Need to change the File Name while getting the Image URL

Here is my code

$url_thumbnail_1x=fly_get_attachment_image_src( $attach_id, array( $imgwidth_1x, $imgheight_1x ), $crop=false);
print_r($url_thumbnail_1x);
exit();
$url_thumbnail_1_5x=fly_get_attachment_image_src( $attach_id, array( $imgwidth_1_5x, $imgheight_1_5x ), $crop=false);
$url_thumbnail_2x=fly_get_attachment_image_src( $attach_id, array( $imgwidth_2x, $imgheight_2x ), $crop=false);
$url_thumbnail_3x=fly_get_attachment_image_src( $attach_id, array( $imgwidth_3x, $imgheight_3x ), $crop=false);
$url_thumbnail_4x=fly_get_attachment_image_src( $attach_id, array( $imgwidth_4x, $imgheight_4x ), $crop=false);

I want to change name of file by default it will takes _width_X_height.png something like this
I want to change it like 1x.png , 2x.png

Please help me in this .
Many Thanks in Advance.

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