About this sample
About this sample
Words: 2923 |
15 min read
Published: Sep 18, 2018
Words: 2923|Pages: 5|15 min read
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
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’ . 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 . Almost 70% of the aluminium components that are manufactured today are by using HPDC .
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 . 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 . 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 . 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.
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 . 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 . 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 . 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 . 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 . 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 
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 .
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.
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 .
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 .
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 . 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 . 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 .
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 . 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.
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 , erosion and corrosion due to melt flow and oxidation, catastrophic failures, force majeure and mechanical instability caused due to cyclic heating . 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 .
The different causes of die failure are:
The traditionally employed repairing methods for dies are:
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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