Plavka SKAKUN1, Igor KAČMARČIK1, Tomaž PEPELNJAK2, Ognjan LUŽANIN1 Aljosa IVANIŠEVIĆ1, Mladomir MILUTINOVIĆ1
Faculty of Technical Sciences, University of Novi Sad, Trg Dositeja Obradovica 6,Novi Sad, Serbia
Faculty of Mechanical Engineering, University of Ljubljana, Aškerčeva 6, Ljubljana, Slovenia
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Abstract:Many different tests for determination of friction coefficient in metal forming have been developed. Some of them are general, while the others are specific for certain metal forming methods. One very often used test, particularly for bulk deformation processes is the ring compression test.
In this paper ring compression test is used for friction coefficient determination for three different lubricants, two conventional (oil for cold forging and phosphated surface with MoS2 ), and one new environmentally friendly lubricant (BonderlubeFL 741). Aim of the test was to prove that new kind of lubricant can achieve good lubricating conditions and can be used as a replacement for conventional environmentally hazardous lubricants.
Key words: ring compression test, environmentally friendly lubricants
1Name of the author, title, company, address and e-mail.
2Name of the author, title, company, address and e-mail.
- INTRODUCTION
Main tasks of lubricant in metal forming processes are to reduce friction, lower the deformation force and to prolong tool life. Lubricants in cold bulk metal forming processes, especially in extrusion, must have abilities to withstand some additional requests. These processes have certain characteristics (high normal pressure between workpiece and tool,large surface expansion of the workpiece etc.) and lubricant is subjected to very sever conditions. Lubricating system which can withstand these conditions is phosphate coating with some additional soap based lubricant. It has been in use for a long period of time (since 1930’s) because it’sability to obtain good lubricating conditions in processes where high pressures, frictionand temperatureoccur.
Use of phosphate coatings has negative environmental impact because of several drawbacks (large amounts of waste water, bath sludge containing heavy metals etc.), and therefore efforts have been made to improve existing lubricating systems and to develop new.
In this paper values of friction coefficient for different types of lubricants are compared.Both environmental friendly and environmental hazardous lubricants were used with the aim to prove that environmentally friendly lubricant can obtain same friction conditions within lubricating system as environmentally hazardous lubricant.
- CONVENTIONAL TRIBOLOGICAL SYSTEM - PHOSPHATE COATING
Wide application and development of cold extrusion is connected with application of phosphate coating. This coating, usually containing zinc, is lubricant carrier. Because of chemical reaction that occurs between lubricant and coating, lubricant adhesion to the billet is improved.
There are several steps in process of zinc phosphate coating forming and they can be dividing in three groups: cleaning of billet, phosphate coating generation and lubrication [7].
First group consists of surface cleaning operations and preparation of surface for phosphate coating. Mechanical cleaning, such as shot blasting and peeling, can prepare surface for better adhesion of the zinc phosphate coating and lubricant, chemical cleaning is applied with the aim to degrease billet, and this is followed by rinsing, pickling, and again rinsing.The pickling operation prepares the surface of the billet for the zinc phosphate coating process, usually sulfuric or hydrochloric acid is used. After this operation billet is rinsed with water in order to neutralize remaining acid.
In the zinc phosphate coating operation, a zinc phosphate coating is formed on the steel surface. It is conversion coating and zinc phosphate is chemically bonded to the metal base. This operation is also followed by rinsing.
The coated part is providedwith lubricant by dipping into a hot bath of alkaline soap (e.g. sodium stearate), that reacts with the zinc phosphate to form zinc stearate. Layer of zinc phosphate has crystalline structure which has dual role, it chemically binds soap to the surface and acts as a physical carrier for the soap. After lubrication, it is necessary to dry the billets.
Althoughlubricating system with zinc phosphate coating shows many advantages, especial in cold extrusion processes, from mentioned procedure it can be seen that from environmental point of view it has negative impact on the environment [3].Those negative impacts are: large water requirements in rinse baths, sludge containing heavy metals, necessity of periodic replacement of baths, large amounts of waste waters containing grease, oils, acid and soap etc. In addition to these reasons phosphating process requires longer treatment time than new trybological systems.
- NEW TRYBOLOGICAL SYSTEMS
New tribological systems which can be used instead of zinc phosphate coating have been developed in two directions: new conversion coatings and lubrication without conversion coating [3].
Basic method for the first group is conversion coating method, only modified and improved comparing to phosphate coating procedure, eliminating many drawbacks related to zinc phosphates. One of these methods is electrolytic phosphate coating. The main advantages of this procedure are: sludge free phosphating bath is obtained, use of acid for pickling may be avoided by electro chemical pickling, cycle time is considerably shortened, working environment is improved. This procedure also makes possible to phosphate high alloyed steels and stainless steel. It can be seen from the Fig, 1, which shows SEM micrographs of conventional, chemical phosphate coating and electrolytic phosphate coating, that electrolytic phosphate coating ensures much more uniform and finer crystalline coating with smaller film thickness.
Fig.1. SEM micrographs of conventional and electrolytic phosphate coating [3]
Another method which belongs to group of new conversion coatings is microporous coating. Different techniques have been developed to produce porous surface layer in which liquid lubricants can be entrapped. Depending on the applied method layer thickness is from 1μm to 5 μm, and friction coefficient which can be achieved is as low as for phosphate coating plus soap lubrication.
Two concepts of lubrication without conversion coating are presented in paper [3]. First is dual bath system, where base coating is adhering to the workpiece surface on top of which over-coating is formed to reduce friction. Second is single bath system, developed in several variations. One of them is method called “dry-in-place” where double coating is formed consisting of a lubricant carrier as base with a lubricant film on top. This lubricant consists of an inorganic salt as base component and a wax as a lubricant. Process is simple for application (dip and dry process) and obtained coating can achieve similar lubricating conditions to the coating formed by phosphating and soap. Similar lubricant, Bonderlube FL 741 was used in experiment described in this paper,
- RING COMPRESSION TEST
Experimental tests for determination of friction coefficient between materials of tool and workpiece within the metal forming processes are numerous. There are tests developed for specific manufacturing processes, (rolling, extrusion, deep drawing etc., ) or tests of general purposes.In this investigation general test was used (ring comperssion test). Results of the test was used to compare friction coefficient values for different lubricants.
The idea for the ring compression test was firs presented by Kunogi [5] and later improved by Mail and CockCroft [6], Avitzur [1] and many other researches. Basic technique of this test relates dimensional changes of a ring specimen to the coefficient of friction. When a ring specimen is compressed between two flat dies, depending on the value of friction coefficient, value of inner diameter can increase or decrease. High friction coefficient results in inward flow of the material, while low friction coefficient results in an outward flow of the material. To determine friction coefficient it is necessary to measure change in height and in inner diameter of the ring, to calculate percentage value of nominal strains εh i εd and to compare εh -εd curve to calibration curves. In this research calibration curves, calculated acording Avitzurs equations [1] were used. In Fig 2 calibration curves are presented.
Fig.2.Calibration curves
- EXPERIMENTAL DETERMINATION OF FRICTION COEFFICIENT
Ring compression experiment was conducted in order to determine friction coefficient for three different kinds of lubricants, two conventional (oil for cold forging and phosphated surface with MoS2), and one new environmentally friendly lubricant (Bonderlube FL 741).
5.1.Preparation of specimens
Material of rings was Č4320. Phosphated rings was lubricated with MoS2,
To apply Bonderlube lubricant at the ring surface certain procedure was followed.
According to the manufacturer instructions phases of the Bonderlube application are: shot blasting, immersion in hot dematerialized water and Bonderlube application followed by hot drying.
If shot blasting can not be applied as a surface preparation, it can be replaced by usual surface preparation for phosphate coating forming.
Before application Bonderlube needs to be diluted by adding demineralized water. Concentration of dilution depends on products types and expected deformation rate, and can be between 60% and 90%. Recommended standard concentration is between 70% and 80%. Temperature of the process is 60ºC, during application bath must be gently moved to avoid product stratification. Also localized overheating must be avoided, temperatures over 70ºC, lead to product coagulation. Time of application is from 3 to 7 minutes, what depends on workpiece dimensions.
After application of lubricant, workpiece needs to be dried at the 100ºC
5.2.Experiment
Common geometry of a ring specimen which is used in this test has proportion of outer diameter to inner diameter to height 6:3:2.This proportion was used in current experimental investigation too. Dimensions of rings were: outer diameter D = 18 mm, inner diameter d = 9mm and hight h = 6 mm. Ring specimen for compresion test is presented at Fig 3. Experiment was conducted on Sack&Kieselbach hydraulic press, nominal force 6.3 MN, using flat dies.
Fig.3.Specimen for ring compresion test
At Fig. 4ring with applied Bonderlube FL 741 lubricant on its surface is presented.
According to experimental values of height and inner diameter for each ring, percentage value of nominal strains εh i εd are calculted and εh -εd curve is compared to calibration curves. At Fig.5 comparison of experimental and calibration curves are presented. Values of experimentally determined friction coefficients are in table 1.
a)
b)
Fig.4. Ring with BonderlubeFL 741 lubricant on its surface a) before test b) after test
Fig. 5. Calibration and experimental curves
Table 1 Experimentally determined friction coefficient values for different lubricants
No /Lubricant
/Friction coefficient
Oil for cold forging / ≈ 0.11Phosphated surface + MoS2 / ≈ 0.11
BonderlubeFL 741 / ≈ 0.09
- CONCLUSION
Ring compression test was used for determination of friction coefficient for three different types of lubricants.
Two convencional lubricants were used (oil for cold forging, phosphate coating with MoS2,) and one environmental friendly lubricant , Bonderlube FL 741. Comparing with conventional ways of lubrication there are several advantages in both economical and environmental way, when Bonderlube FL 741 lubricant is used. Experimental results showed that value of friction coefficient for Bonderlube lubricant is lower (μ≈0.09), compared to conventional lubricants (μ≈0.11).
Future work implies determining and comparing friction coefficient of old and new lubricants using test for specific method, e.g. cold extrusion. In this way it would be possible to evaluate lubricant suitability for specific method.
ACKNOWLEDGEMENT
This paper is a part of the investigation within the project EUREKA E!5005
REFERENCES
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1Name of the author, title, company, address and e-mail.
2Name of the author, title, company, address and e-mail.