HPDC DEFECTS PART2

GAS POROSITY
Porosity is one of the biggest problems in die casting. High volume production and very near net shape make the process very attractive, but the reputation of die castings as having high porosity has heavily restricted use of the process. Strong customer pressure for better quality has forced an
increased use of engineering and technology, which is rapidly reducing the traditional porosity problems. Currently, there are successful applications for heat treatable die castings that would not have been considered even a few years ago. Shrinkage porosity and gas porosity are the major types of internal porosity in die castings. There is another type of porosity, called flow porosity,
which will be discussed later. Gas and shrinkage porosity require almost completely opposite actions for corrections, hence a person doing troubleshooting must first identify the type of porosity in question; otherwise the corrective action is likely to be of little or no value. However, even after identifying the type of porosity, the exact cause will not be determined that easily. Corrections will
often require a judgment based on the most probable cause. But the troubleshooter should not jump to a likely probable cause without making a best effort at determining the type of porosity. This will save a lot of very expensive trial and error, and it is recommended that samples be prepared for examination under magnification before starting correction efforts. A magnification range of 5 to 50 power will handle most questions, though, much of the time, a hand-held magnifier with 5 to 10 power will be satisfactory. A stereo microscope with up to 50 power magnification is very desirable to settle questions about whether the porosity is shrink or gas, and are relatively inexpensive. Training is desirable, but someone working in this area can quickly learn to recognize the differences in the types of porosity with continued use.


SOURCES OF GAS POROSITY

Gas porosity is the largest single problem in die casting. This is an unwanted byproduct of the high velocity injection methods used. Going away from these injection methods would be the first choice; but unfortunately these methods are a requirement to fill the complex, thin wall parts that die

casting does so well. It would be ideal if die casting could use the minimum turbulence fill methods that are common in other processes, and thus greatly reduce the trapped gasses, but this hasn’t been possible to date. Squeeze casting and other processes have increased the options, but the thin walls and complex shapes must still be done by the high velocity quick fill methods. However, rapid progress is being made in developing methods to reduce the amount of gas trapped in the casting and to use the non-turbulent methods of filling in the die casting. Current high vacuum methods can

almost eliminate gas in the cavity, resulting in castings that can be heat treated or welded. Thixotropic fill methods are showing considerable prom ise in extending the usefulness of less turbulent fill methods.

Most die castings are made with the same methods used for years, which involves atomized flow. This results in gas entrapment and castings that have a hard time passing the blister test. This test holds castings above 600°F for an hour or more. Those castings full of gas porosity will blister

and deform from the pressure of expanding gas in the porosity. This test is frequently used as a way of qualifying a casting for general porosity levels. It is expected that much more extensive use of die castings will become the norm as the process is improved and gas porosity greatly reduced

in the next few years.

SOURCES
Gas porosity comes from three main sources in die casting:
• trapped air • steam • burned lubricant