Simulation of High Pressure Die Casting (HPDC)

via

STAR-Cast

STAR Global Conf. 2012, 19-21 March, Noordwijk

Romuald Laqua, Access e.V., Aachen

High Pressure Die Casting: Machines and Products

Common Materials:

● Aluminum alloys

● Magnesium alloys

● Zinc alloys

● Copper alloys

HPDC process cycle, horizontal cold chamber machine

Parts considered for simulation

HPDC process cycle: 1. closing moving die parts

HPDC process cycle: 2. shot sleeve filled with melt, starting plunger movement

HPDC process cycle: 3. completed shot

HPDC process cycle: 4. ejecting and removing solidified casting

HPDC process cycle: 5. spraying of lubricant, casting cycle finished

Challenges in HPDC Simulation:

Moving plunger in filling chamber – moving mesh model necessary

Thin walled, complicated and large castings – challenging enmeshment and high cell count

Multi physics: melt, solid, gas – VoF model with HRIC scheme, combined with surface

tension model and correct wetting angle

Short pouring times, leading to high fluid velocities – small time steps (~0.1ms) mandatory

Extreme pressure ranges from 10 Pa initial cavity pressure up to 1000 bar in melt during

solidification – Compressibility model for melt and gas

Why simulate?

Goals & Objectives of HPDC Simulation:

Reduce iterations in tooling development: Cost for one mould insert 50-100k€

Reduce process development time: faster achievement of a stable process window

Better process understanding: helpful when negotiate with customers about necessary part design

changes

Typical defects in high pressure die casting:

Misruns: Melt solidifies before filling is completed

Cold shuts: Imperfect fusing of molten metal coming together from opposite directions in a mold

Porosity: small holes caused by insufficient feeding or dissolved gas

Air and oxides inclusions

Cold flakes: floating crystals, solidified at shot sleeve walls and transported into cast part

High Pressure Die Casting – Overview

Features

● Filling Simulation

● Gas is Compressible

● Liquid is Compressible

● Moving Mesh

● Phase Change

● Conjugate Heat Transfer

Simulation

● Shot chamber is half filled with liquid,

plunger follows shot control curve, pushing

the fluid into the cavity

High Pressure Die Casting: shot curve vPlunger=f(t)

Constant velocity until

mold is filled, followed

by pressure control, up

to 1000 bar

High Pressure Die Casting – Geometry and Mesh

Die parts

Shot sleeve

Cast

Chilled vents

(allow air to escape and

force melt to freeze)

Meshing

Structured (extruded) mesh

in shot sleeve

Polyhedral mesh in cast part and die

Cell count: 1.6 million cells in fluid domains

3.6 million cells overall

Meshing

● Two layers of prism cells on each side of casting-die

interfaces to resolve high temperature gradients

● Water and oil channels are not meshed

Process parameter setup, additional settings related to HPDC process

Shot sleeve components must be identified:

Empty filling chamber Shot sleeve walls Prefilled with melt

Process parameter setup

Shot curve definition:

Enter values or

read from file

Process parameter setup

Pressure curve definition:

Enter values or

read from file

Die cycle warm up simulation

Pure thermal simulation over

at least 5 casting cycles,

including all phases: shot,

solidification, die opening,

ejection and spraying

Final temperature

distribution in die is used as

initial state for main

simulation run with coupled

filling and solidification

Initial temperatures in die and shot sleeve

Cooling cycles in oil and

water channels are

modelled by applying mean

fluid temperatures on

channel wall boundaries

(channels are not part of

computational domain)

High Pressure Die Casting – Results

Pressure on

melt surface

Velocity on

melt surface

Time = 1.96 seconds Time = 2.05 seconds Time = 2.06 seconds

Temperature distribution on melt surface during mold filling

Front view Rear view

Air entrappments in casting after completed shot (2.52 s)

Initial melt temperature = 640°C Initial melt temperature = 680°C

Animated mold filling process with piston movement

Initial melt temperature = 680°C

Tliquidus = 613°C

Tsolidus = 555°C

Future steps of development of STAR-Cast

HPDC process is challenging to simulate seriously, but casting industry

seeks for a more detailed simulation tool with more physics inside

Migration to STAR-CCM+ will simplify the setup process, enhance

postprocessing capabilities and (probably) improve numerical stability