WooCommerce - Default order by SKU

<?php
  // WooCommerce - default catalog ordering by SKU
  // code goes in functions.php for your child theme
  add_filter('woocommerce_get_catalog_ordering_args', 'catalog_orderby_sku');
  function catalog_orderby_sku( $args ) {
    $args['meta_key'] = '_sku';
    $args['orderby'] = 'meta_value';
    $args['order'] = 'asc';
    return $args;
  }