v6 Machining

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Volume6

H .P ers s o n


2/13

H.Persson MMC-Assess Thematic Network

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H.Persson, CSM Materialteknik AB

([email protected])

1 Contents

1 Contents...............................................................................................................1

2 Summary .............................................................................................................2

3 Introd uction ........................................................................................................2

4 Machin ing character istics .................................................................................3

5 Tool characteristics.............................................................................................3

5.1 Polycrystalline Diamond...........................................................................3

5.2 Diamond gr inding .....................................................................................46 Turn ing and Milling ..........................................................................................4

7 Drilling and Tap ping.........................................................................................5

7.1 Drilling.........................................................................................................5

7.2 Tapping........................................................................................................5

8 Grinding and Honing ........................................................................................5

8.1 Grinding ......................................................................................................5

8.2 Honing .........................................................................................................6

9 Sawing .................................................................................................................6

10 non -convention al machining........................................................................7

10.1 Electro Discharge Machin ing (EDM) ......................................................7

10.2 Laser mach ining .........................................................................................8

10.3 Abrasive Water Jet (AWJ).........................................................................8

11 Ind ustry case stu dy........................................................................................8

11.1 Machining fighter air screw pump MMC housing ...............................8

11.2 PCD Circular saw ing of MMC billets .....................................................9

11.3 Diamond grind ing of power semicond uctor heatsink plates ..............9

11.4 Machining an MMC motorcycle racing brake disc...............................9

12 Work piece consid erations............................................................................9

12.1 Heat Treatment.........................................................................................10

13 Economics......................................................................................................10

14 References......................................................................................................11 Volum

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2 Summary

Despite the sup erior m echan ical and therm al pr operties of p articulate metal-matrix composites,their poor m achinability has been the main d eterrent to their substitu tion for metal parts. The

hard abrasivereinforcement ph ase causes rapid tool wear d uring m achining and , consequently,

high machining costs.Within the fram ewor k of MMC Assess, BET2-621, top ic 2, a review of the

available literature on Particulate Metal Matrix Composites (PMMC) have been performed,

with th e objective to gather generalm achining gu idelines for Alum inum based PMMCs.Even if

these guid elines can be considered as general app licable, it shou ld be n oted th at, for a giventool

and cutting condition, the wear rate will depend on the features of composites. From the

review, it is clear that the morphology, distribution and volume fraction of the

reinforcementphase, as well as the matrix properties, are all factors that affect the overall

cutting process, but as yetrelatively few works related to the op timization of the p rodu ctivity

process have been pu blished.Despite the controversy in explaining th e mechanism behind the tool wear at d ifferent feed

rates, allreviewed literature recommends using feed rates and depths of cut that are as

aggressive as possibledu ring the r ough ing op erationsSeveral researchers hav e also indicated

that p olycrystalline d iamond (PCD) tools are the on ly toolmaterial that is capable of pr oviding a

usefu l tool life dur ing the machining of SiC/Al PMMCS. PCD ishard er than Al 2 O 3 , and SiC

and does not have a chemical tend ency to react w ith the w ork p iece material.Furthermore, PCD

tools with a grain size of 25 mm better withstand abrasion we ar by m icro-cutting than tools

with a grain size of 10 m.Non-conventional m achining pr ocesses, such as Electro Discharge

Machining (EDM), laser cutting and Abrasive Water Jet (AWJ), have also been reviewed and

found to be very suitable for roug h cut ap plication.

3 Introduction

Metal-matrix composites (MMCs) form one group of the new-engineered materials that have

receivedconsid erable research since the trials by Toyota in the early 1980s [1]. The most p opu lar

reinforcements are silicon carbide and alum ina. Aluminum , titanium and magnesium alloys are

commonly used as the matrix phase. The density of most MMCs is app roximately one third

that of steel, resulting in high specific strength and stiffness [2]. Due to these potentially

attractive p roperties coup led with the ability to operate at high temp eratures, MMCs comp ete

with super-alloys, ceramics, plastics and re-designed steel parts in several aerospace andautom otive applications. The latter materials, how ever, may not have m uch fur ther capacity for

the inevitable futu re increases in serv ice loads [3].

Particulate Metal-Matrix Composites (PMMCs) are of particular interest, since they exhibit

higher d uctility and lower an isotropy than fiber reinforced MMCs [2]. Moreover, PMMCs offer

sup erior w ear r esistance [3]. While man y engineering comp onents, made from PMMCs, are

produced by the near net shape forming and casting processes, they frequently require

machining to achieve the desired d imensions and surface finish. The m achining of PMMCs

pr esents a significant challenge, since a nu mber of reinforcement materials are significantly

hard er than the commonly u sed h igh-speed steel (HSS) and carbide tools [4]. The reinforcement

phase causes rapid abrasive tool wear and therefore the widespread usage of PMMCs is

significantly imped ed by th eir poor machinability and high m achining costs.

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4 Machining characterist ics

Although often categorized as difficult to machine, MMCs are actually readily machinable.

They form short cutting chips, cutting forces are moderate and the range of machining

param eters at which they can be machined is quite wide.

However, MMCs are highly abrasive and tools can wear rapidly. Providing machiningparameter

recomm endations are adh ered to, particularly with regard to surface speed w here an absolute

maximum of 1000 m min -1 [5,6,7] shou ld be observed , the most sign ificant effect on tool life

comes from selecting the most su itable of polycrystalline diam ond grad e (PCD).

The most common ly used way of differentiating betw een d ifferent grad es of PCD is by using

different micrometer sized d iamond particle sizes to form the PCD layer.

The particle sizes typically fall within the range 2-25 mm. As the grain size increases, the

concentration of diam ond by volum e increases. This results in an increase in resistance to

abrasive wear . This observation h as been rep orted in the literature [13]. Und er less abrasive

machining conditions the selection of an ap prop riate PCD grade is d etermined by consideringfactors of tool fabrication, tool design, wor k p iece material and machining op eration.

5 Tool characterist ics

5.1 Polycrystalline Diam ond

Several researchers [7] have ind icated that polycrystalline d iamond (PCD) tools are th e only

tool material that is capable of p roviding a u seful tool life d uring the m achining of SiC/Al

PMMCS. PCD is harder than Al 2 O 3 , and SiC and d oes not have a chemical tend ency to react

with the work p iece material. PCD cut ting tool consists of a sintered layer of micrometer sized

diam ond particles bond ed to a tu ngsten carbide su bstrate. Polycrystalline d iamond is typically

prod uced as a flat roun d d isc, from w hich any shap e or size of piece can be cut an d brazed on

to a cutting tool body. The PCD is then p rocessed by grind ing, to produ ce the required cutting

edge geometry. Polycrystalline diamond tools, for turning, boring, milling, drilling, reaming,

tapp ing and sawing, are all possible to manu facture, and are widely available. A number of

different grad es of PCD are available, and are classified in term s of the average m icrometerdiamond particle size within the PCD layer. In most cases, selecting the ap prop riate grade of

PCD is a balanced d ecision between the life of the tool, the quality of machined su rface requ ired

and , sometimes, considerations of the tool man ufacturing p rocess. PCD tools with a grain size

of 25 mm better w ithstand abrasion w ear by micro cutting than tools w ith a grain size of 10 mm

[6,7]. Fur ther in creases in PCD grain size d o not ben efit the tool life, bu t rath er cause sign ificant

deterioration in th e su rface finish. This is because PCD grains w ith size > 25 m are easily

pu lled ou t of the cutting edge. Other forms of diamond cutting tool materials exist, namely in

the forms of polycrystalline chemically vapour deposited (PCVD) diamond and

monocrystalline d iamond . Polycrystalline chemically v apou r d eposited diamond is available as

a coated tool, usually applied to a tungsten carbide substrate, or as a thick film tool. Tools

coated with PCVD diamond usually have a thickness of < 50mm, which is worn throughrelatively quickly when m achining an MMC. Tools with thick film PCVD diam ond are usu ally

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of the order of 0,5 mm thick diam ond brazed to a tu ngsten carbide substrate and perform w ell

wh en m achining MMCs. However, wh ile these materials app ear to have considerable potential

for machining MMCs, they are still very mu ch at the developing stage.

5.2 Diamond grindin g

Diamond grinding is a very well established material removal process and is widely used in th e

manufacture of a vast range of engineering components. As a general rule, work piece

materials, which are currently ground with diamond, are non-ferrous, hard and/or highly

abrasive. Glass and tu ngsten carbide are two examp les in the engineering indu stry. Grind ing

operations are carried out w ith bonded tools, in which d iamond particles are sup ported in a

bond matrix (resin, vitrified and metal are the most comm only used bond s), wh ich essentially

acts as a carrier. As the bond erodes, new particles come to the su rface to continue the gr inding

pr ocess. A p articular typ e of grinding tool performan ce can be achieved by selecting an

app ropr iate diamond grit type (typically classified by shap e, crystallinity, and stren gth) and

grit size. The concentration of the diamon d gr it and a su itable bond type are other p arameters

employed to v ary the cutting characteristics of the wheel and to p rod uce a particular surfacefinish, material removal rate, and wheel life.

6 Turning and Milling

The natu re of machining h ighly abrasive MMCs means, in reality, that tool life considerations

are of overrid ing imp ortance. Coarse grained PCD (25 mm ) is significantly more w ear resistant

than finer grain size grades of PCD when machining MMCs. A summary of recommended

machining condition is given in Table 1. Despite the controversy in explaining the m echan ism

behind the tool wear at d ifferent feed rates, all reviewed literature recommend s using feed rates

and dep ths of cut th at are as ag gressive as possible du ring th e roug hing op erations [5,6,7,8,17].

Table 1: Recommend ed p arameters for machining

Cutting sp eed, m

min -1

Feed rate,

mm rev -1

Depth of cut,

mmCoolant

Turning 300-900 0,2-0,6 < 5,0 Yes/No

Milling 200-1000 0,1-0,4 < 3,0 Yes/No

DURALCAN [6] recomm ends r oughing at th e maximum practical feed rate and dep th of cut

(DOC), followed by a single finishing pass at a lower feed rate and DOC. For general-purposeroug hing, where su rface finish is n ot critical, start at 500 m/min, 0.4 mm /rev, and a 1.5 mm

DOC. Flood coolant shou ld be ap plied on ly if chip clearing or built-up edge (BUE) is a problem.

Tool wear is more rap id d uring finishing, owing to the lower feed rate and DOC. The actual

settings w ill depen d up on th e critical radii of certain features an d on th e required surface finish.

Typical param eters are 600 m/min, 0.13 mm/rev, and a 0.5 mm DOC.

The DOC shou ld never exceed half of the insert's leg length , and the feed should not exceed h alf

of the nose rad ius. These precautions w ill redu ce the likelihood of fracture of the PCD cutting

edge and of poor surface finish on the work piece. (Note that th e rake and clearance angles for

these inserts will generally be the sam e as for an u nreinforced alu minu m alloy.)

Other results ind icates [7,8], that th e cost of PCD tools could be justified by u sing d ry cutting at

feed rates as high as 0.45 mm rev - 1, cutting sp eeds of 894 m m in -1 and a d epth of cut of 1.5mm. With these cutting parameters, the relatively small built-up edge formed on the tool

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protects it from further w ear by abrasion an d micro cutting. Polycrystalline tools with zero rake

angle and large tool nose radii are recomm ended for the roughing op erations

7 Drilling and Tapping

7.1 Drilling

HSS twist drills are not recommen ded to u se to dr ill composites. Stand ard-twist WC-tipped or

solid drills are suitable for p rototyping. How ever, PCD-tipped dr ills are recomm ended to be

used in pr odu ction. Because of the abrasive nature of the MMC, point d well by the d rill mu st be

prevented . Adequ ate feed is essential to d rill performance. For d rills 3-15 mm in d iameter, a

good gu ideline is to program a feed ofd 1.5 /75 (in m m/rev) [6], where d is the drill diameter in

millimeters. Evacuation of the abrasive chips is another, more complicated problem. Flute

design, point d esign, and th e use of coolant are all impor tant. Coolant-fed dr ills offer the most

effective chip clearing from deep holes, but flood cooling w ith 5% water-soluble oil is usua lly

adequ ate for shallow holes. Point grinds w ith comp oun d an gles and extra relief can break chipsinto smaller, easier-to-clear curls. Finally, never drill more than three drill diameters deep

withou t retracting th e d rill to allow chip clearing.

7.2 Tapping

Owing to the physical constraints of the process, tapping is the most difficult machining

operation with composites. The most cost-effective process for prod ucing thread s is, accord ing

to Dur alcan [6], form tapp ing. Stand ard HSS form tap s can prov ide over 75 cm of thread ed

holes (ISO-6H class of fit) at one-third th e cost per h ole of carbide cutting taps. Althou gh HSS

wor ks well for form taps, HSS cutting taps sh ould never t eeused ; their high wear rate results in

un pred ictable tap breakage w ithin the first five holes.

Because ad ditional lubricity is needed, the lubricant ratio should be increased from the usu al

1:20 to 1:4 for tap ping. On a d edicated tapp ing machine, special tapping oils should be u sed

rather than the w ater-soluble cutting fluid, used in general-pu rpose CNC machines. Although

through -the-tool lubricant is preferred, flood lubricant w ill also work if applied correctly.

For cutting tap s, a fairly low-cobalt microgram WC, yields the best comp romise between w ear

resistance and rupture strength. A tap with six straight flutes and a plug chamfer offers

sup erior performan ce, because of its decreased land area, to the more usu al three- and four -

flute d esigns.

Frequent tap and thread inspection is strongly encouraged. Since tap wear occurs on the

leading teeth , thread form in the bottom of the hole will usu ally fail first. In thr ough holes, form

taps can comp ensate for this with an ad ditional revolution or two. But for blind holes or cutting

taps, thread wear on the chamfer teeth represents an absolute limit on tool life.

8 Grinding and Honing

8.1 Grinding

Diamond grinding is norm ally used wh en machining hard , abrasive materials. Metal matrix

composites with a combination of soft and h ard p hases, are unu sual materials to grind. When

machining low volume fraction MMCs, electroplated diamond grinding wheels or hones

should preferably be used. These tools have a single layer of diamond particles held in a m etal

layer, usually nickel. Lower volume fraction MMCs do not p ossess the necessary hard ness or

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the abrasive quality for the bond of the wheel to be eroded away and do not allow an

impregnated tool to be used efficiently. Higher volume fraction MMCs (i.e. >50%) are more

abr asive and cause the bond to be eroded efficiently and the diamon d p articles to work fully.

Any attempt to optimize tool performance has to be based on the individual machining

operation, machining conditions, and the critical performan ce criteria, i.e. optimu m su rface

finish or extend ed tool life. However, the following recomm end ations app ly [5]:

The coarsest grit size should preferably be used that gives the r equired surface finish, Diamon d

grit, wh ich is more friable and wears by m icro chipping, is likely to p erform mu ch better than a

stronger and more crystalline product when grinding MMCs with a bonded tool. Precise

recommendations as to grit type are difficult because of the wide range of machines and

conditions available.

8.2 Honing

Metal-bond ed diam ond stones are essential for honing of PMMCs [6]. Althou gh grit size varies

with finish requ irements, grit concentration should not exceed C75, to pr event the stones fromloading. Stones should not exceed 4 mm in width (unless there are holes, splines, or other

interrup tions), and the su m of their width s shou ld n ot exceed 20% of the bore circum ference. If

guide sh oes are required, specify a metal bonded diamon d (with a grit size equal to or finer

than that of the cutting stones) or carbide shoe for the best wear resistance and accur acy. The

bore should be flooded with a high lubricity ("fatty") oil, containing extreme-pressure (EP)

add itives. (Note th at EP add itives such as chlorine and sulph ur should be in the "inactive" form

to prevent staining of the work p iece.) Filtering the honing oil below 5 m is recommend ed to

prevent micro scratching of the w ork p iece by re-circulated fines. Typically, a part (su ch as a

cylinder liner) should first be bored with a d iamond -cutting tool to within 0.05 mm of finished

dimen sions. Rough h oning w ith a 150-grit ston e for 1-2 min removes 90% of the remainin g

stock. Finish hon ing w ith a 600-grit stone for 30-60 sec rem oves the final few micrometer s ofstock and plateaus the su rface. (Excessive pressure shou ld be avoided to prevent stone loading.

Stroking speed and rotation can be calculated from the d esired crosshatch angle, bore diameter,

and length.) If additional cond itioning is requ ired, a bru sh-type hon e may be useful.

9 Sawing

For heavy sections of greater than 20 cm 2 , its recomm ended to use a heavy-du ty horizontal

band saw with flood cooling (5% water-soluble oil) [6]. A WC-tipped blade with greater or

equal to 1 tooth/cm running at 60-80 m/min with a moderate cutting pressure and feed can

make several squ are meters of cu ts at less than $0.01/cm2 (dep end ing on the saw capacity and

work piece size). Use the maximum blade thickness and wid th that the saw is capable of, to

redu ce blade deflection an d improve cut quality. A rotating in-line wire brush combined with a

triple-chip or raker-set blade offers the best chip-clearing performance and reduces tooth

damage from recut chips. DURALCAN billets [6] over 30 cm in diameter are routinely cut in

this manner. New band -saw blad es mu st be seasoned by m aking four or five cuts each at 50%

and 75% of the norm al operating speed. The pressure and feed sh ould be kept light (20-30% of

the maximu m v alue similar to that used for low-alloy steels) while the blade is still sharp . As

the blade w ears, the pressur e and feed shou ld be increased to 40-60% of the maximu m value

(similar to that used for high-speed tool steels). When the blade nears the end of its life,

pressure and feed may approach values normally used for stainless steel and nickel based

alloys. If a blade loses cu tting efficiency before its normal lifetime, try the following techn iques

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in the order listed: (1) increase the feed settin g by 10% of the full range; (2) increase the pressure

by 10% of the full range; (3) increase or d ecrease the cutting speed by 10% of the cur ren t value.

If the blade is still not cutting p roperly after repeating th ese steps a second time, retract it until

the teeth are barely in the kerf. Stop th e saw an d carefully rotate the work p iece 30-60 degrees

in the d irection opposite the blade motion w hile maintaining prop er blade alignment. Return to

the original cutting p arameters and restart the saw. Cold sawing at 5 cm2. For small degating jobs, V-grade

resinoid-bond cutoff wheels of 24-grit Al 2 O 3 (e.g., A24-V-B5) offer an acceptable G-ratio an d

MRR.

Four basic sawing gu idelines shou ld be observed [6]:

1. For heavy sections, use a WC-tipped blade with low speed, moderate pressure and

feed , and flood coolant.

2. Maximize tooth spacing and gu llet size wh ile keeping at least three teeth in th e kerf.

3. Season a new blade w ith a few cuts at 50% and 75% of the p lanned cutting speed.

4. For light sections, use a d iamond -grit-edged blade at high speed , moderate pressur e

and feed, and flood coolant.

10 non-conventional machining

10.1 Electro Discharge Machin ing (EDM)

The reviewed resu lts [14,15] ind icate that Al/SiC PMMC's can be m achined using EDM, despite

the low electrical cond uctivity and the h igh therm al resistance of the SiC p articles.

The EDM process is how ever slow an d the m aterial removal rate d oes not exceed the value of

V=35 cm 3 /m in un der th e conditions used. The material removal rate increases w ith increasing

discharge curr ent and increased pu lse du ration up to an optimal value and th ereafter decreases.

It has also been sh own that th e material removal rate decreases with increased SiC ceramic

content s. This can be explained by a nu mber of factors. Firstly, the electrical cond uctivity of the

aluminu m m atrix decreases du e to the p resence of the ceramic reinforcement. Furtherm ore,because of th e low therm al cond uctivity, and th e mu ch higher therm al resistance of the SiC, the

aluminu m alloy between the ceramic particles is p referentially rem oved.

It was observed that th e SiC-particles were n ot melted d uring the m achining process since their

full size and sh arp corners were still visible in the m achining debris as w ell as in the recast

layer. This appears to su ggest that the rem oval of the composite material occurs throu gh the

process of melting and v aporizing the matrix material around the ceramic p article and at som e

point the entire SiC-particle becomes detached . This shielding " effect of the SiC ceramic is

followed by a d ecreased rem oval rate w ith increased SiC. The machined su rface of a material

generated u sing EDM is composed of many m icroscopic craters associated with th e rand om

spark d ischarge between the electrodes.

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10.2 Laser machining

To date MMC has been u sed in th e prod uction of relatively thick comp onents bu t more r ecently

attemp ts have been mad e to use this material in thin sheet form.

Conven tional cuttin g u sing d iamon d tools is costly and /or techn ically difficu lt. Since Wire-EDM

is slow, laser machining can offer significant productivity advantages for rough cut-offapp lications. It is app arent th at a laser is very su itable for h igh feed r ates [14] (up t o V=3000

mm /min) and can p rodu ce a cut w ith a narrow kerf width (W k d0.4 mm ). Reinforcing the

aluminu m m atrix with SiC ceramic particles improv es the m achinability of the Machining

guidelines of Al/SiC particulate MMC composite, du e to the redu ction in the optical reflectivety

of the ma terial. How ever qu ality of the laser cut su rface is relatively poor. Striation p atterns on

the cut surface and burrs at the exit of the laser (dross attachment) were observed [14].

Significant therm al indu ced microstructural changes w ere also observed within th e PRMMC.

10.3 Abrasive Water Jet (AWJ)

Machining with an Abrasive Water Jet (AWJ) has many advantages compared to othermachining

technologies. In comp arison to therm al machining p rocesses (laser, EDM) AWJ d oes not induce

high temp eratures and as a consequence there is no therm ally affected zon e. Furth ermore since

high feed r ates are p ossible [14] (feed rates u p to v=450 mm/m in), AWJ can be consider ed to be

a very efficient machining process. The surface quality of the AWJ cut is characterized by a

rough surface, wh ich is one of the dr awbacks of this machining p rocess. A smoother su rface can

be obtained w ith lower feed rates. The formation of striation is presu med to be as a result of a

cutting lag and step rem oval [16] and is generally present w hen m achining thicker material.

The absence of striations also ind icates that the rem oval pr ocess occur s mainly by cutting the

material and d eformation w ear is negligible. The ductile shearing of the aluminu m is evident

from the abrasive scooping and plough ing path (micro-cutting) of the garnet.One of the m ajor ad vantages of the AWJ process is the absence of any therm al dam age to the

surface. Furth ermore, no bur r attachment w as observed [14]. The top edge of the cut s ur face is

however characterized by a rad ius as well as slotted ed ge dam age.

The kerf is characterized by a larger width (average kerf wid th arou nd Wk=1.5 mm) than that

obtained w hen u sing the laser process.

11 Indust ry case study

11.1 Machin ing fighter air screw pu mp MM C housingScrew type p um ps op erate at high speed [9] and th erefore require a closely tolerance hou sing to

ensure correct and even fluid flow across the outside of the screw an d the inner w all of the

housing. Where the w all thickness of the hou sing is only 2,5 mm th e diameter an d length are 65

mm and 127 mm respectively, and the material in question is a 20% SiCp (particulate)

aluminu m, then machining can cause problems.

If diam eter tolerances of + 6 m and roun dn ess of 25 m are to be achieved, and expensive jig

boring tooling avoided, then machining w ithout d istortion is essential. As well as being highly

wear resistant PCD tools prod uce lower cutting forces than tu ngsten carbide. This is a m ajor

consideration w hen m achining th in walled or intricate components m ade from d uctile, light

materials like MMCs. Other than the tap ping of 2 x 6 mm dia. holes, all machining operations

are carried ou t with PCD.

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11.2 PCD Circular sawing of MM C billets

A manufacturer of 12%SiCp magnesium MMC had the task of sectioning 150 mm dia.

continuou s cast billet into man ageable sized p ieces [10]. The individu al billets are thenextruded into thin w alled r oun d tube sections. These tube sections are clearly very light and

very tough and are currently being prototyp ed as suitable frame material for top of the range

moun tain bikes.

The billets were imp ossible to cut with a tungsten carbide tipped ban d saw . After cutting

throug h < 100 mm of the billet all 1000 teeth on the 26 ft long blade were comp letely worn . An

electroplated d iamond saw blad e proved capable of cutting comp letely through the billet, but

the process was slow, the sawn faces were bowed and a special lubricant had to be used. Finally

a 550 mm d ia. saw blade, tipped with 48 PCD teeth, was evaluated, with imm ediate su ccess.

The PCD tool, requiring n o lubricant, cut throu gh th e billet in one p ass. At a cut loading of 0,4

mm per tooth, the cut was completed in less than 10 s with an acceptable sawn surface

pr odu ced on the cut faces of the billet. The first batch of 40 billets were processed w ith noproblems and no sign of wear or chipping on th e teeth of the blade.

11.3 Diam ond grind ing of pow er semicondu ctor heatsink plates

An MMC has proved to be a highly suitable material to use for mounting large power

semicond uctors [ 11]. An MMC p rovides the best combination of high therm al cond uctivity and

low th ermal expansion r equired in this ap plication. The 70%SiC p /Al plates are mad e by

preforming the ceramic and then p ressure infiltrating the alum inium m atrix. Plates are face

milled w ith PCD to remove between 1-2 mm of material and br ing the p lates rapidly close to

thickness tolerances. The final sizing of the plate and prod uction of the required surface finish

is achieved by diamon d grind ing. A metal bonded wh eel containing D151 sized diamon d at a50 concentration (on a scale w here 100 concentration is 4x4 carats cm -3 of bond ) has proved

most effective in pr odu cing th e ultra sm ooth su rface finish that is necessary for dow nstream

plating p rocesses, and at the same time with acceptable tooling costs and p rod uction rates.

11.4 Machinin g an MM C motorcycle racing brak e disc

Metal matrix composite brake discs manufactured [12] from a 20%SiC, are completely

machined w ith PCD tools. The disc is outside diameter turn ed and faced on a center lathe,

before being put on a vertical milling m achine. Here the location features on the inside

diam eter of the d isc are machined with a slot mill. A ball nosed cutter is used to scallop th e faceof the disc (weight saving). The fixing holes are then drilled and chamfered. The inside

diam eter of the disc is then also cham fered.

12 W ork piece considerat ions

Optimu m m achinability assum es a homogeneou s distribution of the reinforcing p articles in the

aluminium m atrix. But when particles are in random clump s or bands, they can damag e

cutting tool ed ges. Likewise, large part icles are more aggressive than small pa rticles. This factor

even ou tweighs th e extra silicon in the m atrix of the composites.

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12.1 Heat Treatm ent

Although some p arts requ ire stress relieving before machining, most par ts are machined in the

fully heat-treated cond ition to ensure d imensional accuracy. But this may not alway s be the best

practice. For examp le, machining material in a non heat-treated condition can redu ce tool wear

by m ore than 50%. For parts that cannot be qu enched after machining, one comp romise may beto solution heat treat, then m achine, and finally artificially age th e par t to full hard ness. In sm all

work pieces, however, heat generated by machining can accelerate aging kinetics so that

subsequ ent art ificial aging cau ses an over-aged condition, redu cing yield strength by up to 35%.

Thus, wh en strength is critical, always ma chine after aging, or m odify the aging step .

13 Economics

In mass prod uction, machining time far outw eighs cuttin g-tool cost. For conventional

aluminu m alloys an d cast iron, expendable cutting-tool costs are typ ically less than 5% of the

total ma chining cost. For the comp osites, tool cost can increase to 30%, but total machining cost

still comp ares favorably to that for machining cast iron, because diamon d tools permit equa l or

faster cutting speeds. Switching from carbide tools to diamond also changes machining

strategy. First, carbide inserts are usu ally ind exed wh en their w ear causes loss of dimen sion or

finish on the w ork p iece, and a re then discarded after the last index. This is also tru e of thin-film

CVDD inserts. However, worn PCD inserts can be resharpened once or twice, though

subsequ ent performance can vary dep ending on insert cond ition and r esharpen ing technique.

Second , because of the higher initial cost of diamond , it is more imp ortant to p rogram the tool

path to use both edges of each corner for additional savings. Finally, the output of

mu ltiplespindle m achines is generally limited by the slowest operation. Often, the faster speed s

used w ith diamond tools allow balancing these simultaneous op erations, further redu cing the

cycle time.

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14 References

[1] P. Rohatgi, Ad vances in cast MMCS, Ad v. Mater . Process. 2 (1990) 38-44.[2] J.M. Monaghan , The use of qu ick-stop test to stu dy the chip formation of an SiC/Al metal

ma trix comp osite and its ma trix alloy, Process. Adv .mater . 4 (1994) 170-179.

[3] B. Board man , Metal Matrix Comp osites-An Oppor tun ity for the Off-Highw ay Indu stry,

SAE International, OH, USA, 1990.

[4] S. Ramrattan , F. Sitkins, M. Nallakatala, Op timization of the casting and machining

pr ocesses for a met al-matr ix composite, Proc. Canad ian Sec. Mech. Eng. Symp.,

McMaster University, 1996, pp. 624-629.

[5] M.W. Cook, Machining MMC engineering components with polycrystalline diam ond

and d iamond g rind ing, Materials Science and Technology, vol.14-1998.

[6] Duralcan composites, Machining gu idelines, pr esented on Web page, 2001-02-20.

[7] M. El-Gallab, M. Sklad , Machining of Al/SiC particulate meta l ma trix comp osites, Toolper formance, Journal of Mater ials Processing Techn ology 83 (1998) 151-158.

[8] M. El-Gallab, M. Sklad, Machining of Al/SiC particulate metal matr ix comp osites,

Workp iece su rface integrity, Journ al of Materials Processing Technology 83 (1998) 277-

285.

[9] Metal Comp osite Technology, Airscrew How don, and Nixon Indu strial Diamonds: Case

study.

[10] Magnesium Elektron and Supreme Saws: Case stud y.

[11] AEA Technology and Asahi Diamond Indu strial UK: Case stud y.

[12] Metal Composite Technology and N ixons Ind ustrial Diamond s: Case stud y.

[13] C. LANE and M. LENNOX: 98th Casting Cong., Hamilton, ON, Canad a, May 1994,

American Found ry men s Society, 701-705.[14] F. Mller, J. Monaghan. Non-conventiona l machin ing of pa rticle reinforced metal matrix

comp osite, Intern ationa l Journal of Machine Tools & Manufactu re 40(2000) 1351-1366.

[15] N.P. Hung, L.J. Yang, K.W. Leong, Electro discharge machining of cast metal matr ix

comp osites , J. of Mat . Pro . Tech. 44 (1994) 229-236.

[16] A.M. Hoogstrate, C.A. Van Lut tervelt, Op portu nities in abrasive water-jet machining ,

Ann . CIRP 46 (2) (1997) 697-714.

[17] L. Iuliano, L. Settineri, A. Gatto. High-speed tu rning of MMC with coated and uncoated

carbide tools. Automotive Au tomation 1997

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H.Persson MMC-Assess

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MM C-Assess Pub lications

Volume 1: Glossary of Terms specific to Metal M atrix Composit es

Volume 2: Thermal Treatments of A ge-hardenable Metal M atrix Compos ites

Volume 3: Metallographic Preparation of Metal M atrix Composit es

Volume 4: X-R ay Computed Tomography on M etal Matrix Composit es

Volume 5: Qualit y control and nondestructiv e tests in metal ma trix composites

Volum e 6: Machining guidelines of Al/SiC parti culate MM C

Volume 7: Thermophy sical Properties of M etal M atrix Compos ites

Volume 8: Guidelines for joining of metal mat rix composit es

Volume 9: Bomding and interface format ion in M etal M atrix Composit es

Copyright: MM C-Assess Consortiu m, July 2001

Content: H.Persson

Design: P.Prader

Con tact: Insitu te of Materials Science and Testing Vienna University of Techn ology

mm c_assess@ewkm mc.tuw ien .ac.atHomepage: http://mmc-assess.tuwien.ac.at/

v6 Machining

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