Although die casting lubricant is continuously exposed to changing pressures and velocities and differing degrees of wear, a dynamic equilibrium – and, therefore, realistic data – may be attained through careful control.
Excerpt from the May 2018 Issue of Gear Solutions by K.D. Clarke and C.J. Van Tyne
Both hot- and cold-chamber die casting are batch-type processes in which steady-state conditions are never fully achieved and the initial lubricant supply must perform adequately for the duration of the operation. The lubricant is continuously exposed to changing pressures and velocities, and wear or pickup products in the lubricant also continuously vary, although a dynamic equilibrium may be attained through careful control. The absence of steady-state conditions creates challenges for the systematic analysis of lubrication and wear in forging processes.
In many ways, various forging processes are competitive with one another, and the competitive position of each is greatly influenced by the lubrication system employed. Thus, hot forging followed by finish machining may be replaced by cold forging, with all the associated advantages, provided that a suitable lubricant can be found. Indeed, economy of production has often been the major impetus for the development of new forging processes and associated lubrication techniques.
In forging steels, die life is often controlled by abrasive wear. Thus, die wear and die life are often thought to be synonymous. However, there are other mechanisms by which dies are rendered unusable, including plastic deformation and fatigue failures, induced both thermally and mechanically. If the loads are high or the dies relatively soft, plastic deformation of the dies may occur, making it impossible to impart the desired shape to the workpiece. Thermal fatigue, or thermal cycling, gives rise to superficial cracks often known as heat checks. Analogous with thermally induced cracking is mechanical fatigue, or cracking, that results from the cyclic application of the forging loads. Although it is not unusual for several of these mechanisms to contribute to die failure, abrasive wear is common to almost all dies and is a significant factor when considering die life for a given process.
Methods to Measure Lubricant Effectiveness and Wear
Measuring Friction for Lubricant Effectiveness
In forging, one of the most common ways to measure friction and thus determine the effectiveness of a lubricant is the ring compression test. This technique was initially applied to cold working  and was then further developed and adapted for hot working , and offers the great advantage that frictional conditions can be judged from deformation alone, without the need to know the flow strength of the metal. The ring test methodology has been widely studied and implemented to evaluate friction conditions and lubricants in both hot and cold forging applications continuously over the past 60 years, including (for example [3-10]), and remains perhaps the most effective method for evaluating relative friction conditions efficiently.
This test is the forging equivalent of forward slip measurement in rolling and is commonly used for lubricant evaluation because simple measurement of the change in internal diameter is sufficient for ranking of lubricant effectiveness. If the specimen geometry is kept constant and the reduction in height is exactly reproduced, a reduced decrease in inside diameter indicates a reduced resistance to shear and thus a lower friction value. With zero friction at the interface, the ring expands as though it were part of a solid disk, the inside diameter increases, and velocities increase radially over the entire surface. With increasing friction, it requires less energy for some of the material to flow toward the center, and the inside diameter of the hole grows less rapidly. With yet higher friction, the internal diameter decreases and both internal and external surfaces barrel. The variation of decrease in internal diameter for unlubricated ring tests on mild and stainless steel are presented in Figure 1 .