<?php global $current_user; if ( isset($current_user) ) {echo $current_user->user_login;}?>

add_action('admin_init','set_user_default_cat_wpa89057');
function set_user_default_cat_wpa89057(){
    global $current_user;
    if ( isset($current_user) ) {
        update_option( 'default_category',  $current_user->user_login);
    }
}

function setcategory_2_username($id){
   global $current_user;
   $category_id = get_cat_ID( $current_user->user_login );
   wp_set_post_categories( $post_id, array($category_id) );
}
add_action('save_post', 'setcategory_2_username');