Materials & Manufacturing Technology
Casting is a very important process within the automotive industry and Jaguar. It plays a crucial part in the design and engineering of every vehicle jaguar produces. Casting is used within jaguar for making parts of complex shape that would be difficult or uneconomical to make by other methods, such as cutting from solid material. Although there are several different types of casting the principal of the process always remains the same it is a manufacturing process by which a liquid material such as molten metal or plastic is introduced into a mould, allowed to solidify within the mould, and then ejected or broken out to make a fabricated part. There are many different types of casting relevant to which metal is being used, the size and scale of the component, the finish required, the complexity of the structure etc. Within this report I will analyse casting methods, there advantages and disadvantages, defects that can take place, and what treatments are available.
Common casting defects and there possible causes;
There are many different types of casting methods used within jaguar, and with all these different methods come different kinds of defects. Defects can be the result of anything within the process from operator error through to a bad mold or environment. Some of the possible defects and there cause are labelled below.
Fig1. Casting after flash occurred
Flask was disturbed while investment was setting.
Base was removed too soon.
Flask was allowed to partially dry before de-waxing.
Incorrect de-waxing or a furnace malfunction.
Flask burned out and allowed to cool below (500oF (260oC) before casting reheating, flask allowed to cool between de-wax and placement in preheated oven. Flask was improperly handled or dropped.
Speed was set too high on centrifugal casting machine.
Patterns were placed on one plane.
Incorrect water powder ratio was used.
Not enough investment was placed over the patterns.
Flask was placed too close to heat source in burnout oven. Flasks were not held at low burnout temperature long enough.
Fig2. Casting after mold shift occurred
Fig3. Casting after porosity occurred
Pattern is improperly sprued. Sprues may be too thin, too long or not attached in the proper location, causing shrinkage porosity. Not enough metal reservoirs to eliminate shrinkage porosity. Metal contains gas.
Mold is too hot.
Too much moisture in the flux.
Too much re-melt being used. Always use at least 50% new metal. Metal is overheated.
Poor mold burnout.
Fig4 casting with inclusions
Patterns were improperly sprued to wax base or tree or not filleted, causing investment to break at sharp corners during casting. Flask was not sufficiently cured before placing into burnout oven. Improper de-waxing cycle was used.
Flask was not cleaned from prior cast.
Loose investment in sprue hole.
Molten metal contains excess flux or foreign oxides.
Crucible disintegrating or poorly fluxed.
Improperly dried graphite crucible.
Investment was not mixed properly or long enough.
Contaminants in wax pattern.
Flask was not held at low burnout temperature long enough. Flask was placed too close to heat source in burnout oven. Gas
Fig5 casting with gas pockets
Fig6. Short casted part
Hot tears are cracks that initiate during solidification (i.e. at non-zero solid fraction). They represent a major defect commonly encountered during the casting of large freezing range alloys and can lead to catastrophic cracking of the cast parts.
In Figure 7, a cracked rolling sheet ingot of very large dimensions is shown. In this example, two hot tears that were initiated during the start-up...