Sad to say, the inspector is probably correct. Aluminum welds are somewhat prone to what are called fine-line LOF discontinuities. The term fine line refers to where the weld lies firmly against the joint preparation or underlying weld pass without melting into it and fusing. This discontinuity can be extremely difficult to detect, even with radiography (see Figure 1). It is very subtle, but if you look carefully at the left fusion line of the weld, near the top in the pass before the cap passes, the straight fusion line at a 45-degree angle is actually an LOF. I think you can see why such discontinuities are hard to detect. We can attribute the cause to a few possibilities. The aluminum might not burn through if excess oxide exists on the joint preparation before welding or on the weld bead before you make the next pass. Be sure to clean the area or the previous pass with a wire brush to remove the oxide before welding. Next, it’s possible that your welding parameters are set too cold to melt the surrounding material. The weld in Figure 1 shows that the inside welds and the outside welds are not aligned. The welder just held the torch too far to one side and never really fused to the left-side weld preparation. There is a practical consideration here in qualifying aluminum welders. Most codes allow the fabricator to qualify welders using either bend tests or radiography. Most welders prefer to take bend tests because they think the bend test is easier to pass. It isn’t. As you have seen here, radiography can easily miss LOF discontinuities. On the other hand, if you bend test an aluminum weld that contains any LOF, the discontinuity will pop right open.