Abrasive Grain Efficiency and Surface Roughness Under Wheel-Loading on Machining Magnesium Alloys

Nguyen Tien Dong, Koji Matsumaru, Masakazu Takatsu and Kozo Ishizaki

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AZojomo (ISSN 1833-122X) Volume 6 August 2009

Topics Covered

Abstract
Keywords
Introduction
Experimental Procedure
Results
Discussions
Conclusions
Acknowledgements
References
Contact Details

Abstract

Wheel-loading is one of the prime factors that determine the grindability of the grinding wheel. It can lead to an increase in grinding forces and grinding temperature to a level which causes deterioration in the surface integrity of ground samples. Newly developed cup-type diamond-grinding-wheels with hexagonal pattern were used to grind a magnesium alloy AZ31B in the present work. The number of abrasive grains which pass through a unit length of a sample surface for each grinding pass, Ng was calculated to evaluate the ground surface and the abrasive grain efficiency of the grinding wheel under wheel-loading conditions. Without wheel-loading, surface roughness data forms one curve in roughness vs. Ng graph for all hexagonal wheels, and forms another curve for the conventional grinding wheel. The difference of two curves indicates that the number of effective working abrasive grains in hexagonal wheels is about 5 times higher than that of the conventional wheel. When wheel-loading occurs, ground surface is damaged, i.e., becomes rougher surface in spite of higher wheel rotation speed and higher Ng. Roughness data for hexagonal wheels under loading conditions fall on the line of the conventional wheel in roughness vs. Ng graph. The results indicate that number of effective working abrasive grains in hexagonal wheels under loading conditions decreases, and reaches a similar value for that of the conventional wheel.

Keywords

Wheel Loading, Magnesium Alloys, Hexagonal Structure, Cup-type Diamond Grinding Wheel, Surface Roughness, Grinding

Introduction

The need for light metals as materials for lightweight constructions has arisen from the effort of the automobile industry to achieve lower fuel consumption. Magnesium alloys are the metallic materials with the lowest density used in construction (ñ = 1.8 g.cm^-3) and offer a potential for weight reduction of up to 35%, even in relation to aluminum [1]. This enables car manufacturers to meet regulatory requirements for lighter weight vehicles and a corresponding reduction in emissions, despite the demand for more and more on-board equipment [2].

The main concern in grinding these light materials is the "wheel-loading", which can be defined as the state of a grinding wheel when the particles of the work-piece material either adhere to the grits or become embedded in the spaces between abrasive grains on the grinding wheel. Under this situation, grinding forces and grinding temperature can rise to a level leading to deterioration in the surface integrity of the grinding wheel and higher wheel wear rate, owing to fracturing of grits or bonds.

Yossifon et al. revealed that the area of metal transferred to a wheel when loading occurs in operating increases as the hardness grade of the wheel increases [3]. Yossifon et al. proposed the way to detect active grains on a grinding wheel surface and mentioned that the number of active grains decreased by adhesion of work-piece material to active grains under loading conditions [4]. However, active abrasive grains are defined as those which act directly to ground surface. On a grinding wheel surface, abrasive grains distribute randomly with different protrusion heights. Many abrasive grains moving one after another grain and pass through to grind a sample surface. Only grains located at different position of groove formed by a previous grain or grains with high enough protrusion can touch and form another groove to remove ground material. Other grains, which are located to go through the same groove formed by previous grains and/or have lower protrusion height, do not touch ground surface. In other words, those grains do not work effectively. Therefore the way to detect number of active grains proposed by Yossifon et al. could not satisfy real number of active grains. Nguyen et al. have been developed a new method to detect number of active grains by surface roughness measurement [5]. By evaluating the number of abrasive grains which pass through a unit length of a sample surface for each grinding pass, they revealed that the abrasive grains work effectively for newly developed cup-type diamond grinding wheels with hexagonal pattern is about 5 times higher that those of the conventional wheel on grind a hard-to-machine ceramics [5].

In the present work, a light metal, which was represented by a magnesium alloy AZ31B (ASTM - American Society for Testing & Materials), was ground by using newly developed cup-type diamond grinding wheels and the conventional wheel. The aim of this paper is to evaluate active abrasive grains under wheel-loading conditions for machining light metals.

Experimental Procedure

A regulated-force-feeding (RFF) grinding system, which was developed by Kim et al., was employed in the present work to evaluate grinding speed [6, 7]. The table feeding force was kept constant at 3.8 N during process. Samples were magnesium alloys AZ31B (Al 3%, Zn 1%, Mn 0.45%) (Kamado Laboratory, Nagaoka University of Technology, Nagaoka, Japan) with dimensions of 50 x 40 x 1.5 mm, and was placed on a vacuum vise. Grinding stone ratio, R is defined as the ratio between the hexagonal edge area containing abrasive grains and the total area of the wheel surface. Four newly developed diamond grinding wheels with different R (13%, 19%, 25% and 36.0%) and a conventional wheel (R 100%) (#200, vitrified bond 20 weight %, NanoTEM Co. Ltd., Nagaoka, Japan) were used as shown in Table 1.

Table 1. Grinding stone ratios, R for hexagonal wheels and a conventional wheel.

Grinding wheel rotation speeds, n were 500, 1500 or 3000 rpm, cutting depth was 10 µm per pass, and total cutting depth was 200 µm. The wheels were re-dressed before each grinding experiment. The coolant water was sprayed into the contact zone between grinding wheel and sample during truing, dressing, and grinding process with 0.5 l/min flow rate. Grinding conditions are listed in Table 2.

Table 2. Specifications of grinding wheels, grinding conditions and sample.

During the grinding process, the grinding time for each grinding pass was recorded to obtain table feeding speed, Sf. Since the table is fed at constant force, the table feeding time is governed by the grinding conditions, such as grindability and grinding depth. The sample surface roughness, Ra was measured by using a profile-meter (Surfcom 3000A, Tokyo Seimitsu Co. Ltd., Tokyo, Japan).

Results

Using RFF machining system Sf can be calculated as:

where D_O, D_I outer and inner diameter of grinding wheel respectively, L_s 50 mm length of sample, and t is time to grind for each grinding pass.

Figs. 1 (a) and (b) shows S_f and R_a as a function of R for different n (500, 1500 and 3000 rpm). Wheel loading does not occur for n = 500, 1500 rpm, but only for 3000 rpm. For all grinding wheels, at a given n, S_f is reduced as R increases. S_f becomes faster as n increases. Surface roughness decreases as n and R increase. For a given n, higher Ra, i. e., a rougher surface is obtained by the conventional wheel. When wheel loading occurs for n = 3000 rpm, ground surface is damaged and surface roughness increases in spite of higher wheel rotation speed n.

Figure 1. (a) Table feeding speed, S_f as function of R in percent for different n, 500, 1500 and 3000 rpm. For all grinding wheel, at a given n, S_f reduces as R increases. (b) Sample surface roughness, Ra as function of R for different n, 500, 1500 and 3000 rpm. Ra decreases as n or R increases.

Discussions

The Effective Working Abrasive Grain

The number of abrasive grains passed through a unit length of sample surface, Ng is calculated as:

where L_A is Archimedes' spiral length, and z_a abrasive grains in a unit area (about 30 grains / mm² for the present case). The detailed method of calculation is presented elsewhere [5].

Surface roughness data for all hexagonal wheels without loading form in one curve, i. e., Curve (I) as shown in Fig. 2.

Figure 2. Surface roughness, R_a versus number of abrasive grains that passed through a unit length of a sample surface, Ng for hexagonal wheels and a conventional wheel without wheel loading. Surface roughness data for all hexagonal wheels form on one curve, i. e., Curve (I).

Star marks show a R_a data for conventional wheel. Ng of conventional wheel is larger than that of hexagonal wheels at same Ra. Ra data for grinding wheels under wheel loading conditions were added in Fig. 3. R_a data for conventional wheel and Ra data for hexagonal wheels under wheel loading conditions form another curve, i.e., Curve (II) as show in Fig. 3. When wheel loading occurs, ground surface is damaged, i. e., R_a increases for hexagonal wheels also.

Figure 3. Surface roughness, R_a versus number of abrasive grains that passed through a unit length of a sample surface, Ng for grinding wheels with and without loading conditions. R_a data for hexagonal wheels under loading conditions and data of R_a for conventional wheel form one curve, i. e., Curve (II).

The possible cause of this phenomenon is the different number of effective working abrasive grains between conventional wheels, hexagonal wheels and the grinding wheels under loading conditions. By multiplying one fifth to the value of Ng of Curve (II), Curve (II) superposes on Curve (I) as show in Fig. 4.

Figure 4. Surface roughness, R_a versus number of abrasive grains that passed through a unit length of a sample surface, Ng for each grinding pass. Curve (II) of the conventional wheel is superposed on Curve (I) of hexagonal wheels by multiplying one fifth to the value of Ng on Curve (II).

In other words, the number of effective working abrasive grains of hexagonal wheels is about 5 times higher than that of the conventional wheel and hexagonal wheels under wheel loading conditions. The number of effective working abrasive grains in hexagonal wheels under loading conditions behaves a similar manner to that of the conventional wheel. The possible cause of this phenomenon is adhesion of work-piece material to abrasive grains and filled up the spaces between grains on grinding wheel thus many abrasive grains cannot work to remove work-piece material. By multiplying one seventh to the value of Ng of conventional wheel under wheel loading conditions, the Ng superposes on Curve (II) as show in Fig. 5.

Figure 5. Surface roughness, Ra versus number of abrasive grains that passed through a unit length of a sample surface, Ng for each grinding pass. Wheel-loading point of conventional wheel at 3000 rpm is superposed on Curve (II) at 500 and 1500 rpm without wheel-loading by multiplying one seventh to that wheel-loading point.

Therefore the number of effective working abrasive grains of conventional wheel is about 7 times higher than that of the conventional wheel under wheel loading condition. The grindability of grinding wheel can be known by this evaluation method. This is a new way to design a higher efficiency grinding wheels.

Conclusions

In this work, a magnesium alloy is ground by a conventional and newly developed hexagonal diamond grinding wheels. The following can be concluded:

1. The evaluation method for abrasive grain efficiency by surface roughness measurements allows to design more efficient grinding wheels, such as hexagonal grinding wheels. Soft light metals such as magnesium alloys are ground efficiently by the newly developed grinding wheels.
   
2. The number of effective working abrasive grains of hexagonal wheels without loading condition is about 5 times higher than that of conventional wheels in grinding magnesium alloys.
   
3. The number of effective working abrasive grains in hexagonal wheels under loading conditions decreased and reaches a similar number as a conventional wheel.
   
4. The number of effective working abrasive grains of conventional wheel without loading condition is about 7 times higher than that of conventional wheel under wheel loading condition.
   
5. The evaluation method by surface roughness measurement shows the advantage to determine active abrasive grains under loading condition in comparision with other methods up to now.
   

Acknowledgements

The author wishes to express his gratitude to the Ministry of Education, Culture, Sports, Science and Technology and the Japanese government for partially supporting this work through the 21st Century Centers of Excellence (COE) Program and City Area Nagaoka of Promotion of Science and Technology in Regional Areas.

References

1. B. L. Mordike and T. Ebert, "Magnesium Properties - Application - Potential", Materials Science and Engineering A302 (2001) 37-45.
   
2. T. Friemuth and J. Winkler, "Machining of Magnesium Workpieces", Advanced Engineering Materials, 1 [3-4] (1999) 183-186.
   
3. S. Yossifon and C. Rubenstein, "The Grinding of Workpieces Exhibiting High Adhesion, Part 1-Mechanism", Transactions of the ASME Journal of Engineering for Industry, 103 (1981) 144-155.
   
4. S. Yossifon and C. Rubenstein, "Wheel Wear When Grinding Workpieces Exhibiting High Adhesion", Int. J. Mach. Tool Des. Res. 22 [3] (1982) 159-176.
   
5. T. D. Nguyen, K. Matsumaru, M. Takatsu and K. Ishizaki, "Abrasive Grain Efficiency and Surface Roughness for Machining Ceramics by Newly Developed Cup-type Diamond-Grinding-Wheels", Adv. in Tech. of Mat. and Mat. Proc. J. (ATM), 10 [2] (2008) 77-84.
   
6. H. Kim, K. Matsumaru, A. Takata and K. Ishizaki, "Grinding Behavior of Silicon Wafer and Sintered Al₂O₃ by Constant Force Feeding Grinding System", Adv. in Tech. of Mat. and Mat. Proc. J. (ATM), 5 [2] (2003) 50-53.
   
7. H. Kim, K. Matsumaru, A. Takata and K. Ishizaki, "Reduction of Ceramic Machining Defects by Regulated Force Feeding Grinding System", Adv. in Tech. of Mat. and Mat. Proc. J. (ATM), 6 [2] (2004) 290-297.
   

Contact Details

Nguyen Tien Dong, Koji Matsumaru, and Kozo Ishizaki
Nagaoka University of Technology
Nagaoka, Niigata 940-2188
JAPAN

Masakazu Takatsu
Nano-TEM Co. Ltd.
Shimogejo 1-485
Nagaoka, Niigata 940-0012
Japan

This paper was also published in print form in "Advances in Technology of Materials and Materials Processing", 11[1] (2009) 19-24.