A customer said that I fabricate high-end, custom, aluminum-framed bicycles. I was able to buy round tube made from alloy 7075 in the correct sizes to make bicycle frames. I checked the mechanical properties and found out that the strength of this alloy is very high. However, I have been unable to find the correct filler metal for this alloy.why?
7075 alloy is non-weldable
I’m afraid that you have made a very common, and in your case a very costly mistake. The problem is that while most aluminum alloys are very weldable using a fusion welding process (any process that melts the aluminum, such as GTAW, GMAW, and laser welding), some are not. This alloy falls into the unweldable category. Additionally, alloy 2024 falls into the unweldable category as well. Both are very high-strength aerospace alloys used to fabricate airframe structures that were developed without any consideration of their weldability. There is a good reason that these structures are riveted and not welded.
The reason why this alloy is non-weldable
Why aren’t these alloys weldable? It all comes down to their chemistry. It’s not that you can’t get them to stick together—you can. However, the heat-affected zones of the weld are extremely prone to cracking and stress-corrosion cracking while in service. So, you could weld a bicycle frame from 7075. While it may look fine, it will most certainly fail the first, the fifth, or the 10th time you ride it. I’m sure you don’t want that.
In fact, while the alloys in the 1XXX, 3XXX, 4XXX, 5XXX, and 6XXX series are virtually all weldable, most alloys in the 2XXX and 7XXX families are not. There are exceptions to this. In the 2XXX family, 2014, 2219, and 2519 are readily arc weldable. The rest are not. In the 7XXX family, 7003, 7005, and 7039 are arc weldable. The rest are not. Please note that when I say weldable I am talking about fusion welding processes. All are weldable using more exotic processes that don’t melt the base material, like friction stir welding. But those are beyond the scope of this column.
So, back to your question. What should you do? My recommendation is to scrap this alloy you bought or divert it to applications that don’t require welding. I know that this hurts. However, if it makes you feel better, I once had to make the same recommendation to a fabricator that had bought this alloy and spent $500,000 in making parts from it before he asked the same question that you did.
My further recommendation is that the next time someone asks you to weld 7075 or 2024, run out of the room screaming and don’t look back. However, the 7005/7018/7029/7100 alloy is weldable.
Any questions please feel free to contact me.