Mechanical Characterization and Fracture Toughness of

Electroplated Cadmium Coating of Al –Zn Alloy, T6

Mohan Kumar S1,a), Ravi Kumar V1, b), Shashi Kumar M E1, c),

Govindaraju H K2, d) 1Department of Mechanical Engineering, Amrita School of Engineering, Bengaluru, Amrita Vishwa

Vidyapeetham, Amrita University, India a) mohanssbg@gmail.com

b) hsrravi@gmail.com c) shashikumar91@gmail.com

2 Department of Mechanical Engineering, BMS Institute of Technology, Bengaluru, India d) anchithgraj@gmail.com

Abstract -Fracture toughness, Hardness and Tensile Strength of the Aluminum alloy 7075-T6 coated with Cadmium with

varying thickness of 10µ and 20µ were investigated. Electroplating Cadmium coating process gives excellent corrosion

resistance, provides low coefficient of friction which increases the surface hardness of the material. It also provides a uniform

and dense coating, in many cases, maintains surface finish as it was before plating. The specimens prepared in line with ASTM

E-8M and E-399 standard were subjected to various tests. The results shows thatthe hardness of the EC coted specimens has

increased by 10% and increases with increase in thickness, butthe thicker the EC coating, more brittle is the material. The

Ultimate Tensile Strength also increased by 5% compared to the uncoated counterpart. There was a steep increase in plain

strain fracture toughness with the increase in the coating thickness for the Aluminum 7075–T6 alloy in TL orientation.The

crack growth was Unstable due to the strong adhesion between the EC coating and the alloy.

Keyword-Fracture Toughness, Electroplating, solution aging, CT specimen

INTRODUCTION

The fact that the Aluminum and its alloy possess lower density of 2.7 gm/cm3 as compared to that of steel which

has 7.9 gm/cm3 and is resistant to corrosion in some environments, including the ambient temperatures, makes

the aluminum and its alloy a most appropriate material in various applications [10]. Aluminum alloys have an

elastic modulus of 70 GPa, which is about 1/3rdof the elastic modulus of steel. In the process of new products

design, the design choices are governed by the special manufacturing technologies. Extrusions technique is the

best manufacturing method as the ease with which aluminum alloys, particularly for the Al-Mg-Si series can be

produced.Aluminum7075, is an aluminum wrought product which has the highest strength of all aluminum alloys.

The T6heat treated aluminium alloys have a tensile strength of 600 MPa, which is higher than many mild steels

[11]. This makes the 7075 alloy usable for structural parts which are highly stressed. Applications includeworm

gears, keys, aircraft fittings, regulating valve parts, gears, missile parts, various other commercial aircraft and

defence equipment [12]. Fracture based design is suitable to make use of even a defective material and it is used

where human criticality is involved especially in automobile and aerospace industries. Fracture toughness based

design is most appropriate design approach to estimate the life of a component. Hence it becomes important to

estimate the fracture toughness of the proposed alloy 7075coated with EC in TL orientation. According to the

ASTM E-399 standard, the cadmium coated CT specimens of different coatingthickness are tested in the first

Mode, the fracture toughness is further estimated using the principles of LEFM (Linear Elastic Fracture

Mechanics) and also the mechanical properties such as tension and hardness properties could be evaluated. The

stress corrosion cracking was characterised by fracture mechanics approach by Mohamed Ragab Bayoumi [1].

Experiments on different aluminium alloys have been carried out and their mechanical properties and plane strain

fracture toughness (K1c) have been estimated by this approach [5]. The effect of solution treatment temperature,

quenching media and artificial aging on tensile strength and hardness on 7075 T6 alloy have been recorded.The

correlation between the tensile strength and hardness was successfully obtained by Clark and Coughran [2], [14].

Lot of investigation has been carried out in order to protect high strength steels from corrosion and erosion by

Cadmium coatings. Ion vapor deposited aluminum coating, aluminum/chromium coating as well as cadmium

coating on high strength 4130 steel have been characterized using different electrochemical methods, chemical

analysis and surface characterization techniques [3, 13]. Mohan Kumar et all, investigated the fracture toughness

and mechanical properties of EN coated 7075 aluminum alloy with a coating thickness of 10µ, 20µ. From the

SEM analysis the research concluded that because of the coating there is a stable crack growth compared to

uncoated alloy [8]. R. T. Foley made a detailed study on Localized Corrosion, because of corrosion cracking

(SCC), pitting, crevice corrosion (cc) of Aluminum Alloys [9].

MATERIAL COMPOSITION AND HEAT TREATMENT

The chemical composition available for Aluminum alloy 7075-T6 rods areshown in Table 1.

Table 1. Material composition of Al 7075 alloy

Elements Zn Mg Cu Fe Si Ti Mn Cr Sn Ni

&Pb Al

% of wt 5.9 2.8 1.9 0.19 0.09 0.02 0.03 0.2 0.02 <0.01 Remainder

Solution and Artificial Aging

T6 Temper heat treatment has been a proved optimal heat treatment for 7075 alloy. This heat treatment involves

solution aging; furnace will be pre-heated up to 4800C after that Aluminum alloy is loaded in to the furnace. Due

to this some amount of the heat is absorbed by alloy, so it reduces the furnace temperature. In order to maintain

the 4800C temperature again furnace will be heated, under this temperature alloy is soaked fora duration of 2

hours. After that alloy should be dipped into glycol solution for rapid quenching. After the solution aging, finally

age hardened at 1210C for 24 hours and alloy is then air cooled [2].

Electroplating Cadmium Coating Process 1) Surface preparation:The specimens are cleaned by using series of cleaning chemicals such as bases and

acids to have good adhesion. Pre-treatment is followed by rinsing with water to remove chemicals on the surface.

Degreasing is the process of removing oil from the surface and acid cleaning removes scaling. The process

involves dissolution of anodeand the deposition of cadmium on the cathode [6].

2) Cadmium Coating: Electrolytesolution of cadmium salts in barrels is used in electro deposition process. This

electrolyte solution contains alkaline cyanide. When current is made to pass through the electrolyte, cadmium is

deposited in the cathode material. The cadmium from the anode gets into the solution and gets deposited on the

cathode material [7]. The cathode is removed and is finished with an anti-oxidation chemical (sodium, dichromate)

and is rinsed in pure water to remove stains. The surface is degreased using Steelex K-20 Solution, De-rust in

H2SO4 40% Solution and Passivated in sodium-dichromate solution and water swilled twice [4].

MECHANICAL TESTS

Tensile Test

Specimen prepared as per ASTM E8M standard of dimensions 72mmX12mm were machined and is shown in the

Fig 1 and 2. The tests were performed on a computerized servo hydraulicuniversal testing machine. The readings

of load against the displacement were measured.

Fig 1. Dimensions of Tensile specimen Fig 2. Tensile specimen

Hardness Test

Rockwell “B” test was performed on the specimen with dimensions of dia 25mm and height 15mm. The type of

indenter used is hardened steel ball having a dia of 1/16th of inch and the total load applied is 100 kg and the time

of application of load is 15sec.

Experimental Evaluation of Fracture Toughness (KIC)

This test method is used to determine the plane strain fracture toughness (KIC) of metallic materials. In this

method, a variety of fatigue-cracked specimens are used. The Linear Elastic Fracture Mechanics (LEFM) as

mentioned was used in the analysis. The entire specimen was deformed elastically and the small strain theory

with linear stress strain relationshipwas assumed. One of the common and simplest methods to test for Fracture

toughness is by using a Servo hydraulic UTMequipped with load cell, stroke transducer.Among the various kinds

of available specimen configurations, CT specimen was selected due to its fabrication simplicity. CT specimen

consumes less material and while testing it does not demand any special fixtures to mount on the jaws of testing

machine. The CT specimen is prepared in T-L orientation as per ASTM E399-90 standard was used and is shown

in Fig 3 and Fig 4. Initially a straight through type notch is introduced till a length of ao=10mm by CNC using

wire cutting machine. Later a plastic zone is generated at the vicinity of the notch by applying fatigue loading by

using Dynamic testing machine.

Fig 3. Dimension of CT Specimen Fig 4. CT Specimens

A 2kN was the maximum and 0.6 kN was the minimum load maintained during the application of the fatigue

loading. The fatigue pre-cracked specimen was obtain by introducing a crack in plastic zone at the vicinity of the

notch by the application of fatigue loading. The length of the pre-crack introduced was ap=2.99mm. Cycles of

fatigue loading used for obtaining the crack length of 2.99 mm was 85,000 cycles and the time duration for the

same was 1.57 hrs. This pre-cracked specimen was then subjected to external load, the cracked tip opened due to

the tensile load. Due to the opening of the tip, there is a rise to the stretched zone and voids begin to form until a

critical size is reached. The crack starts to propagate by beginning to link from one void to the other. If the

Pmax/Pq maximum value is lesser than 1.05, it confirms the validity of KIC obtained.

RESULTS AND DISCUSSION

Tensile Test Results

ASTM standard tensile tests were performed and the results of the same are presented in the Table 2

below.

Table 2. Tensile Test Results

SL

No. Material

σU,

MPa σYS, MPa % elongation

1 7075, T6 608.15 559.19 4.80

2 7075, T6

EC 10µm 616.83 572.14 3.43

3 7075, T6

EC20µm 625.29 577.38 3.05

The fig 5 shows that UTS of a10µ and 20µ thickness EC coated Aluminum 7075, T6 alloy which exhibits higher

tensile strength than compared to that of the base aluminum 7075, T6 alloy. It is also seen that 20µ thickness EC

coated alloy tensile strength is nearly equal to the stainless steel tensile strength.

Fig 5. Variations of Tensile Strength

Hardness Test Results

The Rockwell hardness test conducted on the specimen according to the ASTM E18 standard is shown

in the Table 3.

Table 3. Hardness Test results

Sl. No. Material HRB

1 7075 –T6 82

2 7075 – T6 EC 10 µ 87

3 7075 – T6 EC 20 µ 94

Fig 6 shows variation of HRB, it is seen that HRB of 20µ thickness EC coated alloy exhibit high hardness number

than that of the others and it shows that hardness increases with the increasing thickness of the EC coating.

Fig 6. Variations in Hardness Number

Fracture Toughness Test

Under the plane strain condition and opening mode loading, Fracture toughness test were conducted in

accordance with ASTM E399 standard with the aid of Servo hydraulic UTM. Compact Tension (CT) specimens

were prepared according to the standards. Fatigue pre-crack is measure as per the ASTM E 647 standard. The

values of the fracture toughness obtained from LEFM are tabulated in the Table 4. Table 4. KIC Test Results

595

600

605

610

615

620

625

630

7075, T6 EC 10µ EC 20µ

UTS

in M

pa

7678808284868890929496

7075,T6 EN 10µ EN 20µ

HR

B

SL

No Material

ao

(mm)

ap

(mm)

a

(mm)

PQ

kN

Pmax

kN

KQ

MPa√m

KIC

MPa√m

1 7075, T6 10 2.99 12.99 0.511 9.96 4.39 4.66 22.08 22.08

2 7075, T6-

10µ EC 10 2.99 12.99 0.511 9.96 4.44 4.72 22.28 22.28

3 7075, T6-

20µ EC 10 2.99 12.99 0.511 9.96 7.21 7.62 36.07 36.07

From the above results it is concluded that Fracture toughness increases with the increase of the Electroplating

Cadmium coating thickness. 20µ EC coated Al 7075; T6 alloy exhibits higher Fracture toughness properties. Fig

10 shows the stable crack growth of CT specimen after test and only a very small yielding at tip of crack at the

beginning of the test was observed.

Fig 7. Variations of Fracture toughness results Fig 8. CT Specimen after test

Fractographic Study

Fig 9 Illustrates a typical Aluminum 7075 alloy substrate taken parallel to the axis observed by means of

optical microscopy, it indicates that the structure isconstituted of elongated grains and shows the dislocation

densities. Fig 10 reveals the small size grain Cadmium particles on the surface and also voids due to the un evenly

distributed grain particles throughout the surface. Fig 11 shows the increasing grain size of Cadmium particles

and also the uniform thickness coating throughout the Aluminum 7075 alloy substrate with the very less voids.

Fig 12 shows Optical microscopy analysis of the electroplating coating that the layer constituted by metallic

cadmium was uniform, adherent and thick up to 10µ thickness. Fig 13 illustrates a cross-section view of the

coated sample, which indicates the deposition of a uniform filmwith a mean thickness of approximately 20µ and

acoating-substrate interface apparently free from cracks and pores. Fig 13 Adherent between the Aluminum alloy

and the cadmium coating results voids in the thickness of 20µ EC coating, which results very small amounts of

peel out but it does not make any effect to strength of the alloy.

Fig 9. Microstructure of AL 7075 alloy Fig 10. Microstructure of 10µ EC AL 7075 alloy

0

5

10

15

20

25

30

35

40

7075, T6 10µ 20µ

Frac

ture

To

ugh

ne

ss M

pa√

m

Fig 11. Microstructure of 20µ EC AL7075 alloy

Fig 12.EC 10µ thickness Fig 13. EC 20µ thickness

SEM images were taken on the fracture surfaces. The uncoated Aluminum 7075 alloy showed both stable as well

unstable crack growth regions, the EC coated specimens showed only unstable crack growth region due to the

strong bonding between the alloy and coating. The stable crack growth region is comparatively flat, smooth and

will not show any deep dimples as shown in SEM Image (Fig 14). The SEM Images of the coated specimens

shows rough regions with small dimples and little pulled regions which indicate unstable crack growth. This could

be the reason for the fractured surfaces of the coated specimens being brighter compared to fractured surfaces of

the uncoated specimens (Fig 14, Fig.15and Fig 16).From the SEM it is foundfor a coated specimen, the nature of

failure is completely brittle with very little pulled crystalline structures with some voids on the fractured surface.

Fig 14. SEM AL 7075 alloy Fig 15. SEM of 10µ EC AL 7075 alloy

Fig 16. SEM of 20µ EC AL 7075 alloy

CONCLUSIONS

The hardness test it is clear that the uncoated Aluminum 7075, T6 alloy has 80 HRB and EC coated 10 and 20µ

EC coated alloy has 87 and 94 HRB respectively. The tensile test shows that the UTS of EC coated alloy increases

by 5% comparedto that of uncoated Aluminum alloy 7075, T6. The plain strain fracture toughness of Aluminum

7075–T6 alloy in TL orientation is 22 mMPa and 10, 20µ thickness EC coated alloy Fracture toughness is

22.3 and 36 mMPa respectively. Hence it could be inferred that increased thickness of Electroplating Cadmium

coating will increases the Fracture toughness and Mechanical properties. Also with the increase in the EC coating

thickness, the material shows brittle failure. Unstable crack growth occurs due to the strong bonding between the

EC coating and the Al7075 alloy.

REFERENCES 1. Mohamed RagabBayoumi,“Mechanics and mechanisms of fracture in Stress corrosion cracking of aluminium Alloys,” Mechanical

Engineering Department, Assiut University, Assiut, Egypt, 1996.

2. Yoon Y. Petry. Hansen DC,“Corrosion Protection of Cadmium and CadmiumReplacement Coatings on High-Strength Steel,”Materials

Engineering DivisionUniversity of Dayton Research Institute 300 College Park Ave. Dayton, OH 45469 USA. 3. Plachy J, “Cadmium,” in Minerals Yearbook 2000, Reston, US Geological Survey, 2000, available at

http://minerals.usgs.gov/minerals/pubs/commodity/cadmium/140400.pdf.

4. Legg K, “Cadmium Replacement Options,” The Welding Institute, Cambridge, UK, October 2003. 5. Vallejo G,“Compliant Electroplated Aluminum for Aerospace Applications,” Proceedings of the Surface Engineering for Aerospace and

Defense Conference. Orlando, FL, January 2008.

6. Navinsek B,“PVD Coatings as an Environmentally Clean Alternative to Electroplating and Electroless Processes,”Surface and Coatings Technology. Elsevier; Vol, 116-119, pp. 476-487, 1999

7. Shahin G,“Alloys are Promising as Chromium or Cadmium Substitutes,”Plating and Surface Finishing Journal.American Electroplaters

and Surface Finishers Society.August 1998. 8. Mohan Kumar S,“Evaluation of Fracture Toughness and Mechanical Properties of Aluminum Alloy 7075, T6 with Nickel Coating,”

Procedia Engineering, Vol 97, pp. 178-185, 2014

9. Foley RT,“Localized Corrosion of Aluminum Alloys—A Review,” Corrosion Journal. Vol, 42(5), pp.277-288, 1986 10. Lai MO, Ferguson WG,“Fracture toughness of aluminium alloy 7075-T6 in the as-cast condition,” Materials Science and Engineering,

Vol 74(2), pp.133-138, 1985

11. Robert E,Zinkham,“Anisotropy and thickness effects in fracture of 7075-T6 and -T651 aluminum alloy,” Engineering Fracture Mechanics, Vol 1(2), pp 275-276, 1968

12. Kaufman JG,“Fracture Toughness of 7075-T6 and -T651 Sheet, Plate, and Multilayered Adhesive-Bonded Panels,”Journal of Basic Engineering. Vol 89(3), 2011

13. B Jasim M. Salman, Shaymaa Abbas AbdAlsada, Khadim F Al-Sultani,“Improvement Properties of 7075-T6 Aluminum Alloy by

Quenching in 30% Polyethylene Glycol and Addition 0.1%,” Research Journal of Material Sciences, Vol 1(6), pp 12-17, 2013 14. Das P,Jayaganthan R, Chowdhury T Singh, “Improvement of Fracture Toughness (K1c) of 7075 Al Alloy by Cryorolling Process,”

Materials Science Forum. Vol, 683, pp 81-94, 2011