Thixomolding of Magnesium

Transcription

1 Thixomolding of Magnesium

2 Basic Features of Magnesium ASTM Mg alloy designation system - Alloys- Major alloying ingredient, aluminum Secondary alloying ingredient, zinc Fourth composition of this alloy registered with ASTM A-E, X1 A:aluminum E:rare earths M:manganese S:silicon Z:zinc H:thorium K:zirconium Q:silver W: Yttrium AZ91D Nominal percentage of major ingredient Nominal percentage of secondary ingredient

3 Basic Features of Magnesium - Alloys- Table Nominal compositions of typical magnesium alloys. Al Mn Zn Zr Other Fe Ni Cu AZ91D AM60B AM50A AS41B Si AE42F RE WE54A 0.7 WE43A Y 3.0RE 5.2Y 3.0RE AZ31B Compositions are in %, impurities are in ppm.

4 Basic Features of Magnesium Advantages of magnesium over plastic - Material property- Stronger (Tensile strength : 28 MPa) Stiffer (Young s modulus : 45 GPa) Higher heat resistance (No deformation at 100? ) EMI & RFI Shielding (No plating is required) Better heat transfer and electrical conductivity characteristics Less warp Better recyclability

5 Basic Features of Magnesium Viscosity - Material property- Viscosity (P) Mg alloy (AZ91D) Nylon 6-Nylon PP GF30% + PP PE ABS Temp. ( o C) Figure Temperature dependence of viscosity of several plastics and AZ91D Mg alloy.

6 Principle of Thixomolding - Molding technology- Casting Low pressure casting Investiment casting Sand or plaster casting Permanent mold casting Mgnesium Molding High pressure casting Die casting Hot chamber DC Cold chamber DC Squeeze casting Extrusion Semi-solid forming Thixomolding Rheocasting Thixocasting Forging Press forming Forging Figure Different molding processes utilized for magnesium products.

7 Principle of Thixomolding Die casting - Molding technology- (a) Cold chamber machine (b) Hot chamber machine Figure Die cast machines for magnesium.

8 Principle of Thixomolding - Semi-solid forming- Solid phase Liquid phase Solid phase Liquid phase Stationary state (without shear force) Viscosity High Fluidity Low Low High Thixotropic state (with shear force) Figure Schematic illustration of evolution of structure during solidification with vigorous agitation.

9 Principle of Thixomolding - Semi-solid forming- L (liquid phase) Conventional casting Temperature Semi-solid state a a + L Semi-solid forming a + b Press forming L + b Figure Concept of semi-solid forming. A Composition B

10 Principle of Thixomolding - Semi-solid forming- Advantages of semi-solid forming Less casting shrinkage Fine grain structure Increased dimensional accuracy Higher material efficiency Improved mold filling - solid front -

11 Principle of Thixomolding - Thixomolding- Chipped raw materials Products Argon atmosphere Band heater Hopper Volumetric feeder Rotary drive Mold Screw Barrel Slurry temperature ( ? ) High speed shot system Figure Thixomolding machine for magnesium.

12 Principles of Thixomolding - Thixomolding- Figure Thixomolding machine (JLM220-MG).

13 Principle of Thixomolding - Thixomolding- Slurry Temp. Solid phase Figure 605? 595? 585? 575? 565? <1% 3% 20% 35% 55% 500µm Microstructure of thixomolded AZ91D at different slurry temperature.

14 Application of Thixomolded Products - Magnesium market - Mini-Disk player Others Camera Video Camera Mg alloys PC Figure Application for magnesium products.

15 Application of Thixomolded Products - Automobile applications - Figure Examples of magnesium products for automobile parts

16 Application of Thixomolded Products - Electronic applications - Figure Examples of magnesium products for electronic housings.

17 Thixomolding vs. Die casting Parameter Melt temp.(?) (AZ91D) - Machine - Table Machine specification. Die casting Thixomolding Hot chamber Cold chamber Inj. speed (m/s) Inj. press.( kgf/cm 2 ) Intensifi. press. (Holding pressure) No Yes Internal defect Few Medium Many Machine size <1600t <900t <4500t Application Hot runner Available Possible for small parts Impossible

18 Thixomolding vs. Die casting Table Product quality. - Product- Parameter Thixomolding Die casting Hot chamber Cold chamber Flow marks Few Few Medium Shrinkage Small Medium Medium Surface Excellent Medium Good (high-speed inj.) Shrinkage Dim. precision / 1000 Good / 1000 Medium 7 8 / 1000 Not good Distortion Small Medium Large Mech. property Good Medium Good Corr. resistance Good Medium Not good

19 Thixomolding vs. Die casting Corrosion rate (mg/cm 2 /day) Corrosion - 0 TM DC(1) DC(2) DC ( Lit.) Figure Weight loss corrosion rates of thixomolded (TM) and die cast(dc) AZ91D specimens.

20 Thixomolding vs. Die casting - Strength - Y.S. and T.S. (MPa) Die cast (T.S.) Die cast (Y.S.) Barrel temp.:898k Injection velocity:1.4m/s Die cas t(el.) Shot No Elongation (%) Figure Stability of mechanical properties (AM50A).

21 Thixomolding vs. Die casting -Microstructure- 50µm Figure Microstructure close to thixomolded and die cast surface.

22 Thixomolding vs. Die casting - Accuracy - Deviation from the mean value (mm) Thixomolded Diecast A B C D E F Position D E A C F B Figure Dimensional accuracy of die cast and thixomolded PC housings.

23 Requirements for Thixomolding - Product design - Draft: Zero Draft is possible, Normal draft.5 to 3 degrees. Pattern shrinkage: 4-4.5/1000 Ribs: Radii and Fillets: Uniform Cross Sections: Ejector Pin Marks: Die Steel Sections: Reduce Assembly: use ribs for strength and thin down the walls avoid sharp corners too large a fillet cause shrink avoid rapid change in sections..shrink and flow problems identify areas where pin marks are prohibited or allowed avoid designs that produce thin die steel sections..die life design several components into one thixomolding

24 Requirements for Thixomolding - Product design - Factors influencing the production of thin wall Mg housings Product design -thickness variation -slide & core -others Molding condition -melt temperature -mold temperature -injection speed -others Optimum thickness (0.6-1 mm) Surface treatment Production yield Tool design - gate design - overflow location - hot runner - others Materials - alloy - lubricant - others

25 Structure of Long Nozzle Sealing IH Heater Insulating ring Heater Advantages Improving material yield Reducing shot cycle time(10%) Making quality of molded parts high (For cosmetic parts) Reduce Runner Weight (30-50%) Plug Nozzle Structure of Long Nozzle

26 Comparison Cold Runner Hot Sprue Long Nozzle Sealing Pressure Touch Pressure Tough+ Thermal expanding seal Thermal expanding seal Seal face Seal face Seal face Tool cost Low(1.0) High(1.2) Low(1.0) Material Yield bad (1.0) Good (0.5~ 0.8) Good(0.5~ 0.8) Shot Cycle Long(1.0) Short(0.9) Short (0.9) Life time Fair(1.0) Short (0.8) Long (1.2) Operation Easy (1.0) Bad (0.8) Fair? (0.8) (1.0)

27 Shot Sample Cell phone parts: Key Pad Front Shot weight: 41g Shot weight: 30g Cell phone parts: LCD Shot weight: 115g Shot weight: 61g

28 Concluding Remarks Magnesium is steadily recognized as a structural material in automotive and electronic industries. Thixomolding process offers high quality magnesium products with the environmentally-friendly process. New process technology such as a hot runner system or a powder lubricant, is expected to bring higher yields with further cost down.