Abstract- A high production rate and cost

effective method primarily used in the production of non-ferrous metals is the

Pressure Die Casting (PDC) technology. It is widely used in the manufacturing

of automobile components of complex geometry and intricate forms and shapes

that may be difficult with the other conventional manufacturing processes. The

paper gives an insight into the types of pressure die casting techniques. It also

describes the recent trends and developments done in the pressure die casting

technology. Numerical simulation is one of the cost effective methods that can be

used in optimization of the casting process. The various simulation methods

available for numerical simulation of castings are discussed. The paper also

depicts the use of integrated CAD/CAE approach and parametric design approach

that makes the design process easier. The study made in the paper also

discusses the importance of residual stresses and their effects on the fatigue

life of cast components. The most important tool of the pressure die casting

operation is the ‘die’ that consists of the mold cavity where the molten metal

is forced under pressure for the required component to be cast. The causes of

failure and repair option for dies have been discussed.

Keywords- Pressure Die Casting, Numerical

simulation, Software simulation, Residual stresses, Die failure

I.Introduction

The pressure die casting process is characterized by

forcing the molten metal under a high speed and high pressure through complex

gate and runner system into the mold cavity of the tool called ‘die’ 1.

The cavity in the die is of the shape to be formed. The process has

capabilities of producing complex shapes with good dimensional accuracy,

surface finish and high material yields. It is widely suited for casting non-ferrous

metals like Zn, Cu, Al, Mg, Pb and Sn based alloys. Depending on the pressures

being employed, the die casting process can be of two types mainly High

Pressure Die Casting (HPDC) or Low Pressure Die Casting (LPDC). According to the

injection mechanism used, HPDC is categorized as the Hot Chamber HPDC Process

and Cold Chamber HPDC Process. In the Hot Chamber process, the injection

mechanism is placed inside the metal furnace where the components are in

constant contact with molten metal. It ensures minimum contact of metal with

air thus reducing chances of gas entrapment defects but reduces the life of

components. Whereas, in the Cold Chamber process, the injection system is kept

outside the furnace and metal is poured by means of a ladle

manually/automatically. It increases the life of components but increases chances

of gas entrapment defects 2. About 70% of the

aluminium components that are manufactured today are by using HPDC 3.

HPDC is most widely used in the automobile and

communication industries in forming thin walled, complex shaped and high

quality cast components at low cost 4.

A number of parameters like the geometrical design of the product, design of

runner gate system, temperatures of die and metal, flow velocity, flow pattern,

heat flow and solidification rate have been found to affect the quality of die

castings 5.

A major challenge while designing a die is to determine whether or not the

final part has defects. A number of software packages like MAGMA, PROCAST, and

FLOW 3D, FLUENT, etc are available for simulation of the casting process. They

aid in the optimization of the design parameters and enable the designers to

quickly and accurately identify and locate defects that allow parts to be

produced with higher quality in shorter amount of time

6.

The optimum design of gating system and die geometry is crucial for the

homogenous filling of the dies which closely affect the final quality of cast

components.

II. Existing Developments In Pressure Die Casting

Technology

A.

Numerical

Simulation in Die Casting Process

The

quality of the castings produced by pressure die casting process mainly depends

on the filling pattern of gating system used. A homogenous mould fill pattern ensures

good quality castings. Also, despite the design of the runner gate system,

their proper location and size plays a very important role in controlling

defects like porosity and cracks. A poor gating system design usually results in

production of castings with defects like gas and shrinkage porosity, blowholes,

cold shuts, incomplete filling, flow lines and a poor surface finish 7.

These casting defects have been proved to have an influence on the static and

fatigue strength of the die cast alloys which limits the use of cast parts in

critical high strength applications 8.

The parameters like the filling pattern, pressure, fill rate, cooling rate and

solidification largely affect the formation of defects in cast components. The

most frequently encountered defect in castings is porosity which is very

closely related to the casting process parameters and has a severe impact on

the cost of the casting process by scrap loss 9.

The mould filling process is a typical liquid-gas two phase phenomenon. The

interaction of the molten metal and gas in the complex moulds play an important

role in the formation of gas entrapment defects. Numerical simulation tools can

help in the quantitative prediction of such defects 10.

It also enables us to visualize progressive cooling from inside of the casting

to the external environment. It helps to understand the changes that can be

made in the design parameters so that we obtain a homogenous mould fill pattern

and optimize the design. The high filling speed, high temperature of the liquid

metal, opacity of the metal mould and high metal pressure create difficulties

in the direct visual evaluation of the mould fill process. Thus the design and

modification of the runner gate system using numerical simulation depends on the

trial and error approach.

B.

Simulation

methods available for numerical simulation of die casting

A

number of methods and software packages are available for simulation and

analysis of the casting filling process. The software packages are usually grid

based and employ the volume-of-fluid method (VOF) to track the free surfaces 1.

Methods such as Finite Difference Method (FDM), Finite Volume Method (FVM),

Finite Element Method (FEM), Lattice Boltzmann Method (LBM) and Smoothed

Particle Hydrodynamics (SPH) are used for solving the governing fluid flow

equations of the mould filling process. Among the Eulerian techniques are the

Mark and Cell (MAC) method, level set method, Volume of Fluid method (VOF) and

arbitrary Lagrangian Euler method that are used to study the free surface flows

10

In

the Marker and Cell (MAC) method, Lagrangian markers are placed on the

interface at the initial time. As the interface moves and deforms, markers are

added, deleted and reconnected as necessary. The evolution of the surface

between the different fluids is tracked by the movement of the markers in

velocity field. It is difficult to maintain mass conservation and to determine

a good surface interpolation in three dimensions. However this technique does

not suffer from numerical diffusion and gives accurate results in two

dimensions.

In

the Volume of Fluid (VOF) method, the volume of fluid in each computational

cell is represented by employing a colour function. The use of colour functions

to represent interfaces makes them prone to suffer from numerical diffusion and

numerical oscillations. According to the advection equations, the volume fractions are updated, and free surfaces of the

fluid with fractional volume should be reconstructed for each time step.

This type of reconstruction is difficult in three dimensions but due to the

relative ease of implementation and its basis in volume fractions, this method

is well suited to incorporate other physics and is the most popular and widely

used method 11.

SPH

is a Lagrangian method that does not need a grid to compute its spatial

derivatives and uses an interpolation kernel of compact support to represent

any field quantity in terms of its values at a set of disordered points which

are the particles. The computational frame work on which the fluid equations

are solved are the particles of flow. The particle information allows

calculation of smoothed approximations to the physical properties of the fluid

and provides a way to find gradients of fluid properties. This method is

applicable in multi dimensional problems and is particularly suited for complex

fluid flows because of its Lagrangian nature. Fine details such as plume shape,

frequency and phase of oscillation and the correct relative heights of all the

free surfaces can be captured using SPH.

C.

Software

tools available for numerical simulation

The

numerical simulation results can be validated using water analog experiments or

software simulations. Various commercial CAE software packages are available

that facilitate the simulation and analysis of flow processes. With the rapid

advances in computer technology, different kinds of finite element software

including both the casting professional software and general analysis software

are coming into use in practice across the world. Casting professional software

such as Germany’s MAGMA, United States’ PROCAST, and FLOW 3D, Tsinghua

University’s FT-STAR, etc have been increasingly employed for the numerical

computation of flow fields and temperature fields. The analysis results of general FE software are more

accurate and reliable whereas most casting professional software is expensive

which does not meet the needs of most manufacturers and researchers. Therefore purchasing and using general FE

software remains an ideal choice in the competitive markets. America’s large

general analysis software ANSYS is being widely used and has become very

powerful for calculation of three dimensional flows. As CFX and FLUENT had been

purchased by ANSYS, a FLUENT calculation module is a part of ANSYS that enables

effective simulation of free surfaces of fluid in three dimensional 6.

Germany’s

MAGMAsoft is also extensively used in the die casting industry particularly in

foundry applications. It is a three dimensional solidification and fluid flow

package that employs modelling of molten metal flow and solidification in dies.

The heat and mass transfer equations are solved on a rectangular grid using

finite difference method. This software tool has strong material capabilities

and as it provides useful information about the filling pattern. It is very

useful for analysis of a permanent mould. It facilitates accurate analysis of

features like premature solidification, air entrapment, velocity distribution,

runner and gate effectiveness. Despite such capabilities, the rectangular grid

artificially introduces staircases along curved and sloping boundaries. Also

artificial diffusion and mass conservation issues are introduced because of the

VOF formulation for modelling free flows.

D.

Integration

of CAD/CAE System of Die Casting and semi automated parametric design of gating

system

With

the increasing competitiveness and increased demand from market, a powerful

impact is exerted on designers to reduce casting defects and improve the

quality, production rate and life of dies. Depending upon characteristics like

the type of die casting machine, the geometry of the casting and the properties

of the alloy, the die designers can determine location, shape and dimensions of

runner gate system of a die using appropriate CAD packages like Unigraphics,

CREO Parametric, Catia, etc. By integration of CAE package with CAD, the

parameters like optimal injection pressure, gate velocity, fill time, defects

related to casting filling and solidification process etc. can be obtained 12.

Recent

advances have incorporated parametric design approach into various CAD/CAE

systems. In the parametric design approach, the variable dimensions are treated

as control parameters that allow the designer to modify the existing design by

simply changing the parameter values. This approach facilitates the efficient

design of part families whose members differ only in dimensions, reducing the

work of creating parts repeatedly from scratch. Because with the parametric approach,

a single parameterized model can be developed to represent a part family. In parametric

design, a gating model database (or feature library) is already constructed

which includes the original parametric gating models constructed using a 3D CAD

tool. These models can be easily retrieved from the database, modified with

certain specified parameters and locations and then attached to the die casting

part. The parametric design approach serves thus reduces time and makes design

update easier and faster 13.

E.

Residual

Stresses in casting and their effects on Fatigue and Fracture

Heating

is inevitable in the die casting process and the temperature differences in the

casting along with other loading conditions result in the formation of residual

stresses. These are the stresses that remain in the casting after ejection from

the mold cavity. The formation of residual stresses in casting is associated

with causes like temperature gradients due to continuous heating and cooling in

the casting, hindrance of contraction by the mould and rapid solidification of

the mould 14. Residual stresses if present in

the cast component significantly reduce its fatigue life and result in shape

changes and cracks in castings. However, they can have either a life enhancing

(positive) or life reducing (negative) effect which depends on the sign of the

residual stress relative to that of the applied stress. Tensile residual

stresses are found to be most dangerous as in service they lead to fatigue

crack initiation and growth 15. During the

cold phase of die casting cycle, these tensile stresses appear on surface and

lead to local plastic deformation on die resulting in crack nucleation and

growth 16.

The

residual stress measurement can be done either experimentally or often with a

combination of simulation using advanced numerical analysis techniques. Optimal

design of the die along with correct machining and heat treatments could keep

the residual stresses minimum 17. Some most common

methods for residual stress measurement are X-ray diffraction, hole drilling

and sectioning methods. The X-ray diffraction and Hole drilling methods are non

destructive but they are sensitive to the microstructure and geometry. However,

Sectioning is a destructive method that is very much suitable for measuring macro

stresses in the components. The knowledge of residual stresses is significant

to analyze their influence on fatigue and fracture performance so as to combat

failure.

III. Die Failure Causes and Repair

options

Different

types of tool steels with/without surface treatment are used to manufacture

dies. The life of dies and moulds in industries is improved with the timely

repair of damaged surfaces. The degree and severity of the damage is decided by

the requisite precision in shape and size of dies and the operating conditions

of the tool. The life of the die at a given geometry, material and heat

treatment largely depends on die casting parameters. The hot phase of the cycle

produces high compressive stresses that usually retard nucleation and growth of

cracks but are a major cause of local plastic deformation. The filling pressure

additionally increases the compressive stresses in the dies.

Different

types of stresses are produced in the die during operation and the dies fail

when the stress value becomes larger than the strength of the tool steel. The

die surface is rapidly heated with the molten metal injection and the

subsequently cooled by means of the cooling mechanism or lubricant used to cool

the surface. The need for repairing dies originates because of the design and

manufacturing errors, operational defects, wear and plastic deformation. The

life of dies reduces due to thermo-mechanical fatigue causing heat checks on

the surface of die 16, erosion and corrosion

due to melt flow and oxidation, catastrophic failures, force majeure and

mechanical instability caused due to cyclic heating 17.

Thus for the proper selection of the process and optimization of the process

parameters, failure analysis of the damaged surfaces is important. Computer

based design and analysis programs are available that can be used to ensure

perfection in the specific design of the dies 18.

The different causes of die failure are:

1. High

thermal shocks

2. Mechanical

loading

3. Cyclic

loading

4. Heat

checks due to thermal stresses

5. Plastic

deformation

6. Wear

7. Fatigue

Other

causes include improper or faulty design, mishandling, force majeure and

operational accidents 18.

The traditionally employed repairing methods for

dies are:

1. Gas

tungsten arc and plasma transferred arc welding

2. Laser

based material deposition

3. Micro

GTAW and Micro Plasma

4. Electron

beam welding

5. Cold

spray technique

6.

Thermal coatings

18

IV. Conclusion

The

paper thus describes the recent developments made in the pressure die casting

technology. The use of numerical simulation in the casting process can help in

optimization of the runner gate design and reduction of defects produced in

cast components.

Prototype

parametric design system described in the paper can be employed to consider

different castings since gating system design varies from case to case. The

paper also describes the different causes of failure of dies that can be

analyzed in the design stage to increase the life of the tool and prevent early

failure.

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