This happened to me also, I had a good fit up, so I tried to tig it without filler rod. It didn't work, cracked down the middle. Did the same weld with 4043 rod and the weld turned out decent. 6061 will loose it's strength next to the weld where it was hot, watch for that too. Jodi from Welding tips and tricks recently put out a very quality video about this.