Influence of Copper Content on 6351 Aluminum Alloy ... · and magnesium (Mg) as major alloying elements, and they are present roughly in the proportion required for the formation

2351-9789 © 2015 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).Peer-review under responsibility of the NAMRI Scientific Committeedoi: 10.1016/j.promfg.2015.09.014

Influence of Copper Content on 6351 Aluminum Alloy Machinability

Ricardo Augusto Gonçalves1,2 and Márcio Bacci da Silva2 1Federal University of Valleys of Jequitinhonha e Mucuri, Diamantina, Brazil.

2Federal University of Uberlândia, Uberlândia, Brazil. [email protected], [email protected]

Abstract

The main purpose of this work is to study the influence of copper content (Cu) in machining of 6351 aluminum alloy (Al-Si-Mg). The machinability was evaluated from measurements of drilling torque and drilling thrust force in cutting tool and surface roughness of the machined surface during drilling process. Samples of 6351 aluminum alloy were produced with different levels of copper (Cu) (0.07, 0.23, 0.94, 1.43 and 1.93%). Cutting speed and feed rate were varied in five levels (60 to 100 m/min and 0.1 to 0.3 mm/rev). The results showed that increasing the copper content in a Al-Si-Mg-Cu aluminum alloy increase the precipitation hardening through the stabilization of hardening phases like Al5Cu2Mg8Si6 and Al2Cu, also the increase in amount of Al2Cu. Thus, the samples of 6351 aluminum alloy with higher copper content, in general, showed higher hardness and ultimate tensile strength values, while the percentage elongation after fracture was lower. The drilling torque and drilling thrust force during drilling process increased almost linearly with feed rate increase. The samples with higher amount of copper (1.43% and 1.93%) led to higher drilling torque and drilling thrust force for feed rate over than 0.2 mm/rev. Although, the surface finishing of these samples machined holes was slightly worst. Keywords: Copper Content, 6351 Aluminum Alloy, Machinability, Machining, Drilling,

1 Introduction Aluminum is one of the metallic materials most used in metalworking industry and its use has

greatly increased in the aeronautics and automotive areas. Low weight/strength ratio, good electric and thermal conductivity, mechanical strength and good machinability are some of the properties that improved their market share. The aluminum due to its excellent qualities has taken important place in engineering applications, making it the most produced non-ferrous metal in the metallurgical industry.

To attend the necessary requirements for engineering applications, aluminum is usually combined with other chemicals elements in the alloys form. Aluminum alloys 6XXX series contain silicon (Si)

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and magnesium (Mg) as major alloying elements, and they are present roughly in the proportion required for the formation of magnesium silicide (Mg2Si), thus making that alloy heat treatable. Although not as resistant as the alloys of 2XXX and 7XXX series, 6XXX series alloys have good formability, weldability, machinability and corrosion resistance, with medium strength. Alloys in this group can be formed in the T4 temper (thermally treated by solution, but not by precipitation) and hardened after the forming to the properties of the T6 (thermally treated by solution and artificially aged) by the heat treatment of precipitation. The uses include architectural applications, bicycle frames, transport equipment, bridges rails and welded structures (ASM, 1992).

The 6351 aluminum alloy as well as 6082 and 6005A alloys contains a superior amount of Mg2Si than 6063 and 6061 alloys with a substantial silicon excess. A 0.2% Si excess increases the strength of an alloy containing 0.8% of Mg2Si in about 70 MPa (Tiryakioglu & Staley, 1996). Beyond silicon and magnesium, other elements in smaller amounts, like iron, copper, manganese, chromium, zinc, titanium, are also added to Al-Si-Mg series aluminum alloys to give different properties. When copper (Cu) is added to Al-Si-Mg aluminum alloys, the Al-Cu-Mg-Si alloys family is formed with several properties and applications. The aging response of these alloys is often complex due the occurrence of many intermediate phases. Large strength increases can be achieved adding Cu in Al-Si-Mg alloys, in addition of substantial refinement of precipitate structure (Tiryakioglu & Staley, 1996). Increasing the copper content of 1 to 6%, the tensile strength increases from 152 to 402 MPa and the hardness increases from 45 to 118 HB (Zeren, Effect of Cooper and Silicon Content on Mechanical Properties in Al-Cu-Si-Mg, 2005). Man, Jing, & Jie (2007) found that the addition of 0.6% Cu to the 6082Al–Mg–Si alloy clearly increases the peak hardness and reduces the time to reach the peak hardness. The hardness value of the alloy with 0.6% Cu was always distinctly higher than that of the alloy without Cu during isothermal treatment at 250 °C. According to Jaafar et. al (2011) the addition of Cu (0.1 wt. %) results in refinement of needle-shaped precipitates and may also increase the density of precipitates amount. Also Kim et. al (2013), demonstrate that low Cu and Ag additions enhance the hardness and kinetically accelerate the formation of Mg2Si precipitates, which is the main hardening phase in Al-Mg-Si alloys.

Due to the wide use of aluminum alloys, mainly in automotive and aeronautic industry, and their large participation in the market, the producers need more knowledge about the alloys behavior in manufacturing process to provide more technical data for his clients. In this scenario the machining is one of the most used processes in the industry.

The machinability can be evaluated by one or more criteria as tool life, metal removed rate, cutting forces, surface finish, chip shape, temperature, etc. (Trent & Wrigth, 2000). Compared to other materials, aluminum is easy to machine. However, considering the wide range of available alloys, it is necessary to go into detail about the characteristics of machining of aluminum alloys (Johne, 1994).

The machinability of 6351 (T6) alloy was investigated and the results showed that machining tools with PCD (polycrystalline diamond) compared with carbide tools, led to lower feed forces and cutting forces in addition to better surface finish (Reis & Abrão, 2004). Furthermore the amount of silicon in 6351 aluminum alloy has shown influence on cutting force. Smaller values of cutting force were obtained with 1.2% silicon content. The greater the amount of silicon in the aluminum alloy better is the surface finishing of the machined part (Da Cunha & Da Silva, 2012). Zedan & Alkahtani (2013) machining heat-treated (T6) Al–10.8Si cast alloys demonstrate that the increase in the levels of Cu and/or Mg in this alloy has a detrimental effect on drill life. Such an effect may be attributed to the formation of large amounts of the coarse blocklike Al2Cu phase, together with the formation of thick plates of the Al–Si–Cu–Mg phase. Also the chip breakability of the alloys containing the Al2Cu phase is superior to that of the alloys containing Mg2Si. Thus, combined additions of Cu and Mg are expected to further refine the size of the chips produced. According to Elgallad et. al (2010) the addition of Sn or Bi to an Al-2.3Cu-1.2Si-0.4Mg alloy intended for automotive castings decreases its cutting force and moment significantly, which in turn improves the machinability of the alloy. Also the addition of Bi increases the fragility of the chip considerably whereas no distinct change in chip

Influence of Copper Content on 6351 Aluminum Alloy Machinability Goncalves, and Da Silva

684

characteristics was caused by the addition of Sn. Already Barzani et. al (2015) found that the change of flake-like eutectic silicon into the refined lamellar structure increased surface roughness and decreased machinability of Al–12Si–2Cu cast alloy. However, formation of Bi compound which acts as lubricant during turning can be more likely a reason to obtain the best surface roughness and lowest main cutting force value compared to the base and Sb-containing workpieces.

Although there are available data on how the copper addition influence the mechanical properties of Al-Si-Mg aluminum alloys and the influence of this chemical element in other aluminum alloys machinability, more research and knowledge is needed about how this copper addition and consequently modification caused on microstructure and material properties can influence the behavior of Al-Si-Mg aluminum alloys during machining. Thus, the main objective of this paper is to study the influence of copper (Cu) in machining of aluminum alloy 6351. The machinability will be evaluated from measurements of drilling torque and drilling thrust force in cutting tool and surface roughness of the machined surface.

2 Methodology and Experimental Procedures Five bars of 6351 (T6) aluminum alloy thermal treated (solubilized and artificially aged) with

different levels of copper was manufactured by the Brazilian Aluminum Company (CBA) especially for this work. Samples of 6351 aluminum alloy were produced with different levels of copper (Cu) (0.07, 0.23, 0.94, 1.43 and 1.93%), and have chemical compositions of other alloying elements practically constant within the range recommended by the American Society for Testing and Materials (ASTM) for this alloy (as Tab. 1). The specimens used for drilling tests were prepared with dimensions 150 mm x 40 mm x 28 mm.

The drilling tests were carried out on a machining center Romi Discovery 760. The cutting tool was a Dormer drill A920 ¼” HSS-Co with 6.35 mm diameter and 130° drill point angle (Fig. 1). Cutting speed and feed rate were varied in five levels (60 to 100 m/min and 0.1 to 0.3 mm/rev). For each cutting condition a test was conducted with two replications, the results present the average values of torque, feed force and surface roughness obtained. An emulsion of vegetable base oil Vasco1000 was used as cutting fluid with 7% concentration applied as flood.

The equipment used for the measurement of drilling torque and drilling thrust force consists of a rotating dynamometer Kistler, model 9123C1211, amplifier and signal conditioner Kistler, model 5019A, signal acquisition card NI PCI-6036E and LabVIEW 7.6 software from National Instruments.

Table 1: Chemical composition of 6351 aluminum alloys samples

Samples Alloy Al

(%) Si

(%) Fe

(%) Cu (%)

Mn (%)

Mg (%)

Cr (%)

Zn (%)

Ti (%)

1

6351

98.06 0.80 0.19 0.07 0.44 0.40 0.003 0.006 0.026

2 97.92 0.77 0.17 0.23 0.48 0.41 0.003 0.006 0.030

3 97.02 0.88 0.21 0.94 0.50 0.41 0.003 0.006 0.026

4 96.54 0.88 0.21 1.43 0.49 0.41 0.003 0.005 0.030

5 96.06 0.87 0.21 1.93 0.49 0.40 0.003 0.005 0.027

(*) ASTM - 0.7-1.3 0.5 0.10 0.4-0.8 0.4-0.8 - 0.2 0.2 (*) reference value relative to the standard NBR ISO 209 (ABNT - Brazilian Standard


685

In Fig. 1 are shown the cutting tool, specimen and rotating dynamometer mounted on the machine tool. The values of roughness parameter Rq (root mean square deviation) were obtained using a Form Talysurf 50 profilometer by Taylor Hobson Precision with cut-off 0.8 mm and a measuring length of 8 mm.

To characterize the mechanical properties of the five samples of 6351 alloy aluminum hardness

and tensile tests were performed. The Vickers hardness was determined using a Wolpaert Universal tester with 20 kg load using a pyramidal diamond indenter with a square base in a time of 30 s. The tensile tests were performed in a universal testing machine with 300 kN Shimadzu. Three specimens for each sample of 6351 aluminum alloy were machined with measures specified by the standard NBR 6152 (ABNT - Brazilian Technical Standard Association, 2002).

The microstructure analysis of the 6351 aluminum alloy samples with different copper contents was characterized by a TM3000 Hitashi scanning electron microscope (SEM) equipped with X-ray energy dispersive spectrometer (EDS) systems. The polished samples were etched by Kellers reagent.

3 Results

3.1 Microstructures In Fig.2 are shown the (SEM) microstructures of the five samples of 6351 aluminum alloy with

different contents of copper with 400x magnification. Porosity in aluminum alloys is classified into two kinds: (i) macroporosity (~1–10 mm), which is mainly comprised of massive shrinkage cavities, and occurs in long-freezing range alloys, caused by failure to compensate for solidification shrinkage, and (ii) microporosity (~1–500 μm), distributed more or less homogeneously, due to the failure to feed interdendritic regions, and the precipitation of dissolved gases (i.e., gas porosity) (Samuel, Doty, Valtierra, & Samuel, 2013). In all aluminum alloys samples investigated was observed the two types of porosity like marked in yellow arrows in Fig. 2.

As mentioned previously, the addition of copper in Al-Mg-Si alloys form numerous intermediate phases. According to Ji, Guo, & Pan (2008) the main phases in the as-cast microstructure of an Al-Mg-Si alloy with 0.3% Cu are Mg2Si, Si, Al5Cu2Mg8Si6 (Q), Al5FeSi, α-Al(MnCrFe)Si and CuAl2. During the homogenization process, most of Al2Cu, Q and Mg2Si are dissolved, and plate-like Al5FeSi particles are transformed into multiple, spherical α-Al(MnCrFe)Si particles. In a first analysis it was found α-Al(MnCrFe)Si constituents present in an irregular shape such as light grey Chinese script-like marked in black arrows in Fig. 2. In detail is shown in Fig. 3 the microstructure with EDS analysis for the sample 5 (1.93% Cu). It can be observed the main phases light grey Chinese script-like Al(MnCrFe)Si, dark grey block-like Al2Cu and white Al5Cu2Mg8Si6 (Q) plates and particles.

Figure 1: Cutting tool, specimen and rotating dynamometer mounted on the machine tool


686

Figure 2: SEM Microstructures of the 6351 aluminum alloy samples with different copper content; porosity (yellow marked); Al(MnCuFe)Si constituent (black marked)


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3.2 Hardness The average Vickers hardness and their standard deviations for the five samples of 6351

aluminum alloy with different copper content are shown in Fig. 4. Analyzing that graph these samples can be divided into three groups. In the first group the samples S1 and S2 which contain copper content up to 0.23% showed the lowest hardness values among the five samples, 107.7 HV and 108.3 HV respectively. Since the standards deviations of these measures overlap is not possible to say that exist a statistical difference between the hardness of them. The second group is formed by the sample S3 having the copper content of 0.94%. The sample that has the intermediate copper content among the five samples also showed intermediate hardness of 112 HV. And finally, the third group was formed by samples S4 and S5 with higher copper content of up to 1.93%; they showed higher hardness values among the five samples, 121 HV and 125 HV, respectively. With increasing Cu content, the hardness increases due to precipitation hardening (Zeren, Karakulak, & Gumus, 2011). This happens because increasing Cu stabilizes the Al5Cu2Mg8Si6 and Al2Cu phases and also increases the amount of Al2Cu, hardening phases in these aluminum alloys (Chakrabarti & Laughlin, 2004). Again, as the standard deviations of measures overlap is not possible to say that exist a statistical difference between the hardness of them. In general the average hardness of the aluminum alloy tends to increase with increasing content of copper. An increase in the amount of 0.07% to 1.93% caused a 16% increase in the average hardness of the alloy (107.7 HV to 125 HV).

Figure 3: EDS analysis for aluminum alloy sample 5 with 1.93% Cu

Al(MnCrFe)Si

Al2Cu Al5Cu2Mg8Si6


688

Figure 4: Vickers Hardness of 6351 aluminum alloy samples

100

105

110

115

120

125

130

Vic

kers

Har

dnes

s (H

V)

S1 - 0.07% Cu

S2 - 0.23% Cu

S3 - 0.94% Cu

S4 - 1.43% Cu

S5 - 1.93% Cu

Figure 5: Strength versus percentage elongation after fracture of 6351

aluminum alloy samples

0

50

100

150

200

250

300

0 1 2 3 4 5 6 7

Stre

ngth

(MPa

)

Percentage Elongation After Fracture (%)

S1 - 0.07% Cu

S2 - 0.23% Cu

S3 - 0.94% Cu

S4 - 1.43% Cu

S5 - 1.93% Cu

3.3 Tensile Tests In Tab. 2 are presented the average values of the mechanical properties obtained in the tensile tests

carried out on five samples of 6351 aluminum alloy with different amounts of copper. In the Fig. 5 is present the curve strength versus percentage elongation after fracture for one specimen of each sample in order to better illustrate the behavior of deformation caused by the load application to the different samples.

Table 2: Average values of the mechanical properties obtained in the tensile tests for the five samples of aluminum alloy 6351

Sample Yield strength (MPa)

Ultimate tensile strength (MPa)

Breaking Strength (MPa)

Percentage elongation after fracture (%)

1 74.1 251.3 247.5 6.11 2 79.2 262.5 258.7 5.91 3 86.5 262.7 257.9 5.37 4 95.1 258.4 249.7 4.95 5 103.7 274.8 273.4 4.92


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The average values of ultimate tensile strength and its standard deviation are shown in Fig. 6. It can be seen that the sample S1 with the lowest amount of copper (0.07%) was the one with the lowest average ultimate tensile strength (251.3 MPa) among the tested samples. In turn, the sample S5 with higher copper content (1.93%) showed ultimate tensile strength of 274.8 MPa. Comparing samples S1 and S5, which are the extremes with respect to copper content, the increase in copper content of 0.07% up to 1.93% meant an increase in strength by about 9%. In contrast, samples S2, S3 and S4 with intermediate levels of up to 1.43% copper showed intermediate values of mechanical resistance in the range of 258 MPa to 262 MPa, but as the standard deviation of these samples overlap is not possible to say that there is a real difference between the mean values of resistance shown by these samples.

The average values of percentage elongation after fracture with its standard deviation for the five

samples tested are present in Fig. 7. As performed with hardness values, the five samples can be divided into three groups. In the first group the samples S1 and S2, which contains the lowest levels of copper (up to 0.23%) showed the highest values of elongation after fracture in the range of 5.91% to 6.11%. Since the standards deviations of these measures overlap is not possible to say that exist a statistical difference between them. In the second group sample S3 with intermediate copper content (0.94%) had intermediate elongation (5.37%) among the five tested samples. And finally, the third group samples S4 and S5 with higher concentrations of copper (up to 1.93%) had the lowest values of elongation after fracture in the range of 4.92% to 4.95%. The presence of Cu leads to the formation of Al2Cu particles, which when refined and dispersed, improve machinability by causing a decrease in plasticity, and ultimately result in chip embrittlement (Grum & Kisin, 2003). Again, as the standard deviations of these measures overlap is not possible to say that exist a statistical difference between the elongations suffered by them. Thus, by comparing samples S1 and S5, which are the extremes in terms of the amount of copper, it can be observed an apparently small decrease in elongation values of 6.11% to 4.92%. However, this represents that there was a decrease in elongation, and hence the ductility of about 25% when increased from 0.07% to 1.93% copper content present in the aluminum alloy 6351.

Figure 6: Ultimate tensile strength of 6351 aluminum alloy samples

230

240

250

260

270

280

290

Ulti

mat

e T

ensi

le

Stre

ngth

(MPa

) S1 - 0.07% Cu

S2 - 0.23% Cu

S3 - 0.94% Cu

S4 - 1.43% Cu

S5 - 1.93% Cu


690

Figure 8: Drilling Torque (N.m) versus cutting speed (m/min)

0,90

0,95

1,00

1,05

1,10

1,15

1,20

60 70 80 90 100

Dri

lling

Tor

que

(N.m

)

Cutting Speed (m/min)

S1 - 0.07% Cu

S2 - 0.23% Cu

S3 - 0.94% Cu

S4 - 1.43% Cu

S5 - 1.93% Cu

f = 0.2 mm/rev

Like discussed for the hardness values, the results of mechanical strength and elongation could be

explained for the increase in precipitation hardening caused by the increasing in Cu content in the aluminum alloy. Also, increasing the Cu content increase the amount of Cu-rich intermetallic phases, like block like particles Al2Cu, responsible for hardening these aluminum alloys. So, the samples with more Cu content showed high mechanical strength and low plasticity.

3.4 Drilling Torque The average drilling torque obtained in the tests is shown in Figures 8 and 9. In Fig. 8 is present

the variation of drilling torque with increase of cutting speed. It is not possible to observe any tendency of increase or decrease of the torque as the cutting speed increases. For approximately all cutting speeds, sample S2 with 0.23% Cu, intermediate hardness and mechanical strength, which was the lower torque values generated during machining. On the other hand samples S4 e S5, which showed highest values of hardness and strength in addition of lowest elongations, led to higher values of torque during drilling tests.

The behavior of torque increasing the feed rate is present in Fig. 9. In this case it can clearly be

seen that increasing the feed rate causes an almost linear increase in torque during the drilling, with values of 0.6 N.m for lowest feed rate (0.1 mm/rev) and reaching values between 1.4 and 1.6 N.m for

Figure 7: Percentage elongation after fracture of 6351 aluminum alloys

samples

4,6

4,8

5,0

5,2

5,4

5,6

5,8

6,0

6,2

6,4

Perc

enta

ge E

long

atio

n A

fter

Fra

ctur

e (%

)

S1 - 0.07% Cu

S2 - 0.23% Cu

S3 - 0.94% Cu

S4 - 1.43% Cu

S5 - 1.93% Cu


691

Figure 9: Drilling Torque (N.m) versus feed rate (mm/rev)

0,5

0,7

0,9

1,1

1,3

1,5

1,7

0,1 0,15 0,2 0,25 0,3

Dri

lling

Tor

que

(N.m

)

Feed Rate (mm/rev)

S1 - 0.07% Cu

S2 - 0.23% Cu

S3 - 0.94% Cu

S4 - 1.43% Cu

S5 - 1.93% Cu

vc = 80 m/min

Figure 10: Drilling Thrust Force (N) versus Cutting Speed (m/min)

420430440450460470480490500510

60 70 80 90 100Dri

lling

Thr

ust F

orce

(N

)


S1 - 0.07% Cu

S2 - 0.23% Cu

S3 - 0.94% Cu

S4 - 1.43% Cu

S5 - 1.94% Cu

f = 0,2 mm/rev

the biggest feed rate (0.3 mm/rev). For the highest values of feed rate, the samples S4 e S5 showed greater torque values.

3.5 Drilling Thrust Force The average drilling thrust force obtained is presented in figures 10 and 11. In Fig. 10 there is not a

noticeable variation of the thrust force with increasing cutting speed on drilling. It can be seen that the samples S4 and S5 with higher copper content provided higher values of thrust force. These are the samples having higher values of hardness and mechanical strength, and at the same time, a lower percentage elongation after fracture, and consequently lower ductility.

In the Fig. 11 the variation of drilling thrust force due to the increase of feed rate is presented. The

increase in the feed rate increases the area of the material to be sheared, and thus, the thrust force increases almost linearly. Again the samples with higher copper content showed higher thrust force.

The main factors which influence the machining force are the shear strength of the material in the primary and secondary shear plans, and also the areas of these shear plans (Machado et. al, 2011). Thus, the results show that in this case the predominant factor influencing the thrust force component was the shear strength of the material. Samples S4 and S5 of 6351 aluminum alloy with higher copper content were those that provided higher values of this force in apparently all cutting conditions. With respect of ductility, more ductile materials tend to have greatest areas of shear, tending to increase the


692

Figure 11: Drilling Thrust Force (N) versus Feed Rate (mm/rev)

200

300

400

500

600

700

800

0,1 0,15 0,2 0,25 0,3Dri

lling

Thr

ust F

orce

(N

)

Feed Rate (mm/rev)

S1 - 0.07% Cu

S2 - 0.23% Cu

S3 - 0.94% Cu

S4 - 1.43% Cu

S5 - 1.93% Cu

vc = 80 m/min

force components machining. As the samples of the aluminum alloy tested in this study underwent heat treatment and have a considerable amount of alloy elements, their strain capacity is restricted. So, the ductility variation caused by the change in copper content of these samples was not a factor that caused influence in force values, prevailing the difference between the strength of the samples.

3.6 Surface Roughness The variation of the surface roughness parameter Rq (root mean square deviation) as a function

of cutting speed is shown in Fig. 12. In general, the higher the strength and hardness of the aluminum alloy to be machined, the smoother is the surface produced (Johne, 1994). However, in the experiments, the sample S4 with 1.43% copper, one of the samples with hardness and mechanical strength intermediate between the tested samples showed the highest roughness values for apparently all cutting speeds tested. The surface roughness of a machined part depends on several factors, such as the workpiece material, the tool geometry, feed rate, possible vibrations, and presence of built-up edge (BUE), among others. In this case, the tool geometry and feed rate used were the same for all alloys in each test. Vibration signals were not acquired during the tests from external agents. The collected chips could show traces of BUE, but this will not be discussed in this paper. Another important observation is that the expected decrease in roughness with increasing cutting speed, because the missing of built-up edge (BUE) did not occur.

Figure 12: Surface Roughness Rq (μm) versus Cutting Speed (m/min)

0,00,51,01,52,02,53,03,5

60 70 80 90 100Surf

ace

Rou

ghne

ss R

q (μ

m)


S1 - 0.07% Cu

S2 - 0.23% Cu

S3 - 0.94% Cu

S4 - 1.43% Cu

S5 - 1.93% Cu

f = 0.20 mm/rev


693

Figure 13: Surface Roughness Rq (μm) versus Feed Rate (mm/rev)

0,00,51,01,52,02,53,03,5

0,10 0,15 0,20 0,25 0,30Surf

ace

Rou

ghne

ss R

q (μ

m)

Feed Rate (mm/rev)

S1 - 0.07% Cu

S2 - 0.23% Cu

S3 - 0.94% Cu

S4 - 1.43% Cu

S5 - 1.93% Cu

vc = 80 m/min

The Rq roughness showed a significant increase with the feed rate increase (Fig. 13), this behavior is expected, as the peaks height and the valleys deep of the feed marks tend to increase with the feed rate increase (Machado, Abrão, Coelho, & Da Silva, 2011). In this case, the sample S5 with 1.93% which have higher hardness and mechanical strength, and low ductility, resulted in the worst surface finish for apparently all feed rates tested.

4 Conclusions The investigation about machinability of 6351 aluminum alloy with different levels of copper, has

led to some important conclusions about their behavior during the drilling tests developed in this work:

• Increasing the copper content in a Al-Si-Mg-Cu aluminum alloy increase the precipitation

hardening through the stabilization of hardening phases like Al5Cu2Mg8Si6 and Al2Cu, also the increase in amount of Al2Cu. Thus, the samples of 6351 aluminum alloy with higher copper content, in general, showed higher hardness and ultimate tensile strength values, while the percentage elongation after fracture was lower.

• The drilling torque and drilling thrust force increased almost linearly with feed rate increase. • Regardless the influence of copper content in the machinability, it seems that the samples S4 and

S5 with higher amount of copper (1.43% and 1.93%) led to higher drilling torque and drilling thrust force for feed rate over than 0.2 mm/rev. Although, the surface finishing of the holes of samples S4 and S5 was slightly worst.

Acknowledgements The authors would like to thanks the FAPEMIG (Fundação de Amparo à Pesquisa do Estado de

Minas Gerais) and CAPES (Coordenação de Aperfeiçoamento de Pessoal de Nível Superior) for the financial support, CBA (Brazilian Aluminum Company) for aluminum alloys specimens, the LEPU (Laboratory for Teaching and Researching in Machining), the Mechanical Engineering Department of Federal University of Uberlândia(UFU) and the Science and Technology Institute (ICT) of Federal University of Valleys of Jequitinhonha e Mucuri (UFVJM).


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Influence of Copper Content on 6351 Aluminum Alloy ... · and magnesium (Mg) as major alloying elements, and they are present roughly in the proportion required for the formation