forged piston vs alum. piston
04 February 2003, 01:10 AM
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Hi guys!
I'm about to begin a research project (for university). My project is to compare two kind of piston, forged one and aluminum one. The biggest part is to demonstate what influence high temperature (like overboost) can affect the resistence of the material (tensile strenght, shear, torsion, etc). For this, I need to know witch alloy each piston are made of. Can someone can help me a bit to start my project? Any usefull info will be appreciated!!!
Thanks
Frank
I'm about to begin a research project (for university). My project is to compare two kind of piston, forged one and aluminum one. The biggest part is to demonstate what influence high temperature (like overboost) can affect the resistence of the material (tensile strenght, shear, torsion, etc). For this, I need to know witch alloy each piston are made of. Can someone can help me a bit to start my project? Any usefull info will be appreciated!!!
Thanks
Frank
04 February 2003, 01:11 PM
#5
Forged pistons are still made from Alumnium. Are are key reasons you can get increased strength from forging over casting.
1. Casting is restricted to casting alloys. They are optimised for castability and strength, but one always seems to affect the other. The other problem with the casting alloys, is they generally don't respond as well to heat treatment. Forgings are made from forging alloys, which are generally the same as a wrought alloy.
2. The casting process can lead to porosity in the casting, this can lead to decreased fatigue life, reduced toughness, or even direct stress raisers. Forgings start with reduced porosity (less porosity in the wrought alloy), any remaining porosity will be eliminated during the forgin process.
3. The forging process will leave a much better grain profile. It should result in smaller grain size, which is better for strength, despite giving away a little stiffness to the larger grains pressent in a casting. Forging will also promote flow of the grain, just like formed plywood furniture.
4. Cold forging can be used to strain harden a material. Depending on section thickness, you can get limited increase strength from heat treatment alone. Heat treament followed by cold working can further increase the strength, considerably above that of a cast piece. How much of the strength of forged piston comes from strain hardening i can't tell you though.
The main problem with forging is the limited scope to get low wall thicknesses on a consistant basis. And the overall restriction on complex shapes. Many forges pistons will undergo more secondary machining operations to finish them, whereas the cast pistons can have all the features built into the casting.
There is a compromise that uses a cast blank that undergoes some mild forging post process. This can reduce the porosity and promote some shaping of the grain, giving some of the benefits of forgings, whilst keeping some complex shapes and reduces costs.
Hope this is a start for you, you will need to do more research to get a fuller picture. Bosch Automotive Handbook might be a good place to start. Also "Engineer To Win" by Carroll Smith, that gives a good overview on Al alloys, and the processes for manufacture and treatment.
Paul
1. Casting is restricted to casting alloys. They are optimised for castability and strength, but one always seems to affect the other. The other problem with the casting alloys, is they generally don't respond as well to heat treatment. Forgings are made from forging alloys, which are generally the same as a wrought alloy.
2. The casting process can lead to porosity in the casting, this can lead to decreased fatigue life, reduced toughness, or even direct stress raisers. Forgings start with reduced porosity (less porosity in the wrought alloy), any remaining porosity will be eliminated during the forgin process.
3. The forging process will leave a much better grain profile. It should result in smaller grain size, which is better for strength, despite giving away a little stiffness to the larger grains pressent in a casting. Forging will also promote flow of the grain, just like formed plywood furniture.
4. Cold forging can be used to strain harden a material. Depending on section thickness, you can get limited increase strength from heat treatment alone. Heat treament followed by cold working can further increase the strength, considerably above that of a cast piece. How much of the strength of forged piston comes from strain hardening i can't tell you though.
The main problem with forging is the limited scope to get low wall thicknesses on a consistant basis. And the overall restriction on complex shapes. Many forges pistons will undergo more secondary machining operations to finish them, whereas the cast pistons can have all the features built into the casting.
There is a compromise that uses a cast blank that undergoes some mild forging post process. This can reduce the porosity and promote some shaping of the grain, giving some of the benefits of forgings, whilst keeping some complex shapes and reduces costs.
Hope this is a start for you, you will need to do more research to get a fuller picture. Bosch Automotive Handbook might be a good place to start. Also "Engineer To Win" by Carroll Smith, that gives a good overview on Al alloys, and the processes for manufacture and treatment.
Paul
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