Design a wrap around fixture* for aluminum weld specimens. While it may take more gray-matter on your part to design and build, it can also be used for steel alloys. Keep in mind different radii may be required for different material groups and tensile strengths.
*A plunger type does not work well for heat-treated aluminum/aluminum alloys. The specimens will "kink" where the heat treating has been affected (decreased) at the area adjacent to the weld (HAZ).
p.s.: The dimensions for the fixtures are in the AWS structural codes.