超声波检测铝合金方形锻件时,经常会在1次底波后出现一种位置固定的特殊回波。通过对直探头发射的声束在零件入射面和背面的折射和反射行为进行分析，发现由于存在小角度斜入射而引起波型转换，会在1次底波和2次底波之间固定的位置出现一种迟到波。在使用2次波探伤法或者检测带台阶的零件时，应避免把该迟到波误判为缺陷波。

A special fixed echo after the fist back reflection could be frequently received during inspecting quadrate aluminum forging. After carefully analyzing the refraction and reflection of ultrasonic beam of normal probe at test surface and back surface, we found that small angle incident led to wave form conversion, and wave form conversion led to a special kind of fixed delayed echo between first and second back reflection. When inspecting part with step or using double traverse technique, full attention should be paid to avoid mistreating this kind of delayed echo as flaw echo.