You made and fitted wishbones from 7075 aluminium tube, with welded joints. After three race weekends, you find yourself pulling a wishbone that has a crack on it. The crack is right at the weld, understandably. It’s a situation that is guaranteed to occur in a workshop during club racing. Most don’t understand the metallurgy involved and so it is seen as bad luck. It is designed as a bad luck scenario.
Metallurgy is a discipline that studies the properties and structure of metals and alloys. It is concerned with the effects of certain processes on the structures, properties, and behavior of metallic materials. Some of the properties of aluminum metals designed to be at its strongest as a result of a heat treatment. The 7075 category gets its strength from a heat treatment that creates tiny precipitates throughout the structures of the alloys. These precipitates make it so that the structures can withstand dislocations (which enhances its strength). Welding is one of the processes that heats the metal to the point that the heat penetrates (or damages, in this case) the structures of the alloys.
What you get is a strong component away from the weld, but the heat affected zone is significantly weaker. Base metals can have yield strengths of 500 MPa, but right next to the weld, weak spots can be 150 MPa- less than mild steel. The enclosed wishbones have a soft zone to deform under load, but the harder material surrounding the soft zone creates stress concentrations at the boundary between the hard and soft material. That’s where the crack starts.
The solution also isn’t to weld more carefully or use filler rods with a higher strength. There is a possibility of laying the most perfect welds with no defects, but there will still be cracks in the heat affected zone. The real fix is changing the grade of aluminium. The 6061 and 6082 series alloys can be more confidently said to be weldable. They still lose strength in the heat affected zone, but they lose less, and more importantly, the strength transition is more gradual which eliminates the sharp stress concentrations that can cause cracks.
Welds are one of the biggest drawbacks of using 7075 aluminum. One option is to machine the wishbones from a solid billet and avoid the welds. This is what the professional teams do, but it is way more expensive due to the CNC machining requirements. For club racers, avoid the solid billet option unless it is a championship level pursuit.
The other option is to use 7075 aluminum tube for the main wishbone structure and weld mild steel mounting tabs. The steel tabs will absorb the stress at the mounting points, and you will only be welding steel to steel, which alleviates the hassle of welding to aluminum. You will be sacrificing some of the weight, but you are gaining reliability. The aluminum tube will still carry the majority of the load along with the steel, which will help at the high-stress areas.
Post-weld heat treatment can help recover some lost strength in the heat-affected zone, but it requires taking the entire wishbone up to temp and holding it there for hours before controlled cooling. Most club racing workshops don’t have the equipment to do this, and getting the temp and timing wrong can make things worse.
When running welded aluminum wishbones, inspection becomes critical. Cracks usually begin as hairline fractures at the edge of the heat-affected zone, often on the inside radius, where they are difficult to see. Dye penetrant testing every few race weekends will help show you cracks before they reach failure. If racing at a level where safety wire is required, watch for movement or deformation in the wire pattern. This is often the first sign of something flexing more than it should.
Even if you do not have the possibility to perform a full post-weld heat treatment, stress relieving may still be worth doing. Simply heating the welded assembly to around 250-300°C and letting it cool slowly from that temperature will reduce the residual stresses from welding. It will not recover the full strength of the heat-affected zone, but it will reduce the internal stresses that contribute to crack-initiating.
The take-home lesson here os simple, not every grade of aluminium is the same when it comes to being welded. If your design needs welded joints, and you wish to have any kind of durability to it, use 6061 or 6082. If you need the strength of 7075, design the component so that it can be machined from solid, or just use mechanical fasteners instead of welds. Knowing the metallurgy helps to avoid costly lessons, when a more informed choice can be made.
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