The Role of Bench Top Tribometers in Metal Working Fluids Research

There are several manufacturing techniques like bending, deep drawing, drilling, grinding, milling, punching and cutting used for shaping the metals. The productivity of any manufacturing process will depend on the tool life and surface integrity of the finished workpiece. Therefore, metalworking fluids (MWF’s) are used in the process to reduce the tool wear and improve the surface finish of the workpiece. This is due to the performance of MWF’s as lubricants, coolants, and carriers of the wear debris.

     

Figure 1. Images representing different manufacturing processes like drilling, tube drawing, sheet bending and cutting (left-to-right).

Types and Uses of MWFs

MWF’s are classified as straight (mineral oil base) or synthetic (water base) fluids. In both, we will find several types of additives for lubrication purposes, they are known as extreme pressure or antiwear additives. These additives are polar in nature that feed on the surface defects (cracks, dislocations, etc.) generated during the manufacturing process. Adsorption of these polar molecules will decrease the surface energy and thus, the strength of the workpiece (Griffith’s theory of crack propagation).  Thereby making it easy for forming and machining of metals.

Figure 2. Ducom FBT-3 (left) and Reichert Tester (right) used in friction and wear testing of MWF’s. Ducom FBT-3 with a foolproof user interface can execute up to 15 extreme pressure or EP test per hour. Reichert tester allows the user to screen up to 10 MWF’s per hour. Both are known for high screening rates.

Chlorine-based additives are very popular among the lubricating additives used in MWF’s because of its wide range of operating temperatures (200 oC to 700 oC) compared to sulfur (700 oC to 960 oC) or phosphorus (lower than 200 oC). It is considered a general-purpose additive, applicable to a variety of manufacturing processes. However, they are dangerous to our health and environment, for example, short-chain chlorinated paraffins are toxic to aquatic life even at low concentrations (Data: US Environment Protection Agency). Therefore, several key industries in MWF’s are on a hunt for alternatives to chlorine-based additives.

Friction and wear test instruments play an important role in finding alternatives for chlorine-based additives. As shown in Figure 2, the lab-scale instruments like Ducom Four Ball Tester (FBT-3) and Reichert Tester are widely used in screening the lubricant additives. Although the FBT-3 and Reichert cannot reproduce the real conditions during manufacturing, they enable the adsorption dominated lubrication mechanism during the real process. Moreover, they are quick, reliable and reproducible.

Table 1. Tribometer parameters for four MWF’s as determined by Ducom FBT-3 and Reichert Tester. Based on the point system developed by Ducom, these fluids were ranked for its best lubricity.

Ducom FBT-3 is used to generate 12 to 15 extreme pressure tests in 60 min, according to ASTM or DIN standard. Reichert tester can provide friction and acoustic screening for 10 MWF’s in 60 min. Such a high rate of testing, that is, screening or ranking of additives is very important for the current state of research in MWF’s. Lubrication scientists think that any alternative to chlorine may not mimic its effectiveness over a broad range of operating temperatures. As a result, there will be an increase in customizing the formulations for each application (forming and machining) as the general-purpose chlorine-based additives become obsolete. Higher screening rate using the Ducom FBT3 and Reichert Tester will help in reducing the cost of customizations and deliver the best customer satisfaction.

Study Outcomes

In this study, we have used Ducom FBT-3 and Reichert Tester to screen for the best formulation among the four types of neat MWF’s. Two of them had chlorine-free formulation (fluid A and B). The results are shown in Table 1. It covers the tribology parameters like Weld Load, Brugger Number (Reichert), Acoustic Emission, Weld Load Friction Coefficient and Reichert Friction Coefficient for all the four types of MWF’s. The best formulation should have the highest Ducom MWF number. It is calculated by adding all the points per parameter for a fluid. A maximum point of 4 (number of fluids = 4) is awarded to the best tribology parameter. For example, the fluid A and C with the highest weld load will receive 4 points each. Ducom MWF number has shown that fluid A has the best lubricity formulation (extreme pressure and coolant) that can significantly improve the tool life and productivity of the manufacturing process.

Conclusions

Three important conclusions from this study are,

  1. A large number of tribology parameters should be used to determine the Ducom MWF number. It can improve the accuracy of the ranking variety of formulations. However, it does necessarily mean that we have to run more tests, but we should try to acquire more parameters during a single test
  2. In the case of fluids showing the same four-ball weld load value (Ex. fluid A and C), it is important to look at other tribology parameters like four-ball friction coefficient, four-ball temperature rise, Brugger number, etc.
  3. Fluid A was developed after a thorough screening of several chlorine free formulations. It has outperformed the chlorine-based MWF’s used for machining and grinding. This is a step closer to environmentally adapted lubricants

This information has been sourced, reviewed and adapted from materials provided by Ducom.

For more information on this source, please visit Ducom.

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