How can we eliminate chips after machining and surface treatment of aluminum and magnesium castings?
Q. My company manufactures airplane engine gearboxes and has very restrictive requirements for cleanliness. This often becomes really critical for aluminum and magnesium castings. After machining and surface treatments, we use a gun to wash our sand castings with water and detergent. Before the final assembly with gears, bearings and so on, we perform a washing and flushing operation using high-pressure water in a specially designed washing machine that rotates the part and that is equipped with dedicated flushing tools for every duct.
Our castings are full of oil and fuel ducts, and sometimes also have wide areas in which chips and swarf from machining remain trapped. These chips can be 10-15 mm long and rolled into spirals, like springs, making them very difficult to expel from the tank. How can we eliminate these chips before final assembly? T.M.
A. Your assignment is difficult. I have had similar experiences dealing with customer requirements on oil and transmission systems. It appears that you have many sources of contaminants, including the sand casting itself, its machining and the introduction of numerous other components assembled within the casting such as gears and bearings. While time consuming, I would try and bin your contaminants in order to get an understanding of which processes are the primary contributors to the particulate.
One method would be to first characterize the type of contaminant from each of the manufacturing steps to include composition, size and shape of debris. For instance, the first process of sand casting could have the casting sand with a certain shape and size (probably spherical and 5-20 microns in size), molten aluminum (probably spherical and ~20-100 microns) and dross (irregularly shaped and 50-150 microns). These sizes are only a suggestion, and you would need a measuring microscope to better define the size and shape of your particulate. From there you could move onto the machining process, which is likely producing the spring-like particle 10-15-mm long. Each of the incoming components should be examined in a similar way. What is the material and processing of each component added to this gearbox? Then what are the size, shape and compositions of the particles from each of these parts?
While this may seem time-consuming, it will enable you to focus your time on the correct process steps that are contributing to the residue. For instance, if the spring particles are coming from your machining operation, instead of immediately trying to come up with a better cleaning process, you may first want to see what you can do upstream to minimize particulate even entering the part. Are all machining cells vertical end mills? Can parts be oriented in a way so particulate cannot fall into them? Is particulate filtration of the machining fluid as effective as it can be to minimize re-deposition?
Once you are satisfied that you have examined and improved upstream processing as much as possible, you will need to examine the effect on the final process. If parts are still not getting clean enough, you should look at the current cleaning process for optimization (alternate cleaners, filtration, time and temperatures). Hopefully, when that is complete you will be in compliance with your cleaning requirements. If not, you will need to investigate alternate processes. Some possibilities would be an immersion process such as ultrasonics to insure you are wetting and getting good impact onto all surfaces equally (blind holes as well as outer surfaces). An alkaline etch may also be effective to completely dissolve all the aluminum particulate, although this will also dissolve the base metal of the aluminum casting. If you have enough tolerance in the part, this may be effective, but it would require immersion of the part, something you are currently not doing.
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