Thermo mechanical processing

Transcription

1 Thermo mechanical processing Richard Dickson Hydro Metals USA Rosemont, Illinois October 25-26, 2016

2 Aluminum or Aluminium Aluminum was once considered a more precious metal than gold 1821 Bauxite (aluminum ore) discovered at Les Bauxs, France 1825 First (impure) refined Aluminum by Danish physicist 1880 Emperor of France has Al silverware for honored guests (rest had only gold) 1884 Largest Aluminum casting in world was capstone of Washington monument (100oz) 1888 Pittsburg reduction company starts first Hall-Heroult smelter later to be Alcoa Aluminum is like eggs they are good cooked on their own But add a few onions and a chorizo and you get a great omelet OR add some chocolate and patience then you have a soufflé What you add to them makes them quite different

3 Main alloys and alloying elements Al + Mn 3000 (Manganese) Good extrudability and formability Al + Mg 5000 (Magnesium) Marine corrosion resistant Low extrudability Al + Mg + Si 6000 (Magnesium + Silicon) Good combination of extrudability and mechanical properties Al + Zn 7000 (Zinc) High strength

4 What is special about 6XXX We want an alloy that is soft to extrude But makes a hard product 6XXX does this -These alloys are age hardening Soft to extrude and get strong on aging The metal requires a thermal treatment after extrusion to bring up the strength

5 Mg 5 Si 6 what makes 6XXX strong Effectiveness for hardening: Worst -big pieces of Mg 2 Si Best -very small well dispersed pieces This is what we do at ageing But we need all the Mg and Si in solid solution to be able to do this BIGbits are BAD, smallbits are GOOD

6 Life and times of Mg and Si Getting the alloy to come up to full strength requires process control The process needs to have the Mg and Si in the right conditions at each process step So we need to understand how Mg and Si behave in 6XXX alloys at different temperatures Remember general rule BIGbits are BAD, small bits are GOOD BUT what exactly is large and small?

7 Area of this picture is mm2 Mg2Si count in EI report was 191 in.51mm2 156x Alpha Iron Mg2Si 6 microns D longitudinal 1000x

8 ICAA12 Hydro/NTNU/SINTEF papers These are the hardening particles that make 6XXX strong Very small - 1/500 th of a millionth of a meter

9 Mg 5 Si 6 Precipitate after aging Mg 5 Si 6 Precipitate now believed to be the good precipitate This is what we want and lots of them Any Mg and Si left in solid solution is wasted Mg and Si left in big chunks of Mg 2 Si is also wasted So our objective is to have the right amount of Mg and Si Get it all to be small Mg 5 Si 6

10 What controls the Mg and Si 1300 ~1290 F Temperature ( F) ~1076 F 2-3 hrs Extrusion F T solvus Critical temperature range in the process route with respect to the precipitation of coarse Mg 2 Si phase particles 500 ~ 482 F F 5 hrs 100 Casting Homogenization Preheating Cooling Aging Time

11 Mg and Si in the cycle There are 3 main temperature ranges Above around 1000ºF Everything goes in to solid solution Between 480ºF and ~850ºF (depending on alloy) Mg 2 Si grows very fast in to big particles Around 300ºF to 400ºF Mg 5 Si 6 grows small very good particles in as short as 2 hours

12 Stability of billet heating need to check actual vs set point Billet preheat If Billet temperature is not stable each billet is an adventure Hotter billets use less pressure may tear on exit! Colder billets use more pressure shape going to be the same? Die change Unstable billet heating Up and down 35F

13 Billet heating equipment Gas heaters Lower energy and investment costs Lack of flexibility to production changes More difficult to apply a taper Induction heaters Shorter heating time Fixed taper More precise temperature ocntrol Sensitive to cycle variations Energy costs especially in USA Single billet Induction Large flexibility Less surface damage Requires more demanding control High investment costs Billet quench Overheating Flexibility with gas heater Extra cost and space requirements

14 Billet Heaters -Gas Gas heaters Some recommendations: Daily change or sharpen thermocouples!!!!! Weekly - Housekeeping. remove chips/swarf Weekly - billet carrier chain lubrication & check damage Weekly check clean billet exit. Weekly check/maintain bearings, billet transport, chain For temperature stability SHARPEN thermocouple probes

15 Thermocouple sharpening in - 15 F Temperature deviation ( o F) in - 40 F 0.25 in - 80 F Time (s) Old probes under read so billets hotter than you wanted

16 Time in the preheat oven Consider how long the billet is in the red zone growing Mg 2 Si What happens during a press stop or weekend shut down T B Induction Gas 3-5 mins 15-45mins Time

17 Billet taper During extrusion the billet heats up due to friction between container and billet during extrusion To ensure a near constant temperature of billet arriving at the die we apply a taper back of billet is colder Applying taper in Induction ovens is easy but more challenging in gas ovens

18 Billet taper and Exit temperature Exit temp T max Correct taper Too small taper Too big taper Time Taper is a temperature gradient along the billet length Adjust taper to give constant exit temperature Measure exit temperature after first ~30ft and at end

19 Billet taper Most tapers are made by adding more heat to the front of the billet Alternatively the back of the billet can be cooled or quenched Heat whole billet to higher temperature Can be used to over heat put all Mg and Si in solution One of aluminum s best properties in thermal conduction What happens if the billet is left sitting waiting to go to the press??

20 Loss of billet taper Taper disappears with time and front temperature decreases 0.5 mins 4 mins 9 mins Left in heater 0.5 mins 2 mins 5 mins Cooling in air Temp C A billet waiting in the heater for more than 10 minutes should have a higher preheating temperature prior to extrusion Reject billet waiting in air for more than 3-4 minutes

21 Measuring exit temperature Melt sticks give a minimum temperature reading Non contact optical pyrometers can give very accurate exit temperature measurements and on line These need occasional verification with a calibrated hand held probe same as billet ovens

22 Exit temperature is probably the most important parameter to control at the press

23 Exit temperature Need to put the Mg and Si in to solid solution 1300 Temperature ( F) T solvus ( F) Alloy T solvus T Exit F >1000 F F >1000 F 6005/ F >1000 F 6082/ F >1000 F 500 ~ 482 F 300 Preheating Extrusion Cooling Time (min) Hotter is better until the surface degrades

24 Cooling after extrusion Issues Obtain the required mechanical properties after aging Still keep shape tolerances and avoid distortion Parameters Alloy composition Section shape and acceptable distortions Extrusion speed Cooling media

25 What type of quench? EXTRUSION TEMPERATURE Too slow quenching: reduce mechanical properties Too fast quenching: possible distortion WATER QUENCH MIST QUENCH FAN COOL STILL AIR COOL Log TIME Courtesy of Rio Tinto Alcan

26 Wall thickness on cooling rate 1200 Temp.( F) Air cooling Open section in 0.08 in 0.12 in Time (s) Cooling time increases proportionally with wall thickness

27 Section geometry and cooling rate 1200 Temp.( F) Air cooling Wall thickness 0.12 in 600 Hollow section Open section Time (s) Hollow sections need more cooling than open sections

28 900 Verification of quench rates Travel along data loggers Temperature (deg F) Present Configuration 2nd Configuration Original Configuration 0 0:00:00 0:01:00 0:02:00 0:03:00 0:04:00 0:05:00 0:06:00 0:07:00 0:08:00 (hh:mm:ss) Ideal for minute to below 480 F

29 How many ways can we cool? Air cooling cooled with normal fans Air quenching often in connection with a quench box Conventional water spray just water sprayed through conventional nozzles Spray quenching special quench box, mixing air and water Standing wave a wave of water thrown from 2 sides Increasing cooling efficiency

30 Effect of quench cooling time on tensile strength is an European alloy similar to 6061 Tensile strength (ksi) Aging condition: 365 F - 5h Inefficient use of Mg and Si You extruded a harder alloy But got lower properties Due to poor thermal management / Cooling time (min) ( F)

31 Cooling efficiency Heat transfer coefficients (W/m 2.K) Standing wave Conventional water spray Air quenched Still air Spray quenching Normal fans

32 Air-water spray box Side water Top, Air-Water Side water Each side controlled individually 3-4 zones in extrusion direction Asymmetrical cooling possible Good water quality, control, and maintenance required Store and recall settings for each section Bottom, Air-Water

33 Cooling efficiency -Spray 10 Heat flux (MW/m 2 ) 1 Standing wave Air-water mix spray Water flow rate (kg/m 2 /s) Air cooling

34 Cooling for asymmetric sections Use of sprays to control run out 1.7 in 0.16 in 0.12 in 1.3 in 0.3 in Symmetrical quench box settings Asymmetrical quench box settings