=800°C




                                          0.2 mm                                                        0.2 mm
                                                                                                        0.2 mm
                                                     (b)                                                         (b)

 Fig. 6 Solidification structure (a) of 15%Saffil/           Fig. 7 Solidification structure (a) of 15%Saffil/
 AlCu4MgAg cast at Tm=800 °C, Pmax=100 MPa,                  AlCu4MgAg cast at Tm=800 °C, Pmax=100 MPa,
   and VP=30 mm/s, and Tf=400 °C; and the                      and VP=30 mm/s, and Tf=800 °C; and the
    microstructure at the preform bottom (b).                   microstructure at the preform bottom (b).


            Microscopical investigation of the cross sections of two 15 vol.% Saffil/AlCu4MgAg

   composites produced with preform preheats of 400 and 800 °C, a constant melt superheat of 800 °C

   and a plunger speed of 30 mm/s reveal that, a severe preform deformation and a defect rich zone at

   the preform bottom were induced by premature melt solidification before and during infiltration




                                                       9
when the preform preheat was ∼400 °C, as shown in Fig. 6. With a preform preheat of 750-800 °C,

the preform deformation and the defect rich zone were eliminated, as shown in Fig. 7.

       It was also made clear that the appearance of premature melt solidification will adversely

induce non-uniformity of matrix phase constitution and variation of matrix-ceramic binding strength

along the infiltration direction due to the partition of the alloying elements during solidification and

the change in melt-fibre contact condition [Lon.97a]. The employment of a preform preheat

between 750-800 °C is desirable to eliminate the premature melt solidification. Obviously, the

demand of such high preform preheat and melt superheat stems from the intensive heat loss of the

preform and melt to the cold environment during preform transfer and melt pouring.



3-2 Effect of Hydrodynamic Parameters

3-2-1 Squeeze Infiltration Hydrodynamics

       In liquid metal infiltration of a ceramic preform, there are two hydrodynamic parameters

associated with infiltration quality: infiltration speed (Vinf.) and infiltration pressure (Pinf.).

According to the established infiltration hydrodynamics [Lon.95], the effect of Vinf. and Pinf. during

unidirectional infiltration of a preform can be correlated to the physicochemical nature of the melt-

fibre contact system (melt surface tension σmg, melt viscosity µ and the contact angle of the melt

flow to the fibres θ) and the geometrical features of the porous preform as follows:

               σ mg cosθ    40 µVinf
        P =2             +                 Z + Pback
                           3G p (1 − V f )
                                                                            (1)
         inf    Req. max

where, Vf is the ceramic volume fraction of the preform, Req.max is the equivalent radius of the

largest interspaces between the ceramic phase in the preform, Gp is the geometrical constant of the

preform, representing the orientation and size distribution of the interspaces in the preform.

       According to the model, the infiltration starts when the external pressure exceeds the

minimum capillary resistance of the preform to the melt penetration, followed by a stable infiltration


                                                       10
stage featured by the linear pressure-time relationship until the melt penetrates through the preform.

The infiltration is terminated when the pressure reaches its pre-selected maximum value at the end

of the air compression stage. The variation of the pressure during infiltration is schematically shown

in Fig. 8. For a prescribed preform-melt system the infiltration pressure is a hydrodynamic response

to the resistance of the preform to the melt penetration at a constant flux.




                     P

                                        Maximum Infiltration Pressure
                   Pmax                                                                 Pressurised
                                                                                        Solidification




                                    Penetrating-through Pressure
                     Ppt


                    Pcap

                     0
                                                                                                 t
                         Infiltration              Stable               Entrapped Air
                          Initiation             Infiltration           Compression



        Fig. 8. The Time-Pressure relationship during unidirectional infiltration of a ceramic
                 preform at a constant speed. Note the three stages of the infiltration.



3-2-2 Effect of Infiltration Speed

       The infiltration speed determines not only the time required to achieve full infiltration, but

also the infiltration pressure gradient, penetrating-through pressure and the saturation degree of the

infiltrated preform. Therefore, the infiltration speed affects the infiltration quality via:

1. inducing preform deformation and preform delamination when the infiltration pressure exceeds

    the elastic compression strength of the ceramic preform [Lon.95].

2. determining the amount of the air entrapped in the infiltrated preform and the associated non-

    infiltration defects [Lon.97b].

                                                                11
          As the pressure-time curves during infiltration of a 15% Saffil preform with AlCu4MgAg

melt in Fig. 9 indicate, within the pre-selected plunger speed range, an increase in infiltration speed

shortens the time for the melt to penetrate through the preform, and increases the penetrating-

through pressure as predicted by the model. For a Saffil preform with a fibre volume fraction of

15% the penetrating-through pressure at the maximum plunger speed of 90 mm/s is about ∼1.5

MPa, slightly lower than the quasi-elastic preform compression strength of a 1.7 MPa [Lon.99a].

Therefore, no preform deformation is observed in the castings provided that the infiltration is free of

melt solidification. However, it is obvious that if the plunger speed is further raised, the penetrating-

through pressure will become higher than the strength of the preform, resulting in permanent

preform deformation as reported previously [Kann.95] during infiltration of preforms at a speed of

∼1 m/s.

                          5
                                                    V=90        V=60         V=40       V=20


                          4



                          3
                    P
                  (MPa)           Penetrating-through
                          2           Pressure


                          1



                          0
                              2           2.5           3              3.5          4          4.5
                                                            Time (s)


      Fig. 9. Pressure-Time curves for infiltration of 15% Saffil preform with different plunger
          Speeds and Tf=750 °C, Tm=800 °C, TD=280 °C. Note the increase of the pressure
     gradient and the penetrating-through pressure with increasing infiltration speed [Lon.99a].




                                                        12
3-2-3 Effect of Maximum Pressure

       As mentioned above, infiltration pressure is a hydraulic response to the resistance of the

preform to the melt infiltration. It can reach a few MPa for the infiltration of the preforms of various

types of ceramic reinforcement. Once the melt penetrates through the preform, the pressure is used

to overcome the capillary resistance of the non-infiltrated small interspaces and to compress the air

entrapped there to achieve maximum saturation in the course of its rapid increase [Lon.95]. Die

filling and infiltration are performed under displacement control; the control mode is changed to

pressure control at a preset value after completion of infiltration. The subsequently sustained

pressurisation at the maximum value is necessary to feed the solidification shrinkage of the melt in

the composite casting.

       In the light of achieving a maximum degree of preform saturation, the use of the maximum

pressure available from the hydraulic system of the die casting machine is recommended. With

regard to the effect of pressure on the solidification shrinkage, the employment of insufficiently high

pressure will induce shrinkage voids in the composite castings. Previous infiltration practice on a

direct squeeze caster [Zhu.94] and the present indirect squeeze infiltration indicate that a pressure of

≥15-20 MPa is high enough to deform the peripherally solidified matrix shell to feed the

solidification shrinkage in the central part of the casting. However, an effective feeding of the

solidification shrinkage during indirect squeeze casting on a die casting machine relies on the

employment of an optimal gating system, as highlighted in the following section. The use of a

gating system designed for conventional high pressure die casting will inevitably lead to the

formation of shrinkage voids despite of a pressurisation exceeding 100 MPa, as shown in Fig. 10.




                                                  13
                                                                                  25 µm


 Fig. 10. The as-cast microstructure of the central part of a 15 vol.% Saffil/AlCu4MgAg composite
             casting produced on a Bühler SC die casting machine with a conventional
            gating system under 100 MPa maximum pressure. Note the shrinkage voids.


3-2-4 Effect of the Preform-Melt Infiltration System

       According to the infiltration hydrodynamics, the preform-melt system influences the

infiltration kinetics via altering the melt viscosity, the wettability of the melt to the preform and the

size and numerical distribution of the interspaces between the fibres that act as melt flow ducts in

the preform.

                                      5                                                         200
                                              V (m m /s)
                                              P (10% )
                                              P (20% )
                                      4
                                              P (30% )
                                                                                                180
                                              D (m m )
                 P (MPa) & V (mm/s)




                                      3
                                                                                                      D (mm)




                                                                                                160

                                      2


                                                                                                140
                                      1



                                      0                                                         120
                                          3    3.3         3.6     3.9      4.2   4.5     4.8

                                                                 Time (s)


      Fig. 11 Effect of fibre volume fraction of preforms on the infiltration kinetics [Lon.99a].




                                                                    14
       In general, melt alloying manifests its influence on the infiltration via the change in melt

viscosity and the change in melt wettability to the preform by melt-preform interfacial chemical

reaction. Therefore, it has been widely believed that the addition of the elements chemically reactive

to the preform or/and encouraging the formation of eutectic phases is able to facilitate infiltration.

However, alloying melt within the practical range changes the melt viscosity to a limited extent, and

the wetting promotion via melt-preform interfacial chemical reaction requires a period of contacting

time much longer than the practical infiltration time which is less than one second. Therefore, melt

alloying imposes no considerable influence on the kinetics of solidification free infiltration

[Lon.99a].

       The influence of the geometrical characteristics of the preform on the infiltration kinetics is

demonstrated in Fig. 11. As predicted by the model (eq. 1), with increasing fibre volume fraction,

the threshold pressure for the initiation of infiltration, the penetrating-through pressure and the

pressure gradient are raised due to the inverse decrease of the preform interspaces.

       As far as the solidification free infiltration is concerned, the geometrical factor of the

preform plays a much more dominant role in infiltration quality control than melt alloying. To

prevent preform damage, the infiltration speed must be selected to ensure that the penetrating-

through pressure is lower than the elastic compression strength of the preform.



3-3 Gating System Optimisation

       In foundry practice, a proper gating system design is crucial to obtain sound castings, but its

importance in producing sound composite castings on a die casting machine has been largely

ignored previously.

       To visualise the sensitiveness of matrix void formation to the geometry of the gating system,

the solidification behaviour of a composite casting with a gating system for conventional monolithic




                                                  15
die casting (DG) and with a gating system optimised for composite casting (CG) have been

numerically simulated and empirically verified [Lon.99a].

       Fig. 12 shows the solidification behaviour of the composite castings with different gating

systems (a and d) and the castings produced with the respective gating systems (b and c) under the

optimal processing conditions.

       As illustrated by the solid-liquid volume fraction map of Fig.12a, with the narrow gating

system for conventional monolithic die casting, the melt solidification completes first in the gating

system, which isolates the partially solidified composite casting from the pressurised melt reservoir

in the injection chamber. Consequently, the casting surface is seamed due to the resultant

depressurisation, as shown in Fig. 12b, and internal shrinkage voids form, as shown in Fig. 10,

despite of the use of a 100 MPa infiltration pressure. In contrast, as shown in Fig. 12d, when the

gating system is optimised to control the sequence of the solidification completion from the top of

the casting towards the melt reservoir in the injection chamber through the gating system, the

casting has a high quality surface, as shown in Fig. 12c, and is free of internal shrinkage voids, as

illustrated in Fig. 13b.




                                                 16
                           a      b                                 c                       d




 Fig. 12 Solidification behaviour and corresponding surface appearance of the composite castings
 with DG (a and b) and CG (d and c) gating system, respectively, in terms of solid fraction (where
the red colour refers to 100% solid metal and purple colour to 100% liquid.). Note the poor surface
  quality in (b) due to de-pressurisation induced by early solidification of the DG gating system.


3-4 Processing Timing

        As indicated by the strict requirement on the preform preheat and melt superheat, any

processing delay before the initiation of the infiltration will dramatically alter the actual preform and

melt temperatures from the optimal values and leads to the formation of undesirable microstructure

features as reported previously. Therefore, the importance of a strict processing timing will never be

over-emphasised during indirect squeeze casting of MMCs on a high pressure die casting machine

at the industrial scale.




                                                   17
           4. Microstructure and Mechanical Properties of the Composites

       The microstructure and the mechanical properties of the Al-based metal matrix composites

of different types of reinforcement and different reinforcement volume fractions produced under

optimised conditions were characterised by microscopy and mechanical testing.

       The typical microstructures of the composites are given in Fig. 13. Detailed microscopy of

the composites indicates that the die cast composites possess microstructural features similar to

those of identical composites produced by optimised direct squeeze casting in terms of

reinforcement distribution, interfacial structure, matrix precipitation states, and infiltration degree.




                                         15 µm
                                          15 µm                                              15 µm
                                                                                             15 µm
                                                   (a)                                                     (b)




                                                         Fig. 13
                                                         Microstructures of the peak-age hardened
                                                         F500S-SiCp/AlCu4MgAg composite (A), 15%
                                                         Saffil/AlCu4MgAg composite (B), and as-cast
                                                         Altex/Al composite (C) produced on a Bühler
                                                         SC die caster under optimal processing
                                                         parameters. Note the high infiltration quality.


                                    50 µm
                                             (c)




                                                    18
       The mechanical properties of the composite castings produced on a Bühler SC die casting

machine under optimal conditions are summarised in Table 3 in comparison with the properties of

identical composites produced on EMPA's direct squeeze caster under optimal conditions with 130

MPa maximum pressure. As the data indicate, the composites produced by industrial indirect

squeeze casting possess properties comparable to their counterparts produced by direct squeeze

casting in the laboratory.


Table 3        Mechanical Performance of Direct and Indirect Squeeze Cast MMCs
               (σB: 3-pt. bending strength, σT: ultimate tensile strength)

                                 Composite Type                    σB       σT       E   Pressure
          Process                                       Temper
                             Reinforcement Vol-Fract.             (MPa) (MPa)      (GPa) (MPa)
indirect Squeeze Casting F500S-SiCp/AlCu4MgAg                      730     470      198       100
                                                          T6
  (Bühler SC-die caster)              65%
 direct Squeeze Casting      F500S-SiCp/AlCu4MgAg                  750      n.a.    201       130
                                                          T6
          (EMPA)                      65%
indirect Squeeze Casting          Altex/Al (UD)                    n.a.    899.4    120       100
                                                        as-cast
  (Bühler SC-die caster)              50%
 direct Squeeze Casting           Altex/Al (UD)                    n.a.    890.7   119.9      130
                                                        as-cast
          (EMPA)                      50%
indirect Squeeze Casting       Saffil/AlCu4MgAg                    n.a.     523      96       100
                                                          T6
  (Bühler SC-die caster)              15%
 direct Squeeze Casting        Saffil/AlCu4MgAg                    n.a.     514      95       130
                                                          T6
          (EMPA)                      15%


       Usually, the tensile strength of fibrous composites is well predicted by the modified Rule of

Mixtures (RoM) as follows:

       σc = κσfVf+(1-Vf)σm           (2)

       where, σc, σf and σm refer to the strength of composite, fibre and matrix upon composite

failure, respectively; Vf referes to fibre volume fraction, κ=is the geometrical factor with

consideration of the interfacial binding behaviour. For Altex/Al composite under axial tension,

                                                  19
κ==1. For chopped fibrous composites, κ==0.27 or 0.375 has been used previously [Fuk.82, Bad.85]

to account for the random orientation and variation of aspect ratio of the fibres in the composite.

       As compared with the prediction of the RoM, Altex/Al composite produces a strength very

close to the theoretical value of 930 MPa if the component strengths in Table 1 are used. Fig. 14

gives the strength of the peak-aged (T6) Saffil/AlCu4MgAg composite of variable fibre volume

fraction in comparison with the predictions of the modified RoM.

                          650
                                 AlCu4MgAg Matrix
                          600                                                Tests


                          550

                    σc
                   (MPa) 500
                                                                       RoM (k=0.375)

                          450

                                                                         RoM (k=0.27)
                          400


                          350
                             0.05     0.1     0.15        0.2   0.25       0.3       0.35

                                             Fibre Volume Fraction (%)

    Fig. 14 A comparison of the strength of Saffil/AlCu4MgAg with the theoretical predictions
            of the modified Rule of Mixtures for discontinuous composites [Lon.99b].


       As the figure shows, the properties of the die cast composite exceed the theoretical

predictions, which not only indicates the achievement of a high quality composite casting, but also

hints the need for a better understanding on the strengthening mechanisms of discontinuously

reinforced composite castings.




                                                     20
                                           Conclusions

       The present study demonstrates that the production of metal matrix composites via indirect

squeeze pressurised infiltration of ceramic preforms on a shot-control die casting machine is

feasible and that the produced composites possess the desired microstructural features and excellent

mechanical properties provided that the infiltration processing is conducted with the optimal

parameters as detailed hereafter:

       In accordance with the criterion of solidification free infiltration, the main thermal

parameters--preform preheat and melt superheat--should be selected to avoid premature melt

solidification. In practice, 750-800 °C preform preheat and 800 °C melt superheat are recommended

for indirect squeeze casting with a 280-300 °C die temperature and a strict processing timing.

       Under the optimal thermal parameters, the maximum infiltration speed is controlled by the

geometrical features and the elastic compression strength of the preform. For a prescribed preform-

melt infiltration system, the infiltration speed should be selected in such a way that the penetrating-

through pressure is lower than the elastic compression strength of the preform to prevent preform

damage. For the infiltration of Saffil preforms, the infiltration speed should be inferior to 100 mm/s.

To minimise the non-infiltration defects associated with capillarity and air entrapment, employment

of the maximum pressure available on the die casting machine is recommended.

       To eliminate shrinkage voids in the matrix, to which the properties of the composites are

sensitive, the gating system should be optimised to ensure a sequential solidification from the far

end of the composite casting to the melt reservoir in the injection chamber through the gating

system.




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                                       Acknowledgements

       This work was part of the project 2.1B "High Performance Aluminium Matrix Composites"

of the Swiss Priority Program on Materials Research (1995-1999) and supported by grants from the

Board of the Swiss Federal Institutes of Technology FIT Board.



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