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Volume Guidelines for joining of metal matrix composites


									                                                                                Technical report                                 TEK01-0

                      Gu idelines for joining of
                      metal matr ix compos ites
                      MMC - Assess Thematic Network

                                                                                                                                                  Volume 8

         The stated results are relevant only to the object (s) described in the report. The report may not be reproduced or used for reference
         purposes without the permission of CSM Materialteknik AB unless quoted in its entirety.

         CSM Materialteknik AB                                                                                            A Saab-Company
         P. O. Box 1340                   Telephone +46-13 16 90 00               Internet               Reg. No. 556517-3951
H. Persson- CSM Materialteknik
         SE 581 13 Linköping              Fax       +46-13 16 90 20                                              V A T No. SE556517395101
H. Persson                                                         MMC-Assess – Thematic Network

                  Guidelines for joining of metal matrix composites
Our reference: Bernt Jaensson            Telephone: +4613169043 E-mail:
Job No: A00213                           Id No: MMC Assess

1       Summary
One of the obstacles to using high-performing metal matrix composites is the scarcity of known,
proven joining methods; it is difficult to imagine examples of machinery or equipment where no
kind of joint has been used. As an effort to alleviate this situation the following brief guidelines
have been produced, within the framework of the BRITE EURAM project MMC Assess, BET2-
621, topic 2.
The guidelines, which are based on a literature survey, have been arranged according to type of
matrix alloy system, aluminium-based or other alloys (magnesium and titanium). Reflecting the
dominant position of Al-based metal matrix composites, all the joining techniques described in
this report are mentioned under the aluminium sub-heading. Concerning magnesium and tita-
nium-based MMCs, only one and two joining methods, respectively, have been presented.
The full collection of joining techniques has been split into the three categories fusion - solid state
- other processes.

H. Persson              MMC-Assess – Thematic Network

2      Contents
1     Summary                                       1
2     Contents                                          2
3     Introduction                                      3
4     MMC categories                                    3
5     Joining methods                                   3
6     Discussion                                        8
7     References                                        9

H. Persson                                                       MMC-Assess – Thematic Network

3      Introduction
Metal matrix composites (MMCs), as a class, are extremely versatile materials, enabling the de-
signer to combine the properties of various metals and non-metals in one piece. One of their
draw-backs, though, is the difficulty of connecting such pieces to themselves or to monolithic
metals - it has become evident, that the joining process can impair the properties of the MMC to
the level of the matrix material or even lower. The aim of this literature-based report is to give a
systematic presentation of the various joining methods which have been used with MMCs, to-
gether with the precautions and limitations which apply in each case.

H. Persson                                                      MMC-Assess – Thematic Network

4      MMC categories
MMCs can be classified according to type of matrix material and type of reinforcement.
The choice of matrix alloy is broadly determined by the requirements on density and tempera-
ture capability. Aluminium alloys, 2000-, 6000-, 7000- and 8000-type, are the most common ma-
trix materials. Also worth mentioning here are magnesium and titanium alloys.
The reinforcement may be either continuous or discontinuous. The use of continuous fibres or
filaments, although potentially offering very high static and dynamic strength in the reinforce-
ment direction, involves considerable technical complexity and associated high cost. This type of
MMC will only be briefly mentioned in this report.
Particulate reinforcement is most prevalent among the discontinuous-type MMCs. The particles
are ceramics: SiC, Al2O3, B4C and TiC. Particulate reinforced MMCs are isotropic, relatively easy
to manufacture, and also much less demanding than continuous-type MMCs as for secondary
Reinforcement with short fibres falls between the two aforementioned categories what concerns
technological complexity and cost.
With some combinations of matrix and reinforcement material it can be necessary to
apply a protective coating on the latter before processing, in order to avoid detrimental interfa-
cial reactions. As will be described later, such a coating may also be useful with respect to the
joining process.

H. Persson                                                        MMC-Assess – Thematic Network

5         Joining methods

5.1       Joining methods for aluminium based MMCs

5.1.1     Classification
Since aluminium based MMCs form the dominant class of these composites, they are also the
application area for which many of the published joining methods have been developed. It is
appropriate to put these methods into the following groups [1]:
      -   fusion processes
      -   solid state processes
      -   other processes.

5.1.2     Fusion processes (mainly particulate reinforced MMCs)
There are a number of difficulties associated with fusion welding of MMCs [2]:
      -   high viscosity of the melt above the melting point
      -   segregation effects when the melt resolidifies
      -   interactions between reinforcement and matrix
      -   gas evolution
High viscosity tends to make the mixing of filler and molten parent metal difficult. The problem
can be alleviated by using Si-rich aluminium filler wires or, if possible, by employing a matrix
alloy with high Si content [2].
Segregation may take place during fusion welding of SiC-reinforced Al MMC, since the ceramic
particles are rejected by the solidification front, thus causing formation of particulate-free (unre-
inforced) regions. In the case of Al2O3 particle reinforcement, the use of high Mg containing wires
will help [2].
Trying to lower the melt viscosity by increasing its temperature tends to worsen potential prob-
lems with reinforcement-matrix interaction [2]. In the Al-SiC case, Al4C3 platelets and silicon
blocks may be formed. The resulting microstructure is extremely brittle and, in the presence of
water, very prone to corrosion. The reaction has often been reported to take place during elec-
tron/laser beam welding, which tends to create hot weld pools. To avoid problems with forma-
tion of Al4C3 platelets, it is essential to choose the welding parameters carefully. Also the matrix
composition has been shown to be critical [3].
Gas evolution during fusion welding can be a problem if the MMC material has been produced
by a powder metallurgy route. If the occluded gas content is too high, gas (especially hydrogen)
evolution will occur, leading to extensive cracking in the heat affected zone (HAZ) and/or weld
porosity. This problem may be solved by application of proper degassing techniques to the pow-
der [2].
Among fusion welding processes, the following ones are of interest for joining Al base MMCs:
      -   gas-tungsten arc (GTA) and gas metal arc (GMA) welding
      -   laser beam (LB) welding
      -   electron beam (EB) welding
      -   capacitor discharge (CD) welding

H. Persson                                                        MMC-Assess – Thematic Network

               Gas-tungsten arc and gas-metal arc welding
In both these cases, an electric arc is struck between the workpiece and an electrode, non-
consumable tungsten or consumable metal, respectively. The hot/molten metal is protected by
inert gas flowing around the electrode. In the GTA case, filler metal may be preplaced in the joint
or fed into the arc from an external source.
The GTA method has been extensively used for welding Al MMCs based on 6XXX (Al-Mg-Si)
alloys. Low heat input and high silicon filler wires are recommended. In the case of MMCs with
Al2O3 particle reinforcement, Mg-rich wires should be used to prevent the particles from dewet-
ting and clumping in the weld pool [2].
When continuously reinforced Al(6061)-B MMCs was GTA welded without filler wire, the boron
filaments were overheated, leading to fragmentation and dissolution. The problem was solved by
using Si-enriched (ER4043) filler wire [4].
GMA welding, which is often automated with high welding speed, has been found to be more
adaptable to MMC welding than has the GTA method. As an example, it has been shown that
GMA welding produced the best results when both processes were tried for joining 6061 Al ma-
trix composite reinforced with B4C particles, using filler metal addition. The GMA method is
considered a viable joining technique for MMCs [1].

               Laser beam welding
In this process, a beam of laser light is focussed using optical lenses onto the solid material,
which is heated above its melting point. LB welding involves very high power density, some 106
W/cm2. A high power density is necessary to produce the required interaction with the material,
"beam coupling". This beam coupling is about four times greater for MMCs than for monolithic
aluminium alloys. The result is, that the LB method can be used to produce deep, narrow welds
with narrow heat affected zones [2].
Unfortunately, the high temperature and the laser beam’s interaction with SiC particles tends to
produce a deleterious weld zone microstructure containing Al4C3, primary silicon and Al-Si
eutectic [2]. It is possible, though, to limit the extent of this reaction by controlling the amount
and mode of energy input [5]. Another means of dealing with this problem has been found to be
the addition of a strong carbide-forming element like titanium, either by using Ti filler wire [6] or
by placing a Ti foil between the two MMC blocks to be joined by a butt joint [5].
Other reinforcement types like Al2O3 and B4C do not present this kind of problems [2].

               Electron beam welding
In EB welding, a beam of electrons is accelerated through an electric field and focussed by a
magnetic lens onto the joint zone. Since the electrons would be scattered by gas molecules, the
process must take place in vacuum. Heat is generated when the beam hits the weld zone. The
very high power density, of the order of 106 W/cm2, produces a deep, narrow weld [2]. Compared
to the LB method, EB welding has been found to cause less of the unwanted Al/SiC reaction [7].
Still, EB welding has had limited success with Al based, SiC reinforced MMCs. The use of high
speed and temperature controlled welding automation might improve the joint quality, however

               Capacitor discharge welding
Capacitor discharge welding is a special kind of resistance welding, in which the energy comes
from the rapid discharge of electrical capacitors while force is applied over the joint interface.
Because the discharge pulse is short, of the order of 5 - 25 milliseconds, the CD process may pro-
duce less unwanted reactions and provide somewhat better weld properties than conventional
spot welding. This has been borne out by experiments on several types of Al/SiC MMCs [1].

H. Persson                                                         MMC-Assess – Thematic Network

5.1.3   Solid state processes
Under this heading, the following processes will be addressed:
    -   diffusion bonding (DB)
    -   inertia friction welding (IF)
    -   friction stir welding (FS)

               Diffusion bonding
When joining two solid pieces of material by diffusion bonding, the two parts are brought to-
gether and held under pressure at an elevated temperature for a sufficient length of time to allow
a metallurgical bond to be formed by diffusion. For Al-base materials, the temperature range is
325 - 520°C; the time needed depends on the temperature and the material to be bonded. The
surfaces must be prepared to a good finish, better than Ra 0.4 µm, and they must be clean. Vac-
uum or protective atmosphere is also needed during the process [2].
Al alloys are not especially well suited for diffusion bonding, since a tenacious and stable surface
oxide is naturally formed. The presence of reinforcement particles places further restrictions on
process variables. Practical trials have shown, however, that diffusion bonding is a usable joining
method, either without interlayer or using Cu or Ag interlayers. By careful process control a suit-
able amount of mass transport can be achieved, in order to avoid the formation of either particu-
late-rich or particulate-free zones, both of which would cause bond strength impairment [2].
Diffusion bonding is a preferred joining method for heat transfer applications such as heat pipes,
radiators and heat exchangers [1].

               Inertia friction welding
In friction welding, the heat needed is produced by friction between the two parts to be joined. A
subgroup of friction welding, inertia friction welding, is used in cases where at least one of the
parts is rotationally symmetric. This part, fixed to a rapidly rotating flywheel, is brought into
contact with the (stationary) mating part under pressure. Under the influence of the heat
evolved, a soft layer is formed at the interface. Normally, this bonding layer is allowed to cool
under pressure. The formation of the bonding layer involves upset forging and extrusion of ma-
terial from the interface. With MMCs, a higher axial force must be applied than is the case when
joining monolithic materials, since the flow stress is increased by the reinforcing particles [1].

               Friction stir welding
Being a relatively new method also for joining of monolithic materials, friction stir welding ap-
pears to be a promising technique for joining of MMCs. As distinguished from conventional fric-
tion welding, the parts to be joined are not moved relative to each other. Rather, they are firmly
clamped to a backing plate in order to prevent the faces to be forced apart. A cylindrical, rotating
tool is moved along the joint line to produce a plastisized material zone around the tool through
frictional heating. The plastisized material is forced to move from the front to the back of the tool,
thus forming the weld on consolidation. This solid state process enables the retention of chemis-
try and uniform distribution of reinforcing particles in the matrix. The risk of reinforcement-
matrix chemical reaction is minimized by the low welding temperature [1].
So far, the usefulness of FS welding has been demonstrated for butt welding of flat plates. In the
case of SiC-reinforced MMCs, it has been found that it can be used with reinforcement levels up
to 25 volume percent. A problem is that the SiC particles tend to cause a high rate of wear of the
tool [8].

5.1.4   Other processes
The following joining methods will be described in this paragraph:
H. Persson                                                         MMC-Assess – Thematic Network

    -   transient liquid phase bonding (TLP)
    -   brazing (BZ)
    -   soldering (SD)
    -   adhesive bonding (AB)

               Transient liquid phase bonding
The TLP bonding process is similar to diffusion bonding with an interlayer, a lthough in the TLP
case the process temperature is high enough for the eutectic formed by the matrix and interlayer
metals to melt. With aluminium as matrix metal, Cu, Zn or Ag interlayers are used. The eutectic
temperatures of the Al-Cu and Al-Ag binary systems are 548°C and 566°C, respectively [2].
As is the case with diffusion bonding, the surfaces must be deoxidized to facilitate wetting. In
order to improve contact between substrate and interlayer, pressure should be a pplied across the
joint. Time and temperature should be minimized in order to prevent microstructural damage

Similar to TLP bonding, brazing proceeds by melting a metallic interlayer between the two sur-
faces to be joined. In the case of brazing, however, the alloy used for the interlayer has in itself a
sufficiently low melting temperature to liquefy without formation of eutectic.
The most common brazing methods are vacuum furnace brazing and dip brazing.
Vacuum brazing is used for flat-on-flat applications where a large normal pressure can be ap-
plied during the brazing cycle. Dip brazing is carried out using chemical fluxes and is normally
used with self-fixturing assemblies. Surface oxides must be removed prior to brazing. The chemi-
cal compatibility between brazing alloy and MMC matrix material must be checked in order to
avoid liquid metal embrittlement [1].
Brazing is typically used for thermal applications such as heat pipes, radiators and heat exchang-
ers [1].

In comparison with brazing, which it resembles in several respects, soldering is a low-
temperature process. The dividing line with regard to process temperature is 450°C.
There are positive and negative implications of using a low-temperature solder. Heat treated Al
alloys will not be degraded, and thermal distorsion of the structure will be minimized. On the
other hand, the joint strength will be much lower than what can be achieved with brazing [1].
Because of the tenacious oxide which is naturally formed on aluminium, the joint surfaces must
be prepared by plating in order to be wettable by normal solders, or very a ggressive fluxes must
be used. After soldering, it is important to remove residues of the flux which could otherwise
cause in-service galvanic corrosion or liquid metal embritt-lement. However, an active solder
(Sn-Ag-Ti + lanthanides) has been developed, which makes the use of plating and aggressive
fluxes superfluous [9].

               Adhesive bonding
With regard to its influence on the integrity of the MMC parts to be joined, adhesive bonding is
the lowest-risk alternative:
    -   it does not involve high clamping pressure which might cause physical damage to rein-
        forcing fibres
    -   no risk of corrosion due to flux residues

H. Persson                                                       MMC-Assess – Thematic Network

      -   no risk of liquid metal embrittlement
      -   low curing temperature (in most cases ≤ 180°C) means that the bonding process can be
          used with Al alloys in the heat treated condition.
There are drawbacks, too, with adhesive bonding. Outgassing of the adhesive might contaminate
optical mirrors or photonic sensitive equipment present in the MMC bonded structure. Adhesive
joining cannot be used where high thermal or electrical conductivity across the joint is required
When AB methods are employed for bonding of monolithic aluminium alloys, phosphoric acid
anodising (PAA) or chromic acid anodising (CAA) is normally used as a pretreatment to im-
prove the durability of the joint. It has been found, however, that anodising does not work in the
case of particle reinforced material [2]. The reason for this is thought to be that the presence of
non-conducting asperities at the surface disturbs the anodising process which leads to an unsuit-
able oxide structure. A better pretreatment is etching or, at a lower level, grit blasting [10].
The use of silane as bonding promoter did not increase the joint strength, either the short term
strength or the durability [10].

5.2       Joining methods for other MMC alloys
5.2.1     Magnesium alloys
For the Mg alloy ZC71 reinforced with 10% SiC rotary friction welding has been used. At the
bond interface break-up of SiC particles was noted. The accompanying loss of strength could be
recovered by postweld heat treatment [2].

5.2.2     Titanium alloys
With Ti-6Al-4V 40% SiC(fibres), lap joints have been produced using diffusion bonding and an
interlayer of the unreinforced alloy. A linearly increasing bond strength as a function of time was
observed. With a bonding time of 3 hrs, pressure 10 MPa and temperature 900°C, a maximum
bonding strength of 700 MPa was achieved [11].
Capacitor discharge spot welding has been successfully employed for Ti-6Al-4V
35% SiC(fibres) MMC. Fibre degradation as well as fibre displacement could be avoided by using
optimized welding parameters [12].
With some kinds of fibre reinforced MMCs, e.g. Ti/SiC, there is a risk that long high-temperature
exposure may cause degradation of the fibres, e.g. during diffusion bonding processes. One rem-
edy for this could be the covering of the fibres with a multilayer barrier coating of Y2O3/Y/ Y2O3

H. Persson                                                         MMC-Assess – Thematic Network

6       Discussion

6.1     Joint integrity, restoration of mechanical properties
The joining process can affect the properties of the joint in different ways. If the supplied heat has
caused the reinforcing particles, short fibres or continuous fibres to react chemically, as men-
tioned in the paragraph about laser welding, the damage is irreversible. This is true also for seg-
regation effects during fusion welding, leading to the formation of an uneven distribution of the
reinforcing particles.
With friction welding processes, two mechanisms are possible. For one thing, the reinforcing
particles or short fibres may be disrupted or aligned in the joint region. Also this effect is irre-
versible, but its consequences for the mechanical properties are not known in the general case.
(Cf. paragraph 5.2.1.) Another effect, which has been reported with Al-based MMCs is that the
friction-generated heat makes the matrix material overage, thus creating weak zones on both
sides of the joint. This effect is demonstrated by t he hardness profile in figure 1, recorded across a
friction-welded joint in an Al/SiC MMC [14].
If the matrix alloy belongs to one of the heat treatable types - 2XXX, 6XXX, 7XXX and 8XXX - the
original strength of the MMC may be virtually recovered by re-solutionising and ageing, if such a
treatment is feasible for practical reasons [2]. That such a "post bonding heat treatment" (PBHT)
worked in the case mentioned above is also shown in figure 1 [14].

Figure 1       Hardness profile across friction weld in Al/SiC metal matrix composite [14]

6.2     Adaptability of different joining methods with regard to joint character-
The chart below (figure 2) shows a qualitative rating of a number of joining methods with r espect
to their suitability for different applications, as well as their adaptability for MMCs [1].

H. Persson                                               MMC-Assess – Thematic Network

Figure 2     Qualitative rating of joining methods [1]

H. Persson                                                    MMC-Assess – Thematic Network

7      References
    1. Composite Materials Handbook. Volume 4. Metal Matrix Composites. MIL-HDBK-17-4.
       21 Sept 1999
    2. M B D Ellis. Joining of metal matrix composites - a review. TWI Report 489/1994, August
    3. M B D Ellis, M F Gittos & P L Threadgill. Joining aluminium based metal matrix compos-
       ites. Materials World, August 1994, p 415 - 417
    4. J R Kennedy. Microstructural observations in arc-welded boron-aluminium composites.
       Welding Journal, 52, (3), p 120s - 124s, 1973
    5. H M Wang, Y L Chen & L G Yu. "In-situ weld-alloying/laser beam welding of SiCp/6061
       Al MMC. Mat Sci Eng A, 293, (1-2), p 1-6, 30 Nov 2000
    6. K C Meinert Jr, R P Martukanitz & R B Bhagat. Proceedings of the American Society for
       Composites, Seventh technical conference. Technomic Publishing Company Inc, Lancas-
       ter, PA, 1992, p 168-177
    7. T J Lienert, E D Branden & J C Lippold. Comparison of laser and electron beam welding
       of SiCp reinforced aluminium A-356 metal matrix composite. Scripta Met 28, p 1341 -
       1346, 1993
    8. J A Lee, R W Carter & J Ding. Friction Stir Welding for Aluminum Metal Matrix Compos-
       ites. NASA report No NAS 1.15:209876; NASA/TM-1999-209876, Dec 1999
    9. R W Smith, E Lugscheider, I Rass & F Hillen. Joining of aluminum matrix ceramic com-
       posites (Al-MMC's) with a low temperature metal joining alloy. Conf IBSC 2000: Int
       Brazing and Soldering Conference, Albuquerque, NM, USA,
       2 - 5 April, 2000. Advanced Brazing and Soldering Technologies, April 2000
    10. M R Bowditch & S J Shaw. Adhesive bonding for high performance materials. Advanced
        Performance Materials 3, p 325 - 342 (1996)
    11. A Hirose, M Kotoh, S Fukumoto & K F Kobayashi. Diffusion bonding of SiC
        fibre reinforced Ti-6Al-4V alloy. Met Sci and Tech. 8, (9), p 811-815, 1992
    12. A Cox, W A Baeslack III, S Zorko & C English. Capacitor discharge resistance spot weld-
        ing of SiC fibre-reinforced Ti-6Al-4V. Welding Research Supplement, 72, (10), p 479s -
        491s, Oct 1993
    13. R R Kieschke & T W Clyne. Development of a diffusion barrier for SiC monofilaments in
        titanium. Mat Sci Eng, A135, p 145 - 149, 1991
    14. O T Midling & Ö Grong. Proc 3 rd Int Conf on Trends in Welding Research,
        Gatlinburg, TN, USA, June 1992 (ed S A David & J M Vitek), ASM International, p 1147
        - 1151

CSM Materialteknik AB
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H. Persson                                                    MMC-Assess – Thematic Network

MMC-Assess Publications

Volume 1: of Terms specific to Metal Matrix Composites

Volume 2: Thermal Treatments of Age-hardenable Metal Matrix Composites

Volume 3: Metallographic Preparation of Metal Matrix Composites

Volume 4: X-Ray Computed Tomography on Metal Matrix Composites

Volume 5: Quality control and nondestructive tests in metal matrix composites

Volume 6: Machining guidelines of Al/SiC particulate MMC

Volume 7: Thermophysical Properties of Metal Matrix Composites

Volume 8: Guidelines for joining of metal matrix composites

Volume 9: Bomding and interface formation in Metal Matrix Composites

Copyright:   MMC-Assess Consortium, September 2001
Content:     H. Persson - CSM Materialteknik
Design:      P.Prader – Guillermo Requena
Contact:     Insitute of Materials Science and Testing – Vienna University of Technology


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