The default format for TIME to character conversion is the format in effect for the TIME value.
You can convert a TIME value to a character string using a FORMAT phrase. The resulting format, however, is the same as the TIME value. If you want a different format for the string value, you need to also use CAST as described here.
You must use nested CAST operations in order to convert values from TIME to CHAR and force an explicit FORMAT on the result regardless of the format associated with the TIME value. This is because of the rules for matching FORMAT phrases to data types.