Secondary Operations

The purpose of this article is not to discuss technology gains made in secondary operations for powder metallurgy, but to provide useful information in controlling and improving secondary processes using quality control and inspection methodologies. However, development of the following secondary operations is briefly discussed:

• Restrike--densification

• Restrike--sizing

• Machining and grinding

Examples of FMEA and control plans are shown in Fig. 1 and 2, respectively, for the restrike--densification process to illustrate the use of such tools when assigning process control methods and inspection instructions to a process. Additional information on statistical process control concepts and terminology are also discussed in the article "Planning and Quality Control of Powder Metallurgy Parts Production" in this Volume.

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Restrike--Densification. Restrike operations are frequently used to densify parts above the density attainable at compacting. In these instances, more emphasis is placed on attaining the desired material property through densification than on achieving tight dimensional limits (although dimensional control is also important). Adequate control must be placed on the process to ensure density requirements (and dimensions) are met. Because density checks are not readily performed in a production environment, dimensional controls may be used to control density if correlation can be proved to be sufficient. When densifying thin parts, greater dimensional and weight control may be required at compacting to avoid overdensification and damage to the restrike press and/or tooling. Press and tool load monitors are effective process control tools.

Restrike--Sizing. Sizing using a restrike operation is used to enhance dimensional characteristics on P/M parts. In this instance, the density of the part has already been met in the compacting and sintering operations. Sizing is used to meet tighter dimensional limits on diameters, lengths, tapers, and so forth. The quality control methods assigned for sizing operations depend on the characteristic sized and the capability of the machine in achieving the desired limits of size; highly capable processes require very little in-process or post-process inspection while marginally capable processes require significant and frequent inspection.

Heat Treat. The properties from heat treatment (such as hardness and tensile strength) are dependent on many process variables. Carbon potential, atmosphere, heat treat temperature, quench rate and temperature, and draw temperature affect the properties of parts. A FMEA of the heat treatment should be done to list potential failures caused by incorrect settings of the above inputs. To minimize potential process failures, process control of the heat treat operation should include monitoring the inputs as well as the outputs. Minimal variation of these parameters results in significantly reduced variation of product characteristics. Again, depending on the stability of the process, more or less dimensional and metallurgical inspection is required as post-process inspection.

Machining/Grinding. Although machining and grinding are different technologies, process control issues are similar for each. Variation in the process can and should be minimized in the selection of capable machinery, the design of the fixturing, and the selection of the tooling (inserts, cutters, grinding wheels, etc.). Process control and inspection techniques vary depending on the characteristics being generated, the capability of the process, and the quality requirements specified on the blueprint (classification of characteristics). Machine controls have allowed technologies such as automatic compensation and redundant tooling to significantly reduce process variation due to tool wear and other time-related variations. The selection of the machine to be used is a big factor when analyzing the FMEA and developing control plans.

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