Simulate Casting and Avoid a Hot Mess

Designing a mold pattern for a cast iron product isn’t as easy as it sounds. A foundry engineer needs to consider the inherent characteristics of cast iron and factor them into the feeding system.

For instance, the mold design will need to take into consideration pattern shrinkage and draft allowances.

From Liquid to Solid Iron: Expansion and Shrinkage

“Most standard alloys shrink during solidification as there is a difference in density between liquid and solids. This volumetric change will induce pores,” explained Badarinath Kalkunte, casting product marketing and business development manager at ESI Group.

“Cast iron has an ability to expand during the liquid to solid phase change,” he explained. “Expansion is due to the fact that graphite present in cast iron can form nodular structures that occupy more volume. Interrupting the possible volume change due to this expansion could help in reducing pores. This is the reason why feeder necks are designed to arrest the expansion of cast parts and thereby compensate for shrinkage porosity.

Simulations helps foundry engineers precisely assess how metal expands and contracts during the casting process, thereby predicting the final locations of shrinkage porosity. Several other possible defects can occur during a casting process causing imperfection to the final product. To reduce or eliminate these defects, a well-designed gating and feeding system needs to be developed.

Using simulation, foundry engineers  can also visualize the casting filling and solidification inside the mold. This enables them to validate and to improve the locations and sizes of the down sprue, runners, gates, vents, chills and feeders (a.k.a. risers).

Virtual prototyping software such as ESI ProCAST, now in its 25^th year of use as a commercial CAE tool, can be used to help simulate casting processes and ensure that a mold is made right the first time.

Video of a sand casting simulation with ProCAST and QuikCAST.

How ESI ProCAST Simulates a Casting Application

For a CAE tool to simulate a casting application, it needs to have the right geometry, materials and process conditions.

To determine correct cooling rates near the casting-mold interface, it is important to have an accurate  geometrical description.

The casting product and gating design can also inadvertently cause the liquid metal to have a turbulent flow, which will lead to air entrapment and formation of oxide bi-films. For this reason, the geometry will need to be assessed when optimizing the mold.

Typically, the geometry is imported into the CAE software where it is discretized into small pieces called the mesh. ESI ProCAST uses the finite element (FE) methodology for meshing. This mesh can then be used in a simulation to assess the flow, solidification, residual stress, microstructure and mechanical properties of the casting.

Thermal-Fluid-Mechanical Properties of Alloys

The metal alloy of the material being casted must be considered when designing the mold.  The alloying elements and their proportions differentiate the alloy and affect its thermal-fluid-mechanical properties, causing different filling, solidification and mechanical behaviors. Most literature only provides the basic information of standard alloys. However, it is important to have specific material properties for a given cast alloy for the entire temperature range from pouring to cooling.

“Material characterization is thus required and could be expensive. Adding to this, there is always a slight variation of chemical composition for the same alloy across melts,” said Kalkunte. “ESI ProCAST is uniquely linked to a thermodynamic database, which is able to generate temperature-dependent thermal-fluid-mechanical properties based on a user-defined composition, enabling users to have properties closest to their own alloy.”

Inoculation and Magnesium Treatments

The simulation software will also need to keep track of the inoculation and magnesium treatment the metal has undergone.

“Inoculation is a catalyzer, which increases the number of nucleating sites from which eutectic graphite can grow during the cast iron solidification,” explained Kalkunte. “To simulate this requires the coupling of thermal and microstructure models to consider all the physics involved in this process.”

“Castings have a matrix structure composed of different volume fractions of different solid microstructures like ferrite, pearlite, graphite and ledeburite,” added Kalkunte. “These are an outcome of the alloy composition and cooling rates. These structures have inherent mechanical properties. The final mechanical properties are derived based on mixture laws.”

The benefits of the inoculation will decrease over time. This is referred to as fading, or reduction in the graphitization potential. If the graphitization potential falls too low, the iron might undercool during the solidification and add eutectic carbides to the microstructure.

The inoculation will also affect the amount of pearlite and ferrite in the matrix structure of the cast iron. All of this will affect the properties and performance of the casting and finished product.

To assess the effects of inoculation in a casting, foundry workers can use ProCAST. The software has an algorithm that assesses the multiphysics of the system by coupling the thermal-microstructure and thermodynamic solvers.

“ProCAST has a comprehensive porosity model developed over the years accounting for micro and macro porosity, as well as pipe shrinkage,” explained Kalkunte. “The solver takes the effect of the alloy properties, geometry, cooling rates and temperature gradients of the casting. These will all affect the feeding length and the placements of runners, gates, vents and feeders.”