Proceedings of the 5th International Conference on Integrity-Reliability-Failure, Porto/Portugal 24-28 July 2016 Editors J.F. Silva Gomes and S.A. Meguid Publ. INEGI/FEUP (2016)

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PAPER REF: 6290

LUBRICATION INFLUENCE IN THE DRAWING PROCESS OF

ALUMINUM, STEEL AND COPPER ALLOYS Daniel de Castro Maciel

1, Norberto Martins

1, Diego Raimundi Corradi

2, Daniel Januário Cordeiro

Gomes2, Juan Marcos Santos Dutra

1, Gilmar Cordeiro da Silva

1(*) 1Polytechnic Institute of PUC-Minas (IPUC), Pontifical Catholic University of Minas Gerais, Belo Horizonte, Brazil 2University Center Una, Belo Horizonte, Brazil (*)Email: gilmarcordeiro@pucmians.br

ABSTRACT

This work aims to investigate the influence of lubricant types (solid, liquid and paste) in drawing process of metallic bars, using aluminum, copper and steel alloys with an initial diameter of 15.88 mm and, after the process, a final diameter of 14.87 mm, corresponding to a reduction of 6.36%. The evaluated parameters correspond to surface roughness and microhardness. The found results were assessed by statistical calculations of average and standard deviation.

Keywords: Drawing process, lubrication, surface roughness, aluminum alloy, copper alloy, steel alloy.

INTRODUCTION

According to Dieter (1988), drawing process is a manufacturing process where occurs plastic deformation, consisting on pulling a metallic bar with an applied force on outside of a conic tool. Drawing process is, generally, coldly executed. Consequently, the drawn material exhibits a work-hardening, increasing the mechanical strength. Besides it, a loss of ductility can be noted in the material with residual stresses. After the process, it is necessary to be done a heat treatment, aiming to minimize them (MORAIS, 2003).

Drawing process is widely used by mechanical industries. The surface roughness of drawn bars is influenced by the kind of lubricant used during process. This work aims to assess the influence of lubricant types on surface roughness of drawn bars. Other parameters, like elongation and microhardness, were still measured, with the goal of evaluating other possible influences.

COLD DRAWING

For the purpose of minimizing costs, maximizing productivity, being more competitive on global market and avoiding raw material losses when using no specified products, plastic deformation process have been modernized on the last years. Researches have been done about the process variable parameters and their influence on finished products. So, a big progress on cold drawing techniques have been verified on the last years, through automation and continuous thermal treatment improvements, making standardization and finished products quality better. It is a mechanical process which guarantees dimensional precision and best mechanical properties to the manufactured pieces (DIETER, 1981). Cold drawing is a

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conformation process where a bar or a tube is pulled through a tool called drawing die, made from a high wear resistance material like hard metal (tungsten carbide), diamond (natural or polycrystalline) and some ceramic materials (zircon oxide, titanium carbide-nitrate, etc.). The drawing die has a cylindrical external format, containing a center hole for passing the wire. This hole has a decreasing diameter, presenting a conical form profile.

According to Bresciani (2011), the drawing die has to be constructed with the purpose of dealing with high productivity and low wear, large reductions, getting a better result for superficial finishing and having a better dimensional precision. The passage of the wire causes its transversal section reduction. This operation is commonly done on a cold process, occurs a modification on wire material’s mechanical properties because of work-hardening CORRÊA (2004). Modifications that occur on ductility reduction and mechanical resistance increase direction. Thus, it is possible of needing annealing thermal treatment on every part of cold drawing process, decreasing cold drawing effects, and providing it more ductility, so that it can handle the process for longer. Deformation preponderant efforts are compression efforts, done by the tool orifice walls when line is passing through a traction effort applied on its axial direction, from an external origin. Since external effort is by traction, and deformation efforts are by compression, cold drawing process is classified as an indirect compression process.

Fig. 1 - Drawing die’s region description Drawing die’s geometrical characteristics, according to Dieter (1981), has a purpose of: Region 1 has a opener angle to allow a better lubrication; Region 2 has a material draining because of drawing die’s reactions in virtue of traction tension; Region 3 has a zero angle where occur calibration, defining final product quality; and Region 4, where has an opposite final angle when compared to its initial angle, occurring material elastic return, decreasing possibilities of occurring abrasion in case drawing die is misaligned.

According to Akikazu et. al. (2007) and Wang and Gong (2002), deformation can be reached when combining traction and compression tensions, whose are created by traction force on drawing die’s way out and its geometry. Considering its main tensions, different failing criteria can be used. A metal draining is determined by microstructure grain movement, ruled by shearing tensions. Thus, plastic deformation beginning can be determined by maximum shearing tension, based on Tresca criteria CETLIN (2005).

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SUPERFICIAL FINISHING

According to Oliveira (2011), roughness known as superficial texture, is characterized by micro-geometrical defects provided by fabrication processes. Countries like England and Germany were pioneer on studying this area. It is really important to consider roughness when determining industrial equipment components’ useful life, machinery and maintenance costs. It is also important when applied with the purpose of improving the lubrication system between pieces and/or equipments that work on dynamic superficial contact.

Superficial roughness is a function of the finishing type, the fabrication process and the machine/tool.

In Brazil, the norm ABNT NBR 6405-1985 rules superficial roughness concepts which are very important for problems analysis of wear, friction, corrosion, fatigue resistance, fluid flow etc. Superficial roughness measurement unit is expressed by microns.

In this paper, the material roughness was evaluated by the rugosimeter “Taylor Hobson”, whose cut-off was of 0.8 mm. The superficial texture was evaluated by the following amplitude parameters:

Ra: Average Roughness is determined by the medium line on roughness profile graphic.

Fig. 2 - Roughness Average (Ra)

Rt: Total Roughness correspond to the vertical distance between the highest point and the deepest valley, on evaluating length.

Fig. 3 - Roughness Profile (Rt)

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LUBRICATION

According to Keeler (2001) Lubricant is any solid, liquid or gaseous material which has low shear resistance and able to maintain the piece/tool surfaces separated during the conformation process. Lubrication goal is to avoid direct contact between corps and premature wear. The main lubricant functions are: minimize wear, friction and corrosion, refrigeration, shock absorber, electrical insulating, electrical isolator etc.

On cold drawing process, variables such as friction, wear and temperature are factors of big importance to be evaluated and always studied to increase matrix reliability and to obtain quality products continually better. Therefore, in cold drawing specifically, lubricant has to be capable of satisfying the following properties: keep material cold drawn surface and drawing die surface separated during process, not chemically reacting with metallic surfaces, and lubricant has to keep itself stable by virtue of temperature changing. In this work, the lubricants used were: Vaseline, graphite and grease.

EXPERIMENTAL TESTS

The experimental work developed in this research was divided in two stages: one referent to the fabrication of the samples and the other one referent to the material mechanical characterization tests. The steps of the experimental work were: cold drawing, superficial roughness and microhardness test before and after cold drawing tests.

MATERIALS, LUBRICANTS AND FABRICATION OF TEST SPECIMENS

The materials used in this research were SAE 1020 steel, electrolytic copper C1100 with 99.9% of copper and annealed aluminum 6351-T6 alloy. The average chemical composition of steel and aluminum alloys are shown in Table 1 and Table 2.

Table 1- Average chemical composition of steel SAE 1020

Element C Mn Si P S Cr Ni Mo

Average (%) 0,21 0,56 0,12 0,024 0,017 0,10 0,05 0,01

Source: Quality certificate of general steel, 2016.

Table 2 - Average chemical composition on annealed aluminum steel 6351-T6

Element Al Cu Fe Mg Si Ti Mn Others

Average (%) Balance 0,10 0,50 0,6 1,1 0,2 0,6 0,15

Source: Analysis certificate Albemec Metais Ltda, 2016

The specimens geometry used on cold drawing tests is shown in Figure 4.

Fig. 4 - Used specimen geometry on cold drawing tests

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The lubricants used in this work were: powder graphite, solid Vaseline USP, and molybdenum disulphide grease MOLYKOTE. This research was divided in three groups, according to the lubricant type employed on cold drawing tests, according to Figure 5.

Fig. 5 - Source: Research data

The specimens were fabricated from laminated and normalized cylindrical bars of steel SAE 1020, electrolytic copper C1100 and aluminum alloy 6351-T6. The dimensions of the bars were: 1000 mm of length and 16.00 mm of diameter. For each bar, four specimens were manufactured.

For cold drawing and mechanical characterization tests, twenty seven specimens were fabricated, nine of each material. Same dimensions for each specimen used on tests.

COLD DRAWING TESTS

In the drawing tests, a drawing bench was used. It is from Mechanical Forming Laboratory of PUC Minas campus Contagem. The Figure 6 presented the drawing bench.

Fig. 6 - Equipment used.

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SUPERFICIAL ROUGHNESS MEASUREMENT

In this study, average roughness (Ra) and total roughness (Rt) were evaluated. The measurements of those parameters were obtained by Taylor Robson rugosimeter. A sampling length (cut-off) of 0.8 mm was used, according to norm NBR ISO 4287/2002. Figure 7 illustrate the rugosimeter position on the piece, where the measurements were taken.

In this study, previous and post measurements were taken of the cold drawing specimens, being characterized by the non cold drawing and cold drawing regions.

Fig. 7 - Rugosimeter used.

MICROHARDNESS TESTS

Due to its wide use and acceptance in scientific research, this Vickers microhardness models has been adopted in this study as the hardness measurement method. The Vickers microhardness entails the same practical procedure as the Vickers hardness method, except that the applied loads are non-greater than 1.0 kgf. This method uses a diamond indentation with a squared pyramid base and internal angle of 136o among the opposing surfaces. The Vickers microhardness is defined to be the applied load divided among the imprinting area. This area is computed through the imprinting diagonals. The Vickers microhardness value (HV) is defined analytically by equation xx.

(1)

Where P is the applied load (Kgf), L is the average length between the imprinting diagonal (mm), and θ is the angle between the diamond’s opposing surfaces (θ =136°). The samples have been carefully prepared, going through the grinding process and posterior polishing with an abrasive diamond base. Those tests were done at the Mechanical Construction Materials

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Laboratory of PUC-Minas. On those tests the following have been employed: SHIMADZU Microdurometers series HMV with a 98.07 nM (10 gf) load and a 20 seconds imprinting process.

In order to realize the microhardness, two samples of each cold drawing specimen have been taken, totalling 27 samples, they were taken transversally, one from the cold drawing region and other one on the non-cold drawing one. The measurement was taken radially, starting on the boarders and moving inwards towards the sample mean. In each sample, nine measurements were realized, three in each radial direction; the first at 1.0 mm from the surface and the remaining two equally spaced at 3.0 mm. This procedure is illustrated in Figure 8.

Fig. 8 - Illustrative figure showing the adopted procedure with regards to microhardness measurement

EMPIRICAL RESULTS AND DISCUSSIONS

Figure 09 shows the surface of the drawn bar after the process with an expansion of 15x. Microhardness test was used along the transversal samples of the drawn bars to verify if there were any modifications in microhardness results after the process. It is possible to observe in figure 09 that the steel has shown a greater effort throughout the process, due to its greater cold drawing resistance.

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Steel – Graphite Lubricant Aluminum – Graphite Lubricant Copper – Graphite Lubricant

Steel – Grease Lubricant Aluminum – Grease Lubricant Copper – Grease Lubricant

Steel – Vaseline Lubricant Aluminum – Vaseline Lubricant Copper – Vaseline Lubricant

Fig. 9 - 15x Surface Expansion Basing on Table 3, it is verified that SAE 1020 steel had a better superficial finishing when using lubricant grease on cold drawing process. Analyzing the annealed aluminum alloy 6351-T6, Vaseline presented better superficial roughness results. For electrolytic copper C1100, even if graphite presents a higher average roughness value, its standard deviation percentage is much lower when compared to other lubricants.

Table 3 - Roughness

Graphite

Steel Aluminum Copper

Non cold

drawn

Cold

drawing

Non cold

drawn

Cold

drawing

Non cold

drawn

Cold

drawing

Average Ra

(µ(µ(µ(µm))))

2,245 ±1,147 1,215 ± 1,074

1,192 ± 0,721 0,363 ± 0,142

0,969 ± 0,138 0,267 ± 0,270

Average Rt

(µ(µ(µ(µm))))

18,533 ± 8,507 12,39 ± 8,413

10,162 ± 6,569 1,470 ± 0,544

8,292 ± 2,025 3,215 ± 2,143

Grease

Steel Aluminum Copper

Non cold

drawn

Cold

drawing

Non cold

drawn

Non cold

drawn

Cold drawing Non cold

drawn

Average Ra

(µ(µ(µ(µm))))

3,568 ± 1,524

0,800 ± 0,593

0,821 ± 0,090

0,255 ± 0,169

1,081 ± 0,096

0,288 ± 0,369

Average Rt

(µ(µ(µ(µm))))

31,400 ± 10,535

9,620 ± 5,207

6,985 ± 1,812

3,630 ± 2,479

7,570 ± 0,648

3,115 ± 2,414

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Table 3 – Roughness (continued)

Vaseline

Steel Aluminum Copper

Non cold

drawn

Cold

drawing

Non cold

drawn

Non cold

drawn

Cold drawing Non cold

drawn

Average Ra

(µ(µ(µ(µm))))

2,747 ± 1,020

1,507 ± 1,811

1,192 ± 0,721

0,129 ± 0,019

0,969 ± 0,138

0,267 ± 0,270

Average Rt

(µ(µ(µ(µm))))

26,900 ± 6,539

14,117 ± 12,900

10,162 ± 6,569

1,470 ± 0,544

8,292 ± 2,025

3,215 ± 2,143

MICROHARDNESS TEST RESULTS

Figures 10, 11 and 12 present the metallic materials microhardness profile submitted to three lubricant types on cold drawing process.

In Figure 10 is observed that SAE 1020 steel had a notable gain on hardening because of deformation using Vaseline on cold drawing process.

Fig. 10 - SAE 1020 Steel microhardness profile under different lubricants use in drawing process

In Figure 11 is remarked that annealed aluminum alloy 6351-T6 had a hardness increase because of deformation, highlighting the use of graphite on cold drawing process.

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Fig. 11 - Annealed aluminum 6351-T6 Microhardness profile under different lubricants use in drawing process

In Figure 12 is possible to take the fact that electrolytic copper had a hardness increase by deformation, highlighting the use of grease on cold drawing process.

Fig. 12 - Electrolytic copper C1100 Microhardness profile under different lubricants use in drawing process

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Analyzing roughness measurement results jointly using microhardness profile, is observed that even though ABNT 1020 steel has a larger superficial roughness data when using Vaseline, it had, at the same time, an increase on work-hardening results when comparing to other lubricants.

A similar behavior is observed when annealed aluminum alloy 6351-T6 is analyzed. Even presenting a higher superficial roughness result when using graphite, this alloy had a high level of work-hardening when comparing to other lubricants.

Electrolytic Copper C1100 showed high hardening by deformation (work-hardening), when using grease lubricant. On the other hand, the highest average roughness was emphasized when using graphite. And it had a too elevated roughness standard deviation value.

CONCLUSIONS

Roughness measurement has proved that lubricants influence superficial finishing, because it had different Ra and Rt parameters results when using same specimen material, and different lubricant types. When analyzing each material jointly to each lubricant, steel had lower Ra and Rt results using grease, thus a better superficial finishing, whereas aluminum and copper when using Vaseline presented better superficial finishing to drawn pieces. When compared the hardness on measured area to steel, vaseline presented a bigger hardening if related to non drawn material. In contrast, aluminum bar had better results using graphite, showing higher hardening values if compared to non drawn region. And copper using grease had the highest hardening value in measured region when compared to the non drawn region. Thus concludes that lubricants may have chemical or physical affinity on process, because of its different values when varying material and lubricant parameters.

ACKNOWLEDGMENTS

The authors gratefully acknowledge to PUC Minas, Una, Fapemig, Capes and CNPQ.

REFERENCES

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[6]-CETLIN, Paulo Roberto e HELMAN, Horácio. Fundamentos da conformação Mecânica dos Metais. 2ª ed. São Paulo: Artliber Editora, 2005.

[7]-CORRÊA, Elaine Carballo Siqueira. Aspectos do Encruamento de Metais Previamente Deformados a Frio. 2004. 262f. Tese (Doutorado) - Programa de Pós-Graduação em Engenharia Metalúrgica e de Minas, Universidade Federal de Minas Gerais, Belo Horizonte, 2004.

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