I'm going to try to justify some of the craziness that I'm about to apply to these very expensive parts with some background theory. Precipitation hardening is a common way of strengthening 356 aluminum which is a popular casting alloy. To promote this process, certain impurities such as magnesium and silicon must also be present in the aluminum melt. As the castings cool these impurities form simple compounds which gradually, over time, come out of solution (precipitate) and they end up distributed throughout the casting. These precipitates harden the casting by preventing its plastic deformation (bending or stretching) when it is put under stress. These precipitates strengthen the casting, but they also make it brittle. This hardening process is kicked off just after the castings solidify, and it continues for hours to days or even weeks later depending upon the casting's storage temperature. Because of this process's dependency upon time, it is also commonly referred to as age hardening.
A casting that has been age-hardened has little tolerance to bending, twisting, or stretching. If a 356 casting warped during its solidification, and if it requires straightening before it can be finish machined, then it must be annealed. This can be done by heating the casting to about 700F and then allowing it to air-cool. The common shop technique of using an acetylene torch to create a soot coating for use as an annealing temperature indicator also works for 356. If, after straightening, the part is left in its annealed state, significant strength will be lost. For 356 the tensile strength loss can be as great as 10,000 psi. Unfortunately, a 700F annealing is not high enough to kick off another age hardening cycle.
The casting can be re-hardened, though, back to its maximum strength by heating it to 1000F for a dozen or so hours and then quickly quenching it. What makes this difficult to do in a home shop is the fact that aluminum melts at 1035F, and so careful temperature control is required. There is also a chance that the casting will deform under its own weight if it isn't properly supported. In addition, air-cooling needs to be minimized which means the quench tank needs be located within seconds of the furnace.
Unlike the more familiar hardening process associated with steel, precipitation hardening does not occur immediately after the quench. The metal may remain soft enough to be straightened for up to a full day after the quench.
Production castings should normally be straightened by the foundry before age hardening has progressed to any significant extent. An even better solution, of course, is to design the part so warpage is a minor concern, and the casting can be corrected by finish machining. The Merlin castings were not straightened by the foundry, and their thin-wall and complex cross sections make them very susceptible to warping that may not be correctable solely by machining. Straightening, after the castings have been allowed to harden, was therefore left to the end-user.
Since this is a totally new experience for me, I felt it would be best to practice on some scrap cast parts I picked up long ago from my favorite scrapyard. My practice pieces, which are louvered vents, were sand cast from 356 and allowed to age-hardened for many years.
I decided to immediately answer a question that was in the back of my mind, and that was just how much could I permanently deform one of these castings without annealing. After breaking two practice parts, I realized the answer was 'pretty much nothing at all.' The rest of my practice was done using only annealed parts.
I eventually developed a process, after cracking a few annealed practice parts, for controlling the pressures I used to bend the castings. I learned to clamp the parts down firmly and to use positive calibrated stops to quantitatively limit the distance that an edge was being pushed. Rather than using a press I typically used my own strength and body weight in combination with fulcrums, levers, and clamps so I could maintain a hands-on feel for what I was doing. I found it was important to proceed in small deformation steps of .005" at a time and continually return to the surface plate to check my progress. I also decided it was best to not aim for perfection but to stop at the point where measurements showed I could machine the remaining defects away without negatively impacting the part's appearance. Since I had decided to not even attempt age-hardening in my shop, I tried to minimize the areas that I annealed. Before attempting any straightening, I located the major axis of the warpage using a surface plate, and I tried to anneal only a narrow region along that axis. I then applied my straightening efforts across this axis. After a full day of experimenting I had gained enough confidence to start on the Merlin parts.
I first selected the three crankcase-related castings. I was able to machine flat the bottom surface of the main casting with respect to the crankshaft bores in order to obtain a reference surface. The front gear case turned out to be the major problem area on this part. It was out of perpendicular by almost .050" over its 5" height. I annealed a line across the gear case just above the top deck of the crankcase. After clamping the crankcase with its reference surface down to my drill press table, I clamped a long piece of wood to the gear case to which I applied the straightening force. With a pencil mark on the wood as a moving pointer I carefully monitored the distance the part was being pushed. After a half dozen tries which included returning to the surface plate to check my progress after each push, I had finally bent the gear case to within .015" of perfection. At this point I was able to machine its cover mounting surface flat and perfectly perpendicular to the reference surface in order to meet the drawing dimensions with no noticeable impact on appearance. I then machined the rear of the crankcase to its finished dimension. The decks for the cylinder heads will be done later, since they can be cleaned up with just finish machining.
The gear case cover was relatively simple to correct because its major warp was also about a single axis. The documentation warned that this rather rigid part might have to be widened, and the drawings included the design of a complex 'stretcher' to attempt this. I'm thankful that my particular cover, which is a fairly rigid part, didn't require this really scary correction.
The oil pan was considerably more complex and problematic. Being rather flimsy by design, it was warped across two separate axes. In addition, it's width had to be spread to match the crankcase. When checked on the surface plate, one corner of this part was initially almost 1/8" higher the other three. This part required almost a full day to correct, and I ended up annealing practically the whole casting. Fortunately, the oil pan is not a structural part, and the loss in strength that it likely suffered is not important. In the set-up for its final flange machining, the pan had to be packed with plastic modeling clay in order to dampen the chatter created by the mounting flange machining. I used high relief aluminum-cutting Korloy carbide inserts for all the machining operations and was able to obtain excellent surface finishes on both the annealed and un-annealed areas of all three castings. - Terry
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