Abstract: 7A52 alloy plate was welded by means of MIG (metal inert-gas)welding using Al-Mg-Mn-Sc-Zr micro-alloyed wire.The specimens cut down at different regions of the welded joint were diffracted accurately by high-power X-ray diffractometer.The results showed that there are 4 different heat diffusion zones in the welded joint, these zones had themselves substantive characteristics.The first was the weld, and it was the α-Al matrix as cast structure distanced the weld centre line about 0-4 mm.The second was the semi-melted zone, and it distanced the weld centre line about 4-10 mm.The nanocrystalline η'(MgZn₂)phase and Al6Mn re-precipitated in nature ageing process when it was laid at room temperature.The region distanced the weld centre 10-16 mm was the low temperature solution zone, and the strengthening phases redissolved into α-Al matrix partly at the weld temperature but could not re-precipitate at room temperature.The fourth was a soften zone. In this zone, the strengthening phase growth and formed the T phase.It was the reason of strength decreasing.Because the effects of welding heat on zones were different, it lead to different solid sol-ubility, distortion of lattice, the micro-strains and residual stress.