int a[] is an array of integers.
char conv[10] is an array of characters - a single string.
How do you expect this conversion to work? If a is an array of digits, simply add '0' to them. If you really want to have an array of strings, make char conv[6][10] to have 6 strings 10 characters each.
Also, you can't do strlen(a) as a is not a string.