SUMMARY OF CAUSES1. Post homogenisation cooling

2. Press practices a. Billet preheating b. Extrusion speed c. Press quenching d. Handling (soft spots)3. Ageing practices

POOR MECHANICALPROPERTIES

IN 6000 SERIESEXTRUSIONS

1ProblemFailure to attain adequate or uniform mechanical properties such as ultimate tensile strength, yield strength, hardness and possibly ductility for a particular alloy.

IdentificationExtruded sections do not pass mechanical property tests and fail to reach known specified hardness/tensile properties, or properties show excessive variability.

1.POST HOMOGENISATION COOLINGCauseSlow post homogenisation cooling rates (rates are dependent on billet diameter) through the range 450C - 200C (depending on alloy type) promote the formation of coarse Mg2Si precipitates. These coarse precipitates may not redissolve during the extrusion process and consequently do not allow full properties to be reached after artificial ageing.

PreventionExtrude billet that has undergone rapid post homogenisation cooling. Controlled post homogenisation cooling practices are designed to provide the extruder with billet containing fine evenly distributed Mg2Si precipitates . These fine precipitates are readily dissolved if correctly preheated and extruded and will give optimum mechanical properties after ageing.

2. PRESS PRACTICES a. BILLET PREHEATING

CauseBoth the final billet temperature and billet preheat rate affect the characteristics of the Mg2Siprecipitates and thus the mechanical properties of the alloy. Figure 1 shows the effect (on mechanical properties) of induction heating (85C/min) and gas heating (6.5C/min) on a 202mm diameter billet of 6063 type alloy to final billet temperatures of 400C and 450C.

C O M A L C O E X T R U S I O N G U I D E S F O R 6 0 0 0 S E R I E S A L L O Y S 1

2C O M A L C O E X T R U S I O N G U I D E S F O R 6 0 0 0 S E R I E S A L L O Y S

Figure 1 highlights the effect ofpreheat rates on mechanicalproperties. Slow heating rates orholding times at around 400Ccan also result in the growth ofcoarse Mg2Si precipitates. Ascoarse Mg2Si precipitates usuallyfail to redissolve during the extrusionprocess they result in less than optimum mechanical properties. An example of coarse Mg2Si precipitates resulting from incorrect preheat practices is shown in Figure 2.

0

50

100

150

200

250

300

Preheat Rate ( C/min)

UTS (MPa)

400 C 450 C

T5 minimum

properties

o

oo

0

50

100

150

200

250

300

0

50

100

150

200

250

300

Preheat Rate

UTS (MPa)

400 C 450 C

T5 minimum

properties

oo

Figure 1 : Billet preheat rate vs strength. The experimental

conditions used to create this graph were designed to highlight

the difference between the heating methods.

BILLET

LOCATION OF SAMPLESTAKEN FOR MICROGRAPHS

Figure 2: Optical micrographs (500x) of Mg2Si precipitates in 6063. The accompanying schematic shows the origin of the samples.

Fine Mg2Si

Coarse Mg2Si

6063 rapid billet preheat 6063 slow billet preheat

3C O M A L C O E X T R U S I O N G U I D E S F O R 6 0 0 0 S E R I E S A L L O Y S

PreventionRapid preheat rates will avoid Mg2Si precipitate coarsening at temperatures around 400C. When using gas preheaters, avoid slow heating rates or holding times around 400C.

High billet temperatures may ensure that metal can be pushed more readily through a die to initiate extrusion but combined with high extrusion speed, this could result in surface finish defects. A compromise allowing for all factors is usually required .

Rapid preheat rates will avoid precipitate coarsening. However, it should be noted that maximum press productivity can then only be achieved if sufficient Mg2Si has been precipitated during the homogenisation cool-down process. This reduces the billet's break-out pressure and increases extrusion speeds.

2. PRESS PRACTICESb. EXTRUSION SPEED

Cause

The relationship betweenextrusion speed, billettemperature, mechanicalproperties, surface finish andpress capacity (availablepressure) can be summarisedwith a limit diagram, Figure 3.

At all points on the diagrammaximum press capacityis being utilised to obtainmaximum productivity.

As shown in Figure 3, lowmechanical properties can arisefrom a combination of the low extrusion speed A and billet temperature below 450C. This is due to the low extrusion speed and consequently the low exit temperature (below 500C). At these low temperatures, the Mg2Si precipitates do not dissolve and undissolved Mg2Si will not permit an optimum ageing response. Increasing the billet temperature to 500c will increase the exit temperature and properties will be achieved.

Prevention

The exit temperature required to dissolve Mg2Si is generally greater than 500C but this is strongly dependent on precipitate size. This can be easily achieved by raising the billet temperature. However, this may not achieve optimum breakout and speed characteristics.

Faster extrusion speeds increase the heat of deformation and also the exit temperatures. At extrusion speed B and billet temperature 450C an exit temperature exceeding 500C is obtained. Acceptable mechanical properties after ageing can therefore be readily achieved.

The limit diagram also shows that an increase in billet temperature at extrusion speed B may result in a poor surface finish. At the higher temperatures surface melting, die pick-up and tearing may occur.If the mechanical property requirements of the extrudate cannot be met by extruding at adequate speeds with good surface finish, then the alloy selection or property requirement should be reviewed.

POORSURFACEFINISH

INSUFFICENT PRESSURE

EXTRUSION SPEED

BILLET TEMPERATURE

LOW MECHANICAL PROPERTIES

450 C

A

B

500 C

OPERATINGWINDOW

oo

Figure 3 : Extrusion limit diagram.

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c. PRESS QUENCHINGCause

In order to achieve optimum response for ageing and maximum mechanical properties in 6000 series alloys, the majority of Mg and Si must be retained in solid solution after extrusion. Inadequate press quenching can allow premature precipitation of Mg2Si which reduces the response for hardening during subsequent ageing.

PreventionThe most rapid press quench possible must be employed to retain the Mg and Si in solution and prevent the formation of coarse Mg2Si precipitates during cooling after extrusion.

In practice, the quench rate required by the various 6000 series alloys is dependent on alloy type and extrusion shape, Figure 4.

Rapid quenching need only be maintained through the critical range 450C - 200C (depending on the alloy).

Minimum cooling rates and quench methods are given for some Comalco 6000 series alloys in Table 1.

EXTRUSION

TEMPERATURE

Log TIME

6060HE

6061

WATERQUENCH

MIST QUENCH

FANCOOL

FORMATION OF COARSE Mg Si

2

STILL AIRCOOL

Figure 4 : Time-Temperature-Transformation diagram (example only).

Table 1. Recommended Quench Rates and Methods for Various 6000 Series alloys

Alloys Minimum QuenchRate C/min

Solid Sections< 10mm Thick

Solid Sections> 10mm Thick

6060

6063

6061

6082/6351

50

60

300

300

Still Air or Fans Water Mist

Water Mist

Water Mist

Water Mist

Fans

Water Sprays

Water Sprays

Table 1. Recommended quench rates and methods for various 6000 series alloys

d. HANDLING (SOFT SPOTS)CauseSoft spots in extrusions are not usually obvious to the extruder. These soft spots are localised regions of the extrusion where hardness is significantly less than the bulk of the material. If the material is later anodised the soft spots show up as a variation in colour along the extruded length.

The source of these soft spots is the cooling conditions on the extrusion runout table. The carbon or graphite blocks that support the extrusion have a high thermal conductivity. Portions of the extrusion in contact with these blocks may be cooled rapidly if the extrusion remains stationary. The surrounding hot regions re-heat these cooled regions causing premature precipitation which has a detrimental effect on the subsequent age hardening process.

PreventionThe soft spots may be prevented by minimising contact time of extrusions with the blocks on the runout table by moving the extrusions quickly and using fan cooling in place of a still air quench.

3. AGEING PRACTICESCause Under or over ageing may be performed deliberately to obtain specific properties. This section does not cover these conditions, rather, it applies to unintentional loss of mechanical properties during ageing.

i Under ageing Under ageing is the result of using ageing times that are too short, temperatures that are too low, or both. The net result is a reduction in the mechanical properties of the alloy.

Figure 5 shows ageing curves for the alloy 6061 at temperatures of170C, 185C and 200C. As an example under ageing would occur if the alloy was aged for 2 hours. Similarly, the potential response in strength would be reduced if the alloy was aged at 185C or 200C, rather than 170C.

Mg2Si precipitates are still nucleated at low temperatures and short times but they do not reach an optimum size or number that will result in maximum mechanical properties being achieved.

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2 50

3 00

3 50

ARTIFICIAL AGEING TIME (HOURS)

ULTI

MAT

E TE

NSIL

E ST

RENG

TH (M

pa) 2 00 C

1 85 C

1 70 C

O

O

O

Figure 5 : Ageing curve showing tensile strength responseat various ageing times for 6061.

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ii Over ageing Extended ageing times or ageing at relatively high temperatures result in an over-aged microstructure and reduced mechanical properties. As shown in Figure 5 ageing longer than 4 hours at 200C or 7 hours at 185C will result in a reduction in mechanical properties. At 170C the sample hardness is more stable but it will be reduced at some time after 10 hours. Over-ageing results in a smaller number of larger Mg2Si precipitates that have grown past the optimum size for maximum contribution to mechanical properties.

iii Uniform Properties For uniform mechanical properties throughout an extrusion, an even distribution of fine Mg2Si precipitates is required. Rapid heat-up rates to the ageing temperature may not allow time for the nucleation of evenly distributed Mg2Si precipitates which results in non-uniform mechanical properties. Non-uniform temperatures within a furnace will cause a variation in mechanical properties.

PreventionIn the case of Comalco 6000 series alloys the optimum artificial ageing conditions are 6 hours at 185C or 8 hours at 170C.

Although correct ageing temperatures are used, it is still possible for some variation in mechanical properties to occur due to fluctuations in ageing furnace temperatures. Temperature differences of 10C or more between various sections of the furnace can result in some differences in mechanical properties of extrusions within an ageing batch and should be avoided.

Heat-up times exceeding 30 minutes should be used to allow time for all parts of the batch to reach a uniform temperature during the ageing cycle. This condition will provide more uniform mechanical properties.

"Important Disclaimer"

This brochure has not been prepared with any particular reader in mind and therefore, although we believe that the advice and information herein is accurate and reliable, no warranty of accuracy, reliability or completeness is given and (except insofar as liability under any statute cannot be excluded) no responsibility arising in any other way for errors or omissions or in negligence is accepted by the company or any director, employee or agent of the company.

C O M A L C O E X T R U S I O N G U I D E S F O R 6 0 0 0 S E R I E S A L L O Y S 6

As an aid to further understanding of the information in this brochure it is recommended that the reader refer to the Comalco brochure entitled "THE BASIC METALLURGY OF 6000 SERIES EXTRUSION ALLOYS".