Die Casting

PRESSURE DIE PRESSURE DIE CASTINGCASTING

INTRODUCTIONINTRODUCTION

Die Casting as alternative to forging, drawing & machining

Paper deals with requirements of die castingEmphasis on Die Steel Properties, Heat

Treatment & Deep Sub Zero Stress Relieving to Maximise Die Life & Performance

DIE CASTINGDIE CASTING Economical Way to Produce Large Quantities of

Complex, High Tolerance Parts. Replacement of Conventional Alloys with light

Die Cast Components. Requirement of Long Product Runs. Real savings possible by selection of right Tool

Steel, Heat Treatment & Deep Sub Zero Processing

Onus on Good Product & Die Design & improved Die Casting Practice.

DEMANDS ON DIE CAST DEMANDS ON DIE CAST PRODUCTSPRODUCTS

Alloys needed with higher strength, ductility, machinability, weldability & corrosion resistance

Product Design Focussed on– Larger Component Sizes– Thinner Wall Thicknesses– More Complicated Shapes– Closer Tolerances

Hence Advantage Pressure Die Casting.

PROPERTIES OF DIE PROPERTIES OF DIE STEELSSTEELS

Die Casting Dies exposed to severe thermal & mechanical cyclic loading.

Factors Affecting die life are– Thermal Fatigue (Heat Checking)– Corrosion / Erosion– Gross Cracking (Total Failure)

FACTORS AFFECTING DIE FACTORS AFFECTING DIE LIFELIFE

Die life affected by working temperature i.e. Casting Alloy

Die Life depends on – Design of Cast Product & Die– Production Rate & Process Control– Surface Finish of Dies– Die Steel & its Hardening Process– Tolerances on Cast Product

THERMAL FATIGUETHERMAL FATIGUE

Gradual Cracking due to Thermal Stresses arising out of Temperature Cycling

Dies Subject to Alternate Heating & Cooling during operation.

Severe Strains in surface layer of diesLeads to formation of surface thermal

fatigue cracks also called Heat Checking

THEORY OF THERMAL THEORY OF THERMAL FATIGUEFATIGUE

Thermal Fatigue caused by combination of Thermal Cyclic Stress, Tensile Stress & Plastic Strain

Eliminate Any One & Heat Checking can be Avoided

Plastic Strain initiates the Crack & Tensile Stress Promotes Crack Growth in Presence of Thermal Cyclic Loading

FACTORS AFFECTING FACTORS AFFECTING THERMAL FATIGUETHERMAL FATIGUE

Die Temperature CycleBasic Die Material PropertiesStress Raisers

CORROSION BY MOLTEN CORROSION BY MOLTEN METALMETAL

Molten Metal injected into die. Lack of Protective layer leads to diffusion of

Molten Metal into die surface. Iron diffuses from die surface into molten metal Leads to dissolution of steel & formation of

intermetallic compounds & soldering of cast metal on the die surface.

Portion of die will eject during ejection of the cast piece leading to pit formation on die surface.

FACTORS INFLUENCING FACTORS INFLUENCING CORROSIONCORROSION

Temperature of cast metal– Critical Temperature at which metal attacks the

steel– 480 C for Zinc, 700 for Aluminium

Composition of Casting Metal– Pure metals attack tool steel at greater rates

than commercial alloys

FACTORS INFLUENCING FACTORS INFLUENCING CORROSIONCORROSION

Design of the die– High injection speeds leads to washing away of

lubricants– Caused by incorrect gating design

Surface Treatment– Avoid Metallic Contact to reduce corrosion– Oxide, Nitride or Nitrocarburised surface

provides better corrosion resistance

EROSION BY MOLTEN EROSION BY MOLTEN METALMETAL

Mechanical wear caused by hot melt motion Depends on melt velocity, temperature &

composition Higher speeds increase erosion Higher melt temperature causes steel to temper

back Usually see a combination of Corrosion & Erosion Die Steel should have high hot yield strength &

good temper resistance.

GROSS CRACKINGGROSS CRACKINGToughness of die is ability to withstand

stresses without cracking at stress raisersDependent on Die Material & Heat

TreatmentDie Toughness important in all directions –

longitudinal, transverse & short transverseGross Cracking is Total Cracking due to

occasional Thermal Overloading.

TOOL STEEL SELECTIONTOOL STEEL SELECTION

Onus on Understanding Thermal Fatigue Process & relate Heat Checking to Basic Material Properties.

Good Die Steel needs excellent Temper Resistance, High Hot Yield Strength, Toughness & Ductility

Steel must have Good Hardenability & Resistance to Heat Checking, Erosion & Gross Cracking.

TOOL STEEL SELECTIONTOOL STEEL SELECTION Steel Alloyed to Give Optimum Mix of these

properties Medium Carbon Steels alloyed with Cr, Mo & V. Mo & V preferred as they produce small hard

carbides as well as impart high Hardenability to the steel.

Choice of die steel usually dependent on various parameters & commonly use various grades of die steels in various areas in a casting assembly.

TOOL STEEL SELECTIONTOOL STEEL SELECTION

Zinc Die Casting Dies Usually Fail by Erosion hence need a die steel with good erosion resistance.

Aluminium Dies Usually Fail by Thermal Fatigue & call for die steel with good hot yield strength & temper resistance.

DIE MAKING PROCEDUREDIE MAKING PROCEDURE

Machinability of the Die Steel.Heat Treatment of the Die Steel.Deep Sub Zero Stress Relieving.Electric Discharge Machining & Finish

Grinding

MACHINABILITYMACHINABILITY

Machinability influenced by amount of inclusions in the steels.

Improve performance by lowering the impurity levels in the steel.

Optimum microstructure is Spherodised Carbides in a soft annealed Ferritic matrix.

Hardness as low as possible.

HEAT TREATMENTHEAT TREATMENT Stress relieving mandatory after rough machining

to avoid distortions during heat treatment. Steel Properties controlled by Hardening

Temperature & Time. High Austentising Temperature improves Hot

Yield Strength & Temper Resistance but reduces Toughness & Ductility.

Recommended for smaller dies, Cores & Core Pins.

Heating Carried out as slowly as feasible.

HEAT TREATMENTHEAT TREATMENT Protect Steel Surface against Decarburisation. Quenching rate during Hardening very important. Slower cooling rates ensure better dimensional

stability but increase Bainite formation chances leading to loss of Fracture Toughness.

Faster Cooling rate means better metallurgical properties but more post heat treatment operations.

Strike a compromise between the two but prefer faster cooling rates for better die economy . Martempering provides good combination of Surface Finish, Higher Hardness & Lower risks of Distortion & Cracking.

HEAT TREATMENTHEAT TREATMENT Quench at around 500 C & smaller dies can be air

cooled to room temperature while larger dies need to be force air cooled.

Dies should be tempered as soon as they cool to 50 – 70 C.

Tempering temperature designed for optimum die properties. Normally two temperings with the third one defered till after EDM for added safety.

Good operating Procedure would involve Higher Austenitising Temperatures and Higher Tempering Temperatures so as to improve Thermal Fatigue properties

DEEP SUB ZERO DEEP SUB ZERO PROCESSINGPROCESSING

Deep Sub Zero Stress Relieving operation for correction of defects in crystal structure & provide order in chaotic Post Heat Treatment structure.

One time operation affecting the bulk of the material. Elimination of defects such as vacancies, dislocations,

stacking faults, etc. Increasing stress leads to increase in defects leading

to increase in defect concentration which causes cracks to develop & propagate causing failure.


Third Law of Thermodynamics. Materials subject to extremely low temperatures for

Prolonged periods of time leading to equilibrium conditions.

Defects Ironed out, Minimum Entropy state achieved & inter atomic distances reduced.

Steel at room temperature reflects equilibrium defect levels.

Compaction of crystal structure leads to improved abrasive & erosive wear resistance & higher fatigue strength & resilience.


Benefits to Die Casting Dies:– Better Thermal Conductivity.– Increased Resistance to Erosion.– Longer Die Lives– Shorter Machine Run Times.


Process Calls for Extremely Slow Heating & Cooling Rates & Prolonged Soak at Low Temperatures.

Slow rates to eliminate gradients in the surface & minimise stresses in the Die Blocks.

Prolonged Soak necessary for kinetics to proceed to completion.

Avoid heating of any sort & entire heat transfer is effected by Natural Convection only.

ELECTRIC DISCHARGE ELECTRIC DISCHARGE MACHININGMACHINING

EDM use increasing in die manufacture. Electric Discharge between an Electrode & the

Steel in a Dielectric Medium. Steel Surface subjected to very high temperature

causing it to melt & vapourise. Brittle resolidified layer is formed (White Layer). EDM surfaces should be ground & polished & the

tool tempered at 15-25C below the last tempering temperature.

SURFACE TREATMENTSSURFACE TREATMENTS Dies usually used with either a Nitride or Carbo-

Nitride or Oxide surface layer. Prefer to carry out the surface hardening right at

the outset rather than after a few hundred shots as this inhibits die life.

Surface hardness along with a rough surface finish improves lubricant adherence to the die surface and prevents Erosion/Corrosion of the die surface.

Harder Die Surface also leads to better Thermal Fatigue Properties but too thick a Case Depth will worsen die performance as Thermal Properties are affected.

Case Depth usually between 30 – 50 m.

CONCLUSIONSCONCLUSIONS

Need for improved Tooling Economy has led to Better Tool Steels.

Tooling Cost usually around 10% of product cost, hence benefits of using premium die steels are obvious.

Die Steel Cost usually around 10 – 15% of die cost while Heat Treatment & Deep Sub Zero Operations account for another 8 – 10% of die cost.

CONCLUSIONSCONCLUSIONS Steel Quality has improved significantly over the

past 20 years. Need for better attention to heat treatment & stress

relieving operations. Surface treatments to protect die surface &

minimise erosion/corrosion & thermal fatigue. With large variations in quality level at every step,

optimum results can be achieved only by demanding & paying for premium quality all along the line.

Die Casting

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