Investment Casting

I was catching up with the 2009 Interbike coverage, when I came across We The People and Fly Bikes. They were using a process I had not heard about: Investment Casting. After a quick Wiki, I thought it would be easier to find a video. This one appeals to my background in titanium.

So in real world terms, what does this mean for BMX?

• Smoother transitions than weld joints help to reduce stress risers.
• Material can be better placed to reduce unnecissary weight.
• Sexier components. For some reason, I have this feeling that the sexier a component looks the better it will perform. We'll have to see how that one pans out...

However, there are a couple of issues to address. The one that really jumps to mind is metal grain orientation. This is an interesting comparison between casting and forging in industrial steel manufacturing.

As previously stated the forging process produces a part that is anisotropic. This means that the mechanical properties of a forging are better in the longitudinal direction (parallel to lines of flow) direction versus the transverse direction (perpendicular to lies of flow). Conversely a casting is homogeneous this means that the mechanical properties of a casting are the same, regardless of the orientation of test bar material.

In essence, what we want is for the material's grains to be alligned with the long direction running along the length of the tubes and dropouts. This orientation is analogous to fiber orientation in a composite. Below is a rough comparison of grain orientation is a few different methods:

Casting: Grain orientation is not easily controlled. However, more complex shapes can be produced with lower capital investment.

Forging: Grains are aligned perpendicular to th movement of the forging hammer. However, the grains are aligned in the 2-dimensional plane, not along a single line.

Extrusion: As the extrusion is made, the grains are aligned parallel to the pulling direction

Welding: Reliant on the quality of the substrate material. Any metal that is melted and resolidified (which will be the immediate weld zone) will have non-directional grain formation. However, the material of the tube (block, whatever) that is not melted, like the Heat AffectedZone, will retain its orientation.

There is another issue that we ought to address is welding. Even though investment casting can get rid of the 90* weld at the dropout-tube interface, we still need to join the dropout to the frame/fork (see WTP Scorpio forks above).

Fly aslo does this, but in a bit cleaner way (think Klein aluminum frames).

This joint requires high quality welding to be done right. Although BMX has evolved to the point where this is not a leap, I lean more to avoiding joints wherever possible.

Which leads right into the Conclusion: I am very excited to see this process being used in BMX. Fly and Eclat appear to do it a bit cleaner than the standard WTP stuff, but it is good none the less. As noted above, there are still a couple limitations to using this method. As far as forks go, I think the issue has been solved by the Odyssey Director forks. Hopefully they will become more polished as time goes on, and maybe we will one day see one-piece forks *fingers crossed*. The next step is how to do this on the frame (one proposal is to use the seatstays as the top of the dropout, and the chainstays as the bottom, then just weld the middle-front of the dropout).

About-me-tidbit: I worked with a company doing titanium metal deposition using a plasma welding method. One of the major issues we ran into was the uncontrollable grain orientation in the work piece. In some situations this was not an issue, others a blessing, and others still, a real issue. It shows how we ought to design components with specific manufacturing methods in mind.