Die Material Research Articles

Effect of Die Surface Composition on Friction and Adhesion in Deformation Processing

An experimental investigation was carried out to determine interface friction and adhesion characteristics of workpiece and die materials under pressures and temperatures typical of plastic deformation. Evaluations were made using a twist-compression technique, which provided conditions of enforced sliding, and a ring upsetting technique in which sticking friction was predominant. Hot-rolling tests were performed to correlate the results with conventional metalworking practices. Workpiece materials included aluminum, titanium, and molybdenum alloys, low-alloy steel, and a nickel-base superalloy. Tools were made of die steels, tungsten carbide, and diffusion-coated die materials to evaluate the effects of surface chemistry and hardness in unlubricated contact with the workpiece material. Significant variations in friction due to die surface composition were observed, the effects being more marked with forced interface sliding. The most promising die materials were boronized 4140 steel and titanium carbide-coated 4140 steel. The investigation has shown that with the proper choice of die and workpiece material combinations, friction can be reduced and more importantly, adhesion can be minimized thereby avoiding or delaying catastrophic die failure in the event of lubricant breakdown.

THE ROLE OF SURFACE ENERGY OF ADHESION IN METAL WORKING

The concept of surface energy of adhesion can be applied in a number of ways to reduce friction during metalworking operations. Theoretical considerations suggest that the die material should be insoluble in the workpiece material, as indicated by the mutual phase diagram. If a liquid lubricant is used, then it should either form a solid surface film or have a high surface energy and wet the die and workpiece. A solid lubricant should have either a low shear strength or a low surface energy and a thickness in the range 10‐6‐10‐2 cm. The paper describes how these requirements are met in practice.

Surface hardness, compressive strength, and abrasion resistance of indirect die stones

HE common use of the indirect method in the preparation of inlays, threequarter and full crowns, various bridge forms, and other dental castings, together with the appearance on the market of newer type gypsum products for use as dies, indicates a need for more knowledge of some of the physical properties of the quick-setting artificial stones. In addition, an understanding of the best method of die preparation is necessary in order that the stone may be used to its fullest advantage. Every dentist who includes in his practice gold castings, or other precision techniques requiring dies, should ‘have a thorough knowledge of the factors involved in the fabrication of the die which lead to its improvement. Such a study should enable him, with a minimum of extraneous effort, to produce the most satisfactory stone die obtainable by practical methods. Recently, much has been written about the hydrocolloid technique for use in crown and bridge procedures. Special equipment for taking agar impressions has received widespread recognition, and the accuracy of the material is acclaimed by many dentists as superior to that of other impression materials. This method of impression making, however, requires that the operator use artificial dental stone as a die material inasmuch as no other well-accepted method appears practicable. Amalgam cannot be packed readily into the hydrocolloid impression because of the frailty of the impression material. The hydrocolloid impression cannot be copperplated unless special preparations and extremely careful methods are employed, although some operators have succeeded in accomplishing this. The technique is not a simple one, however, nor can it be done with the equipment generally found in the average dental office. The regular and improved dental stones have been found to be reasonably satisfactory as a die material to be used in conjunction with the hydrocolloid impression materials. In the past, objections have been made to the use of dental