As all the postings here are really good information I have to add that I believe a vertical down weld can be just as strong as a vertical up if done properly. I have welded on a lot of 3/4" angle in a vertical down fashion and to this day I have never had a vertical down fail. If a welder is experienced enough then he/she should be able to establish a good vertical down weld. As far as heat transfer well I think that depends on your method of welding, travel speed, etc... I like to use what I call the back stepping method which can help with distributing the heat in a vertical down weld. I do a lot of robot programming and have used both vertical ups and downs and have had success both ways.
