Influence of cutting parameters on surface roughness of as cast and RRA treated AA7075 - A comparative study

For obtaining better combination of properties some of the parameters such as ageing time, solutionization temperature, ageing temperature, retrogression temperature and retrogression time are chosen meticulously. Machining studies were conducted on as-cast and RRA treated samples. The influence of cutting parameters on the surface roughness is studied. The optimum cutting parameters are evaluated through Taguchi DoE.

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INFLUENCE OF CUTTING PARAMETERS ON SURFACE ROUGHNESS OF AS-CAST AND RRA TREATED AA7075 - A COMPARATIVE STUDY

D Venkatesh, N D Prasanna, *Balasubramanya H S,

Sanjith Girish Kamath, Daruk Lulla

Ramaiah Institute of Technology, Bangalore, India

*Corresponding Author: baluhs.md@gmail.com

ABSTRACT

Abundantly available aluminium is being used in various applications such as air craft, automotive industry etc due to its high specific strength and good corrosion resistance To achieve desired results, knowledge of the manufacturing process employed and behavior of metal under various working conditions are utmost important Several engineering applications such as fly motors, airframes, space transports, hypersonic rockets and remote ocean submersibles, and so forth demands materials with tailored set of properties that can't be obtained by employing the ordinary solid materials There is a need for lightweight materials which exhibit high strength, stiffness and high wear resistance which are primarily required in structural, aerospace and automotive industries In present work a sincere attempt has been made to study some of the important properties of heat treatable AA7075 aluminium alloy subjected to an heat treatment procedure known as Retrogression and Re-ageing (RRA) to achieve property enhancement For obtaining better combination of properties some of the parameters such as ageing time, solutionization temperature, ageing temperature, retrogression temperature and retrogression time are chosen meticulously Machining studies were conducted on as-cast and RRA treated samples The influence of cutting parameters on the surface roughness is studied The optimum cutting parameters are evaluated through Taguchi DoE

Keywords: Rattan fiber, composites, manufacturing, car spoiler

Cite this Article: D Venkatesh, N D Prasanna, Balasubramanya H S, Sanjith Girish

Kamath, Daruk Lulla, Influence of Cutting Parameters on Surface Roughness of

as-Cast and RRA Treated AA7075 - A Comparative Study International Journal of

Mechanical Engineering and Technology 11(1), 2020, pp 16-22

http://www.iaeme.com/IJMET/issues.asp?JType=IJMET&VType=11&IType=1

1 INTRODUCTION

7xxx series lightweight aluminium are wide used in applications such as aviation, marine and defence due to their superior mechanical properties The several investigation carried out by researchers focuses on casting of alloy and evaluation of its composition, heat treatment of

microstructure analysis with optical microscope (OM), evaluation of mechanical properties, wear studies using optical and scanning electron microscopes, analysis of results and drawing conclusions AA 7075 is an heat treatable alloy is used in the present investigation If this alloy is subjected to heat treatment, improvement in some of the properties has been observed

as suggested in some of the literature

The surface finish is a critical factor in the production of machine components The surface roughness affects wear resistance and fatigue strength The efficiency of the metal cutting activity is constrained by the surface quality

For light weight components and structures aluminium and magnesium are considered In aerospace applications Al-Zn castings are widely used due to their excellent combination of properties such as better castability, low thermal expansion coefficient, better strength-to-weight ratio, and energy required for recycling is less

Wallace et al started conducting investigation on retrogression and re-ageing in 1980, which aims out at modifications in the existing method so as to extend it to bigger parts [1] they have Also stressed on improved stress corrosion cracking resistance and strength for T6 levels for the process condition at the second retrogression hardening peak Retrogression at lower temperatures took around 360 sec to 180 min, which has influence on the properties so

as to be used for bigger section parts

ES-Said O S et al [2] Considered the Aluminium alloy 2024 for the investigation, subjecting it to solution treatments at 495, 505, and 515°C followed by quenching it in water

It was then artificially aged at 190 and 208°C Hardness, tensile tests, Impact and fatigue tests were carried out on the heat treated alloys Based on tensile test results, fatigue tests (bending-rotation test machine) were carried out The best combination of property is seen for solution temperature of 505°C and ageing temperature of 208°C for 2 h

Taguchi strategy is a marvelous statistical framework It gives basic, proficient and orderly ways to deal with quality and cost Taguchi technique is productive strategy for planning process that works reliably and ideally over a range of conditions

In present study, Taguchi DoE is used for optimizing the process parameters with only a small number of experiments [6] Taguchi design methodology optimizes the process parameters to improve the quality of parts that are fabricated Taguchi's strong plan strategy is

an integral asset for the structure of an excellent framework [7]

Since good surface finish is one of the important requirement in aerospace application, the present study aims at optimizing the cutting parameters Hence machinability study on this material has been carried out Trial ware conducted for as-cast, T6 temper and RRA heat treatment T6 treatment is conducted at a solutionizing temperature of 460ºC for two hours followed by water quenching at 120ºC for twenty-four hours Later it is being followed by RRA heat treatment Retrogression is carried out at 200ºC for fifty min followed by water quenching and reheating the specimen at 120ºC for twenty-four hours Mechanical tests and surface roughness tests were carried out for a batch of as-cast, T6 temper and RRA treated examples

The aim of the current investigation is to study the effect of cutting parameters on surface roughness on as cast and Retrogression and Re-Aged specimen and to determine the optimum cutting parameters to minimize surface roughness when machined on a conventional lathe

2 EXPERIMENTAL DETAILS

The alloy chosen for this work is AA 7075 (Al-Zn-Mg) alloy It is one of the high strength alloys that can be easily heat treated The chemical composition of this alloy and its properties are given in the Table 1 and Table 2

Table 1 Chemical composition of Aluminium alloy (AA7075)

Table 2 Properties of AA7075 alloy

2.1 Fabrication Setup

The melting unit is shown in Fig 1 is an electric resistance-heating furnace of 4KW heating capacity, with a controller to monitor the temperature within ±20 C accuracy The furnace can attain a maximum temperature 12000 C The melt temperature was maintained at 8100 C

Figure 1 Melting furnace

For the development of the composites, AA7075 alloy ingots were first melted in a electrical resistance melting furnace in a graphite crucible The melt was stirred with a graphite stirrer continuously During stirring, speed was increased continuously till vortex was created for the homogeneous mixing of the particles in the matrix Degassifier in powdered form was added to the molten metal to remove entrapped gases Scum powder is added to the melt to remove the impurities collected at the top surface Molten metal was poured in the precoated-preheated cast iron molds and allowed to cool in air at room temperature Samples were prepared for characterization of the composite

2.2 Heat Treatment

Heat treatment is carried out in 2 stages namely (a) Solution heat treatment and age hardening (T6 temper) (b) Retrogression and Re-aging (RRA) As cast specimen are subjected to treatment temperature of 460°C for a duration of 2 hours It is followed by quenching in water bath maintained at 60°C with the time delay less than 15 seconds The specimens were then subjected to age hardening treatment to improve hardening properties The specimens were subjected to precipitation heat treatment at a temperature of 120°C for a duration of 24 hours The specimens were then quenched again in water bath maintained below 600C

The specimens were then subjected to retrogression heat treatment in a furnace maintained

at 200°C The duration of 50min was maintained The specimens were again quenched in water bath maintained below 60°C

This is followed by re-aging heat treatment process in Electrical muffle furnace at 120°C for a duration of 24 hours The specimens were then quenched in water bath maintained below 60°C

2.3 Machining Studies

The experiments were designed based on three levels and three factors These experiments were conducted according to the Taguchi's beta structure and L9 array

The experiments were conducted by employing conventional lathe HSS was used as the cutting tool material for machining the work pieces The work piece used has a dimension of diameter 22 mm and length of 110 mm

3 RESULTS AND DISCUSSIONS

3.1 Machinability Tests

The machinability tests were conducted as per Taguchi’s experimental style and L9 orthogonal array shown in Table.3 Cutting parameters like speed, feed rate and depth of cut were varied and surface roughness was measured for each trial for as cast and RRA heat treated samples and compared Relation between input and output parameters was studied

Table 3 Cutting conditions and response for as-cast sample

ANOVA considering cutting parameters as input and surface roughness as output for as cast and RRA heat treated samples shows the influence of various cutting parameter on

surface roughness, which is provided in Table 4 and Table 5

Table 4 ANOVA table for surface roughness of as-cast sample

Table 5:ANOVA table for surface roughness of RRA samples

Figure 2 (a) and (b) Parameter contribution for as-cast RRA treated sample (%)

Fig 2(a) shows the contribution of cutting parameter on surface roughness for as-cast specimen It is found that feed rate has the significant parameter contributing 83.38% on surface roughness It is followed by DOC Speed has least significant effect on roughness Fig 2(b) demonstrates the contribution of cutting parameters on surface roughness of RRA test sample It is found that feed rate has the most significant impact on the yield contributing to 48.65% Cutting force found to be least significant The higher contribution of error for RRA samples indicates that special care needs to be taken in carrying out RRA in industry These outcomes can be utilized to ideally decide the best arrangement of machining parameters and additionally device geometry particular so as to accomplish the most ideal surface finish

Figure 3(a) Main effect plot for as-cast sample Figure 3(b) Main effect plot for RRA sample

From Fig 3(a) shows the evident that the S/N ratio is more in the event of feed rate has a significant effect on surface roughness for both alloy and the alloy subjected to RRA treatment From this the optimum combination of the cutting parameters obtained are as shown in Table 6 This combination can be employed to control over surface roughness

Table 6 Optimum parameter values for as-cast and RRA sample

4 CONCLUSIONS

The results of the investigation carried out to study the influence of cutting parameters on surface roughness of as cast and RRA treated AA 7075 as follows

The alloy was fabricated successfully through stir casting route The samples are subjected

to RRA treatment The prepared samples are turned on a conventional lathe according the cutting parameters selected from array From the machinability studies conducted using L9 orthogonal array

Machinability (as indicated by surface roughness) of the AA7075 alloy deteriorates on heat treatment and the feed-rate has the maximum impact on surface roughness; as such the required surface smoothness can be obtained by a proper control of the feed-rate

Main effect plot is drawn considering the SN ratio reveals the contribution of cutting parameters on surface roughness It was found that slope obtained for the feed rate is high compared to other parameters

ANOVA technique shows the contribution of cutting parameters on surface roughness It

is found that feed rate offers significant effect on surface roughness Feed rate contributes 83.38% of total yield for as cast and 48.65 for RRA treated condition, which is very important aspect to be considered in aerospace applications to maximize productivity

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