Display logged in user name and lastname on page

I want to display the first an lastname of my users on a welcome site.
I have some code in my functions.php that let me show the first name. How can I change the code to show both first and last?

This is my code for the shortcode:

function custom_shortcode_func() {
    ob_start();
    $current_user = wp_get_current_user();
    echo 'Willkommen ' . $current_user->user_firstname . '<br />';
    $output = ob_get_clean();
    return $output;
}
add_shortcode('current_user', 'custom_shortcode_func');