A: There can be definite problems in transitioning from hammers to screw presses in that the tooling is often under different stress conditions. In my experience with screw presses (particularly with steels), the problems of scale and lack of pre-forming can lead to some unusual problems because of the "one blow" situation.
The use of screw presses for aluminum should be just fine, except when forging grades like 7075 or 7050 (low overheating temperatures). They are excellent for 6061 and other aluminum alloy grades that have high overheating temperatures.
The die lives with aluminum forgings can be affected by the starting surface conditions and the die finishing methods. For example, if the preforms are pickled before forging and the screw press is being used to forge in one blow (versus hammers with two or more blows), the lubricant system is challenged to a greater degree than for hammers (where it is more typical to lube between blows).
If the die finishing is done incorrectly, there is a greater chance of die pick up where aluminum tends to pressure-bond to the die surfaces (die galling). What can happen is that the re-polishing of dies — to remove the galled aluminum — can affect the die life and appear to be excessive tool wear. In fact, the galling often becomes more pronounced on the areas that were re-polished due to the typical "flapper wheel scratch" patterns.
As your follow-up note implied, you are forging horseshoes that are nested. These can result in what are called flow-through defects if the metal gets trapped between the legs of the U-shape. This condition can cause the forgings to be weakened and likely to fail under certain load conditions. Thus, attention to flash and gutter design is essential to avoid this problem. The key is to be sure that the internal gutters do not fill up tight or the flow-through condition will occur.
Your die-finishing methods can be leading to excessive die pickup and, as I indicated above there is more than one way to improve the situation. One method is to etch the forged blanks in caustic solution and then dry them out quickly in a baking oven (to avoid pitting corrosion) before heating for forging. Since alloys 5051 and 6061 contain copper, this process leaves a copper-rich layer on the surfaces of the forging. It serves as a good parting agent as well as assists in the lubrication, along with the graphite lube.
Another way is to dip the forging blanks in a water-based graphite lube and again drying this coating before the forge heat. This method works fairly well for alloys that do not contain copper. Still another method involves re-tempering the dies at about 950°F after polishing to leave a fairly hard oxide on the die surface, which will discourage the die pick up and galling.
H. James Henning answers forgers' technical and operational questions. For more than 40 years he held key technical positions in the forging industry, most recently as director of technology for the Forging Industry Association. He is president of Henning Education Services, Columbus, OH, specializing in customized education and training in forging technologies.
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