Aluminum 7075-T6

Decide on what limits the part. If it is strength- or fatigue-limited, 7075-T6 wins clearly: roughly 500 MPa yield against about 275 MPa for 6061-T6, which is what lets you thin a wall or shrink a section. If it is stiffness-limited, 7075 buys nothing — both alloys have essentially the same elastic modulus (about 70 GPa), so a bracket that deflects too much in 6061 will deflect the same in 7075 at higher cost. If the assembly will be welded, use 6061. Put the temper on the drawing in both cases; "7075" alone is not a specification.

Not by conventional fusion welding. 7075 is prone to hot cracking, and the heat-affected zone loses the T6 properties the alloy was chosen for. Design the joint mechanically instead — fasteners, press fits, threaded inserts, dowels — or move the welded members to 6061 and keep 7075 for the machined, loaded parts. Friction stir welding of 7075 exists, but it is a specialist process, not a machine-shop operation.

It anodizes, but not identically. Type II and Type III hardcoat both work on 7075; the zinc and copper in the alloy make the anodic layer duller and a little darker than on 6061, and colour anodizing comes out less vivid and can vary between heats. Treat anodizing on 7075 as a functional finish — corrosion and wear protection — and put cosmetic colour on 6061 parts where you can. Specify masking for threads and datum faces, and state which dimensions apply after coating. Unlike 6061, bare 7075 should not be left exposed: anodize or chromate any surface that will see moisture.

T651 is T6 plate that has been stress-relieved by stretching. Specify it for anything thin-walled or asymmetric machined from plate: removing material from ordinary T6 stock releases residual stress and the part moves. T73 (T7351 for plate) is over-aged to resist stress-corrosion cracking at a cost of roughly 10–15% strength; call it where the part carries sustained tensile stress in a humid or marine environment, especially across the short-transverse grain direction. Plain T6 is fine for compact, symmetric parts in benign service. Whatever you choose, write it on the drawing next to the alloy.

Distortion, not cutting. 7075 machines cleanly and holds dimension well; the difficulty with thin walls is residual stress in the stock releasing as material comes off, plus cutting pressure flexing the wall. The practical answers are stress-relieved T651 stock, symmetric roughing from both sides, a finishing pass with light engagement, and — for the thinnest sections — a stress-relief step between roughing and finishing. On the drawing, identify which wall thicknesses and flatness callouts are functional so inspection is planned in the intended restraint condition rather than free-state everywhere.

Harder to cut, easier to finish. 7075-T6 runs around 150 HB against roughly 95 HB for 6061-T6, so it takes more spindle power and wears tooling faster — that is the sense in which it is the harder material, and the reason it rates lower for machinability and costs more per part. But it is far less gummy: 6061 tends to build up on the cutting edge and produce stringy chips, while 7075 breaks chips cleanly and holds a finer surface finish, which is why a finishing pass is often more predictable in 7075. The real machining risk on 7075 is neither hardness nor chips — it is distortion as residual stress leaves the plate, and the answers to that are stress-relieved T651 stock, symmetric roughing from both sides and a light finishing pass. Choose the alloy on the load case first; machinability changes the price, not the engineering.