May 2021 Volume 3

MATERIALS

As noted, the machined part moves if uneven amounts of metal are removed as this results in uneven amounts of stresses being released. The redistribution of stresses shown above is simplified. There are actually still high and low areas close to the un-machined bottom side although in lower amounts than before the machining. When this bottomsurface ismachined, the partwillmove yet again, but in a lesser amount than from the first machining pass. Progressive machining can minimize the overall distortion and several machine shops have proprietary techniques to keep this distortion to a minimum. Some parts have non-uniform metal removal requirements to create their finished shapes which leads to unusual problems. The alternate method of distortion control is for the designer to specify the aluminum in the -T652, -T852, -T7352 or -T7452 tempers. The Txx52 added to the end of the temper designation indicates that stress relieving by cold compression beyond the yield point by 1-5% is required. There is also a T7454 temper where the forging is stretched beyond yield point to reduce residual stress. The generation of residual stress which leads to distortion in machined parts occurs within seconds of the start of quench. There is a reversal of stresses from compressive to tension during this process. Control of these internal residual stresses is different for aluminum than for steel or titanium. It involves reversing the as- quenched stress pattern and causing it to cancel itself out. See the article in next month’s issue entitled: Techniques to Reduce Residual Stress in Aluminum Forgings. ■

FIA MARKETING WORKSHOP

AUGUST 30-31, 2021 Hybrid Event | Aloft Hotel Downtown Cleveland This 2-day program provides attendees with sales and marketing strategies to help promote and grow their businesses. Expert speakers will provide overviews of the automotive, aerospace, energy, and construction markets. Registration Opens Soon

Mark Timko Executive Technical Advisor Weber Metals, Inc. Email: marktimko@webermetals.com

FIA MAGAZINE | MAY 2021 31