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Project 2:

The Effect of Over Aging on the Mechanical Properties of 7075 Aluminum

MSE 527L: Mechanical Behavior of Materials Lab

12-2-2015

Group 2:

Rna WahebRameen HassanzadehRyan OhPavan Kumar NanneSiddhesh SawantDhaval Prajapati

Abstract

The paper represents an experimental study on age-hardening of aluminium alloy 7075 to determine the effects of artificial aging on the hardness of aluminum alloy 7075. Age-hardening is a thermal treatment, which consists of heat treatment, quenching and artificial aging process. The experimental study is focused on artificial aging upon which the temperature is kept constant ( 320°F) and samples are aged for different periods of time. The Rockwell hardness test is used to evaluate the hardness of aluminum alloy 7075 before and after the aging process. A tensile test is used to test the samples for tensile strength at the different aging times.

Introduction

For over five decades, aluminum has been considered as a second choice to iron and steel in the metal market. The increasing demand for aluminum is due to its unique combination of properties which makes it one of the most versatile materials for engineering and construction based applications. Aluminum is lightweight. The specific gravity of aluminum is 2.7; which is only 30% heavy of copper and one third that of iron. Besides Magnesium, it is the lightest of all common metals. Some if its alloys have greater strengths than structure steel. Aluminum has good electrical and thermal conductivity and high reflectivity to both heat and light. It is non-toxic and highly corrosion resistant under any service conditions. Aluminum can be given almost any type of surface finish and can be cast and produced into any form of product. It is because of all these outstanding properties that aluminum has become a material of prime importance for engineering.

Heat treatment is the process of heating a metal or alloy to elevated temperatures to change the physical of the metal to suit the needs of its application. The temperature at which the heat treatment is carried out is dependent on the desired physical properties. A process called age hardening is performed to further increase the strength of the metal after heat treatment. This process works by allowing precipitates in the metal alloy to gather together, increasing localized density of precipitates. As a result, these higher density precipitate locations, named GP zones, impede dislocations and defects, which in turn increase the strength and hardness of the metal alloy. However if the alloy is heated long enough, a process known as over-aging will occur, in which the metal alloy will produce precipitates that will result in the loss of hardness. Mechanisms of strengthening in metals include grain size reduction, solid-solution strengthening and strain hardening. While grain size reduction and strain hardening increase the mechanical properties of a material through cold work, precipitation or age hardening increases the strength of a material via heat treatment. This heat treatment process increases the yield strength by increasing the amount of dislocations in a metal. Within an alloy this process is governed by diffusion and must be controlled at the right temperature and time to maximize the size of the precipitates to achieve the desired increase in strength.

Mechanical Properties

The ultimate tensile strength of T6 Temper 7075 is 74,000-78,000 psi and yield strength of at least 63,000-69,000 psi. It has a failure elongation of 5-11%. It is usually achieved by homogenizing the cast 7075 at 4500 C for several hours, quenching and then aging at 1200C for 24 hours. This yields the peak strength of the 7075 alloy. The strength is mainly derived from finely dispersed eta and eta1 precipitates both within grains and along grain boundaries. Aluminum 7075 has a density of 2.810 g/cm³. The mechanical properties of 7075 depend greatly on the temper of the material.

Fig 1 : Aluminium 7075 test sample

Rockwell Hardness Test

Hardness is a characteristic of a material, not a fundamental physical property. It is defined as resistance to indentation, and it is determined by measuring the permanent depth of the indentation. The rockwell method measures the permanent depth of indentation produced by a force/load on an indenter. First, a primary test force is applied to the sample using the diamond indenter. This loads represents zero or a reference position that breaks through the surface to reduce the effects of surface finish. after the preload, an additional load (the major load) is applied to reach the total required test load. This force is held for a predetermined amount of time to allow for elastic recovery. This major load is then released and the final position is measured against the position derived from the preload, the indentation depth variance between the preload value and major load value. This distance is converted to a hardness number. A variety of indenters may be used : conical diamond with a round tip ball for harder metals to ball indenters with diameter ranging from 1/16” to ½” for softer material.

Fig 2 : Load applied through the indenter on the surface

When selecting Rockwell scale, a general guide is to select a scale that specifies the largest load and the smallest indenter possible without exceeding defined operating conditions and accounting for conditions that may influence the test results. These conditions include test specimens that are below minimum thickness for the depth of indentation; a test impression that falls too close to the edge of the specimen or another impression; or testing on cylindrical specimens. Moreover the test axis should be within 2-degrees of perpendicular to ensure precise loading; there should be no deflection of the test sample or the tester during the loading application from conditions such as dirt under the test specimen or on elevating screw. It is important to keep the surface finish clean and decarburization from heat treatment should be removed.

Procedure

Ten tensile coupons of 7075 aluminum were obtained and labeled as samples 1 -10. After identifying the samples, their hardness values were measured. Temperature was kept constant and time was variable in this study. All of the samples were subjected to a temperature of 320°F (160°C) and then water quenched at specific time intervals. When aging for all the samples was completed, their post heat treatment hardness values were measured again. Then, the samples were subjected to a tensile test. Their remnants were then cross-sectioned in transverse and longitudinal directions to evaluate grain structure.

Discussion

Table 1 displays the 10 samples and their respective hardness values. As expected, the hardness values across the samples are approximately the same.

Table 1. Preheat treatment Rockwell B values

Sample Rockwell Hardness Average

1 89,88,89 89 HRB2 89,89,89 89 HRB3 89,89,89 89 HRB4 89,89,87 89 HRB5 89,89,89 89 HRB6 89,89,89 89 HRB7 89,89,89 89 HRB8 89,89,89 89 HRB9 89,89,89 89 HRB10 89,89,89 89 HRB

Table 2 shows the amount of time that each sample was aged in the oven. Samples 1 and 2 were used as reference.

Table 2. Heat treatment Duration

Sample Heat Treatment Time(hrs)

1 02 03 124 125 246 247 368 369 4810 48

Table 3 shows the hardness values measured post over aging, and figure 3 plots the relationship between the post over aging hardness values and time.

Table 3. Post heat treatment Rockwell B values

Sample Rockwell Hardness Average

1 89,89,89 89 HRB2 89,89,89 89 HRB

3 81,82,81 81.33 HRB4 80,81,79 80 HRB5 76,77,70,73,74 74 HRB6 75,77,74 75.33 HRB7 72,73,73 73.66 HRB8 74,73,74 73.66 HRB9 74,75,73 74 HRB10 73,75,77 75 HRB

Fig. 3. Relationship between hardness and time after the over aging process.

Table 4. Post heat treatment conductivity measurements.

Sample %IACS Temper1 32.29 T62 32.25 T63 39.49 T734 39.62 T735 41.07 T736 41.89 T737 41.83 T73

8 41.65 T739 41.57 T7310 41.11 T73

Fig. 4. Relationship between conductivity and time after the over aging process.

Fig. 5. Stress/ Strain Curve of Age Hardened Al 7075 samples

From these stress-strain curves, it is possible to extract multiple properties of the samples. Table

4 shows the Ultimate Tensile Strength, Yield Strength, Ductility, Young's Modulus, and Strain

Hardening Exponent. The analysis methods used for these extractions, as well as a discussion for

error, can be found in appendix sections 1-6.

Table 4. Properties of samples, extracted from experimental stress-strain data.

SampleTime

aged (hours)Tensile

Strength (ksi)Yield

Strength (ksi) Ductility (%)Young

Modulus (ksi)

Strain Hardening Exponent

1 0 81.7 62.8 14.49 1611 0.1752 0 82.5 62.5 13.73 1716 0.1783 12 71.4 57.5 11.52 1659 0.2064 12 72.1 54.4 12.6 1508 0.3015 24 66 50.7 12.6 1359 0.3086 24 66.8 53.9 12 1439 0.2367 36 62.8 50.8 11.3 1397 0.2378 36 63.9 51.7 11.5 1552 0.2419 48 65 51.6 11.8 1455 0.262

10 48 67.3 53 11.5 1509 0.257

Fig. 6. Relationship between tensile strength and time after the over ageing process.

Fig. 7. Relationship between yield strength and time after the over ageing process.

Fig. 8. Relationship between ductility and time after the over ageing process.

Fig. 9. Relationship between modulus of elasticity and time after the over ageing process.

Fig. 10. Relationship between strain hardening exponent and time after the over ageing process.

As can be seen from the graphics, the general trend is that the longer the samples aged

for, the lower their mechanical properties became. From Figures 6-10, σys, and σUTS, were both

shown to decrease as a general trend with time with the exception of the last sample category.

The general decreasing trend is to be expected from overaging, but the increase observed from

the longest aged sample category is not well understood. Ductility was shown to decrease after

any amount of aging, but further aging did not appear to significantly change the overall

ductility.

Additionally, while we do not believe that the the values for the Modulus of Elasticity

and Strain Hardening exponent can be accurately found through this testing method (In the

tension lab, the “laser” method of measuring strain was more accurate), it was theorized that the

general trend observed might be of value. However, as can be seen in figures 9 and 10, the

variability within sample category far outweighs the trend from category to category, and

because of this, no reasonable conclusions can be drawn from this data.

Microstructure

Figure 11. Longitudinal microstructure of Sample 1 100X and 500X

Figure 12. Transverse microstructure of Sample 1 100X and 500X







Applications of Al 7075 Series:

● The 7000 series alloys like 7075 are often used in transport applications, which also include marine, automotive and aviation, due to their high strength to density ratio.

● Their strength and light weight is also desirable in other fields.● The Rock climbing equipment, bicycle components, inline skating- frames and hang

glider airframes are commonly made from 7075 Al alloy.● Hobby grade RC models commonly use 7075 and 6061 for chassis plates.● One of the Interesting application is in the manufacturing of M16 rifles for the American

military. In particular high quality M16 rifle lower and upper receivers as well as extension tubes are typically made from 7075-T6 alloy.

● Desert Tactical arms, SIG Sauer and French armament company PGM use it for their precision rifles.

● It also have common applications in shafts for the lacrosse sticks such as the STX sabre and camping knife and fork sets.

● Since it has High strength, low density, thermal properties and ability to be highly polished, 7075 is widely used in mold manufacture[4].

Appendix

1. General Error

The nature of a tension measurement depends very heavily on the orientation of the sample under load. The tool used lacked any alignment system to ensure that the samples were loaded aligned parallel to the load, and a judgment was instead made by eye.

2. Finding Ultimate Tensile Strength

Ultimate Tensile Strength was taken as the maximum stress measured on the stress/strain curve. In all cases, this is the peak of the plastic region of the stress/strain curve.

3. Finding Yield Strength

An estimation of what region of the stress/strain curve represented the sample still in elastic deformation was taken by eye. The last stress point of this region gives the Yield Strength.

4. Finding Ductility

The maximum strain experienced by the material before breaking (last data point) gives the total ductility.

The difference found in the steel samples compared to published results is not well understood. This could depend heavily on the alignment of the measurement system, and more trials could give an increased understanding.

5. Finding Young's Modulus

The most linear portion of the sample while the material was still in the elastic regime was identified, and a linear fit was made to the data. The slope of that fit gives the young's modulus.

6. Finding the Strain Hardening Exponent

The formula below was used in order to resolve n, the strain hardening exponent. Here, εT

=ln(1+εE ) was used to get the true strain.

References:

1. "Heat Treatments: Introduction." Heat Treatments: Introduction. N.p., n.d. Web. 17 Apr.

2013. <http://www.efunda.com/processes/heat_treat/introduction/heat_treatments.cfm>.

2. "Age Hardening." Bluewater Thermal Solutions. N.p., n.d. Web. 17 Apr. 2013.

<http://bluewaterthermal.com/age-hardening/>.

3. T. V. Rajan, C. P. Sharma, Ashok K. Sharma. “Heat Treatment: Principles and

Techniques” 15ed. New Delhi: Prentice-Hall, 2006. 336-8.

http://books.google.com/books?

id=A2QG3UssqKUC&pg=PA336&lpg=PA336&dq=gp+II+zone&source=bl&ots=Gmso

2xnIks&sig=dsRTDlPIE0tyo2qF4YhpUGNLIsU&hl=en&sa=X&ei=tzJ_UaSRFOLtiwK

qooDgAQ&ved=0CDoQ6AEwAQ#v=onepage&q=gp%20II%20zone&f=false

4. Effect of Ageing on Microstructure and Mechanical Properties of Bulk, Cryolled and

Room temperature rolled Al 7075 Alloy by Sushantha Kumar Panigrahi, R Jayaganthan;

2011.

5. "Rockwell Hardness Test Method | Newage Testing Instruments." Rockwell Hardness

Test Method | Newage Testing Instruments. N.p., n.d. Web. 29 Nov. 2015.

6. "Rockwell Hardness Testing." Rockwell Hardness Testing. N.p., n.d. Web. 29 Nov. 2015.

7. "WWW.BUEHLER.COM." An ITW Company. N.p., n.d. Web. 29 Nov. 2015.

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