Abstract- cavity where the molten metal is forced

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 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

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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). Depending on
the injection mechanism used, HPDC is classified 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. Almost 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 the runner and gate 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 have an
impact on the formation of defects in castings. 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 as 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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