The Effects of Scandium and Zirconium Additions on Aluminum Mechanical Properties

Home , Brazing, Extrusion

The Effects of Scandium and Zirconium Additions on Aluminum Mechanical Properties,

Post-Braze Grain Structure, and Extrusion

A thesis presented to

the faculty of

the Russ College of Engineering and Technology of Ohio University

In partial fulfillment

of the requirements for the degree

Master of Science

Cory R. Williams

March 2012

This thesis titled

The Effects of Scandium and Zirconium Additions on Aluminum Mechanical Properties,

Post-Braze Grain Structure, and Extrusion

CORY R.WILLIAMS

has been approved for

the Department of Mechanical Engineering

and the Russ College of Engineering and Technology by

Frank F. Kraft

Associate Professor of Mechanical Engineering

Dennis Irwin

Dean, Russ College of Engineering and Technology 3

Abstract

WILLIAMS, CORY R., M.S., March 2012, Mechanical Engineering

The Effects of Scandium and Zirconium Additions on Aluminum Mechanical Properties,

Post-Braze Grain Structure, and Extrusion

Director of Thesis: Frank F. Kraft

The purpose of this work was to evaluate the ability of Sc and Zr to inhibit recrystallization and grain growth of cold-worked aluminum parts that are brazed into assemblies. Extruded, multi-channel tube, used for automotive climate control systems, are sized and straightened prior to assembly into heat exchangers. This small amount of plastic strain (cold-work) is the driving force for recrystallization and large grain growth during the brazing process, which takes place at approximately 600°C. This generally coincides with a significant decrease in part strength. A 1000 series aluminum alloy with additions of 0.2% Sc and 0.05% Zr was evaluated with respect to the brazing thermal cycle and a typical amount of cold-work imposed during straightening and sizing. The effect on extrusion was considered, and a comparison was made to AA 3102, a typical alloy for this application.

Approved: ______

Frank F. Kraft

Associate Professor of Mechanical Engineering 4

ACKNOWLEDGMENTS

I would like to thank Dr. Frank Kraft for his guidance on this project, in the classroom, and also in life. While working with Dr. Kraft I learned many subtle life lessons in addition to the regular curriculum. For serving on my committee, I would like to thank Dr. Israel Urieli, Dr. Daniel Gulino, and Dr. Hugh Richardson.

This project would not have been possible without the generosity of Alcan, Inc. and Dr. Nick Parson by providing the material and extrusion data. Alcan, Inc. is a

Canadian mining company and aluminum manufacturer.

For the use of their labs and for sparking my interest in the material field during my internship, I would like to thank Gary Pruett, Robert Bianco, and Hector Cirilo at

Goodrich Corporation. Goodrich Corporation is an American aerospace manufacturing company.

Most importantly, I would like to thank my wife, Sadie, my daughter, Paisley, my parents, Randy and Sandra, my brother, Brad, and my family for always supporting me and helping me achieve everything I have accomplished.

TABLE OF CONTENTS

Page

Acknowledgments...... 4

List of Tables ...... 7

List of Figures ...... 8

Chapter 1: Introduction ...... 10

1.1 Background of Brazed Automotive Heat Exchangers ...... 10

1.2 Background of Aluminum-Scandium Alloys ...... 13

1.3 Objectives ...... 17

Chapter 3: Experimental Approach ...... 20

3.1 Cold Rolling ...... 20

3.2 Simulated Brazing ...... 21

3.3 Aging Heat Treatment...... 23

Chapter 4: Mechanical Testing and Procedures...... 25

4.1 Sample Preparation ...... 25

4.2 Tensile Testing ...... 27

4.3 Burst Testing ...... 31

4.4 Metallography ...... 35

Chapter 5: Results And Analysis ...... 38

5.1 Extrusion ...... 38

5.2 Microstructure Analysis ...... 39 6

5.3 Tensile Testing ...... 42

5.4 Burst Testing ...... 49

Chapter 6: Conclusions ...... 52

Chapter 7: Future Work ...... 54

References ...... 55

LIST OF TABLES Page

Table 1: AlSc alloy composition in weight % ...... 18

Table 2: AA 3102 alloy composition in weight % ...... 18

Table 3: Extrusion parameters ...... 19

Table 4: Measurements taken during sample preparation ...... 26

Table 5: Summary of extrusion results ...... 38

Table 6: Results of burst tested samples ...... 50

LIST OF FIGURES Page

Figure 1: Parallel Heat Exchanger and Tube Cross-Section ...... 11

Figure 2: Pre-Braze and Post-Braze AA 3102 Tube ...... 12

Figure 3: Inner Wall Failure of Post-Braze AA 3102 Tube ...... 12

Figure 4: The Effects of Roll-Sizing on Failure Pressure on AA 3102 Tube ...... 13

Figure 5: Al-Sc Phase Diagram and Solvus Close-Up ...... 15

Figure 6: Alloying Additions Effects on Recrystallization Temperature ...... 16

Figure 7: AlSc multi-channel tube cross-section ...... 19

Figure 8: Thermolyne tube furnace with samples partially inserted ...... 21

Figure 9: Simulated brazing apparatus ...... 22

Figure 10: Simulated brazing thermal cycle on AlSc tube samples ...... 23

Figure 11: Aging heat treatment data...... 24

Figure 12: Tinius Olsen 1000 tensile test machine with extensometer ...... 27

Figure 13: Personal DaqView data acquisition software ...... 28

Figure 14: Yield strength graph ...... 31

Figure 15: Burst testing apparatus using MTS machine ...... 33

Figure 16: Sample graph of burst test pressure over time ...... 34

Figure 17: Mounted and polished samples of AlSc multi-channel tube ...... 36

Figure 18: Microstructure of as-extruded AlSc multi-channel tube ...... 37

Figure 19: Detailed extrusion results ...... 39

Figure 20: Microstructure of as-extruded AA 3102 ...... 40 9

Figure 21: Microstructure of as-extruded AlSc ...... 40

Figure 22: AlSc alloy longitudinal sections ...... 41

Figure 23: Microstructure of AA 3102 samples with 4% reduction ...... 42

Figure 24: Tensile test results for experimental AlSc tube samples ...... 44

Figure 25: Tensile test results for AlSc alloy samples ...... 45

Figure 26: Tensile test results for AA 3102 alloy samples ...... 47

Figure 27: Tensile test results for AA 3102 and AlSc ...... 49

CHAPTER 1: INTRODUCTION

For several decades, there has been a trend toward the use of environmentally- friendly refrigerants for climate control systems, particularly in the automotive industry.

These refrigerants, such as R744 (CO2), operate at higher pressures (13 MPa) than the

R134a based systems (2 MPa). The current systems, particularly the heat exchangers, are not designed to withstand this increased pressure. A solution is sought that would allow automakers to produce a reasonably priced system that would be safer for the environment than the R134a-A/C-systems [1]. The heat exchangers’ multi-channel aluminum tube is one concern due to its low inherent strength after the brazing process.

This low strength is primarily due to recrystallization and extremely large grain formation

[2]. The addition of small amounts of Scandium (Sc) and Zirconium (Zr) to a 1000 series aluminum alloy is intended to inhibit the recrystallization and grain growth such that pre- braze strength can be retained.

1.1 Background of Brazed Automotive Heat Exchangers

Automotive climate control systems contain heat exchangers that have small aluminum multi-channel tubing arranged in rows (Figure 1a). Aluminum multi-channel tubing contains small channels for refrigerant flow (Figure 1b). For optimum manufacturability, precise thickness and width of the multi-channel tubing is required for these parallel flow heat exchangers. To obtain this precision, the tubes are straightened 11

and sized through a sequence of rolls prior to assembly into the heat exchanger and subsequent brazing. This process imposes cold-work, or small amounts of plastic strain, into the tubes.

12.00 0.30

0.40 0.40 ø 0.76 1.30

(a) (b)

Figure 1: (a) Automotive climate control system heat exchanger [3]. (b) Example multi- channel tube profile, dimensions are in mm [2].

While in the brazing furnace, the pieces reach a high enough temperature, around

600°C, to cause the alloy to recrystallize and form very large grains. This grain growth is directly related to cold-work or prior room temperature strain imposed during sizing and straightening operations. Figure 2 shows examples of tube before and after brazing, demonstrating this phenomenon in an AA 3102 alloy (an aluminum alloy that contains manganese) typically used in such heat exchangers.

(a) (b) Figure 2: AA 3102 aluminum multi-channel tubing with 4% reduction in thickness (a) Pre-braze [4] and (b) Post-braze [4].

When these tubes are pressure tested, the first parts of the tube to fail are the more highly stressed, large grained inner walls of the multi-channel tube [4]. Figure 3 shows a cross-section of inner wall failures in a typical AA 3102 tube. The outer wall failure occurred after that of the inner walls.

Figure 3: Cross-section of a typical AA 3102 tube failure [2].

Figure 4 indicates how the cold work imposed during roll-sizing affects failure pressure before and after brazing. Typically, there is a 3 to 7% reduction in tube thickness after straightening and resizing [2]. At about 3% reduction in thickness, there is a 22% reduction in strength due to the large grain formation [2]. The increase in failure pressure of non-brazed tube (with reduction) is due to strain hardening in the walls and an increase in thickness of the walls.

54 7830 Extruded and rolled 52 7540 Extruded, rolled and 50 simulated braze 7250 48 6960

46 6670

44 6380

42 outer wall failures 6090 psi

40 5800

38 5510

36 5220 Failure Pressure (MPa)Failure ~22% reduction 34 4930

32 4640

30 4350 0.00 0.01 0.02 0.03 0.04 0.05 0.06 0.07 0.08 0.09 0.10 0.11 0.12 Roll Sizing Reduction

Figure 4: The effects of roll-sizing on failure pressure for tube in the pre and post braze condition for the profile depicted in Figure 1-b in AA 3102 alloy [2]. Tube failures were at the internal walls unless otherwise noted.

1.2 Background of Aluminum-Scandium Alloys

Adding scandium as an alloying addition is not a new concept. AlSc alloys were patented in 1971 [5] and have been used in recent products such as bicycle frames, 14

baseball bats, tent poles and lacrosse sticks [6]. Very little scandium, <1 weight percent

(wt.%), is needed to increase the strength of aluminum alloys. Scandium strengthens aluminum in three different ways: grain refining, precipitation hardening, and inhibiting recrystallization, or grain growth [7]. The Al-Sc phase diagram in Figure 5 illustrates the coexistence of Al3Sc phase with the -Al phase (pure aluminum) with respect to temperature and composition. 15

1600

1500 Liquid o 1400 1392 C o o 1300 C 1283 C o 1300 1240 C 1200 o o 1220 C o

) 1190 C 1185 C

C 1100 o (

e 1000 970 C

r -Sc

tu 900 a

e 800 p

o m 700 660 C e

T 600

500 2 Sc Sc 2  3

Al AlSc Al AlSc 400 Al 300 200 0 10 20 30 40 50 60 70 80 90 100 Atomic % Sc

(a)

700

650

600

C) 550 o

(  -Al e

r 500 tu a

e 450 p m e 400 T  -Al + Al3Sc

350

300

250 0.00 0.05 0.10 0.15 0.20 0.25 0.30 0.35 0.40 Wt. % Sc (b)

Figure 5: Al-Sc phase diagram. (a) shows the entire diagram, while (b) shows only the solvus and detail of the Al-rich side of the diagram (after [7]).

The Al3Sc phase that forms is of particular importance. The creation of fine equiaxed grains and uniform precipitate distribution is attributed directly to the Al3Sc 16

intermetalic phase [7]. With respect to the recrystallization inhibiting effect of Sc compared to other elements, Figure 6 shows the increase in recrystallization temperature as a function of wt.% additions. AlSc alloys resist recrystallization due to the Al3Sc dispersoids that, even at high temperatures, hold their structure [8].

Figure 6: Common alloying additions and their effect on recrystallization temperature [9].

Scandium by far exhibits the best results, but Røyset and Ryum [7] also found in the literature that zirconium in combination with scandium creates an enhanced environment for resistance to recrystallization. This is due to the formation of Al3(Sc1-x Zrx) dispersoids which provide recrystallization resistance significantly in excess to just a binary Al-Sc alloy [7]. Aluminum alloys with additions of 0.25wt.% Sc and 0.08wt.% Zr having been cold-worked, withstood recrystallization for 8 hours at 520°C and did not 17

recrystallize until around 600°C [10]. The research presented herein seeks to determine the degree to which recrystallization and grain growth in extruded multi-channel tube can be inhibited with nominal Sc and Zr additions.

1.3 Objectives

The objectives of this research were directed toward alloy development for multi- channel heat exchanger tube in brazed assemblies. The objectives were:

 To determine the effects of Sc and Zr additions on post-extrusion (hot)

grain structure

 To determine annealing or recrystallization response to different thermal

cycles up to brazing temperatures

 To compare the microstructure, extrudability, and mechanical properties

of a Al-Sc-Zr alloy to an industry standard AA 3102 alloy 18

CHAPTER 2: EXPERIMENTAL MATERIALS

An aluminum alloy containing ~0.2%Sc and ~0.05% Zr was selected for this project. Table 1 shows the composition of the experimental alloy.

Table 1: AlSc alloy composition in weight %. The remaining % is Al. Analysis was performed by Alcan, Inc. Si Fe Zn Ti Ga Zr Sc 0.11 0.09 0.043 0.011 0.013 0.049 0.192

AA 3102, a typical alloy for this application, was chosen as the material to which a comparison was made. The composition of AA 3102 material used in this work can be seen in Table 2.

Table2: AA 3102 alloy composition in weight %. The remaining % is Al. Trace elements were not evaluated. (provided by Alcan, Inc.) Si Fe Mn 0.07 0.41 0.24

The experimental AlSc alloy was cast by Alcan, Inc. The billets were cast in permanent molds and scalped to 102mm (4 inches) prior to extrusion [2]. The billets were given homogenization heat treatments using heating and cooling rates of 50°C/hr. and 350°C/hr., respectively. Two different homogenization temperatures were used for the billets; 400°C for 8 hours, and 450°C for 4 hours. Two AA 3102 billets were also 19

produced for comparison. Homogenization is a heat treating cycle by which the as-cast microstructure is modified to improve the metal’s workability by reducing heterogeneity.

The two cycles were proposed and performed by Alcan.

The billets were extruded into multi-channel tube sample of the design presented in Figure 7. The extrusion parameters and information are listed in Table 3. The extrusion results are discussed later in the results section. Extrusion of all samples was performed by Alcan.

12.00 0.30

0.40 0.40 ø 0.76 1.30

Figure 7: Experimental multi-channel tube cross-section in mm [2].

Table 3: Extrusion parameters Container diameter 106 mm (4.17 in.) Billet diameter 102 mm (4 in.) Billet length 203 mm (8 in.) Extrusion ratio 817 Billet temperature 500 °C (932 °F) Container/tooling temperature 450 °C (842 °F) Ram velocity 0.7 mm/s (1.65 in/min) Exit velocity ~34m/min (113 ft/min) Quenching Water quench after the press Extrusion run sequence 1st Billet: AA 3102 2nd Billet: AlSc (8 hour, 400°C homo. cycle) 3rd Billet: AlSc (4 hour, 450°C homo. cycle) 4th Billet: AA 3102 20

CHAPTER 3: EXPERIMENTAL APPROACH

Tube specimens produced from the experimental AlSc alloy and a typical AA

3102 alloy were processed in a way that simulates typical manufacturing. These steps include cold rolling to simulate the strain imposed during straightening/sizing and a simulated brazing thermal cycle in a tube furnace. An additional aging heat treatment step was also considered to assess the potential impact on post-braze mechanical properties.

3.1 Cold Rolling

The tubes of automotive climate control heat exchangers are typically processed through rollers for sizing and straightening prior to assembly. This processing induces cold work (plastic strain at room temperature) into the pieces. In order to simulate this processing the pieces were roll sized to various thickness reductions between 0% and

10% using a laboratory rolling mill. This allowed plastic strain to be assessed in amounts that well exceeded the range of typical manufacturing. The initial tube thickness was

1.34 mm.

A ~5% reduction in thickness was also targeted (typical production). This batch was used to compare mechanical properties between as-extruded samples and cold rolled samples of AlSc with different thermal cycles.

3.2 Simulated Brazing

Aluminum heat exchangers are brazed at approximately 600°C in continuous furnaces. In order to simulate this process, tube samples were held at 600°C, inside a tube furnace (Figure 8), for two minutes, and then removed to air cool.

Figure 8: Thermolyne tube furnace with samples partially inserted.

A Thermolyne tube furnace was used to simulate the brazing cycle on the tube samples. Three K-type thermocouples were attached to a dummy sample to monitor and control sample temperature (Figure 9). Figure 10 shows an example of a typical simulated brazing thermal cycle. 22

For a better understanding of the recrystallization response at different temperatures samples were prepared for tensile testing with thermal cycles of 500°C, 550°C, 575°C,

600°C, and 625°C. This range of thermal cycles was applied to the samples that were given a 5% reduction in thickness.

Figure 9: Simulated brazing apparatus with embedded thermocouples loaded in the middle position. AlSc multi-channel tube samples from billet 2 are loaded in the outer positions.

600

550

500 C) o 450

400

350 Temp.1 (°C) Temperature( Temp.2 (°C) 300 Temp.3 (°C)

250

200 0 1 2 3 4 5 6 7 8 9 Time (min)

Figure 10: Simulated brazing thermal cycle on AlSc tube samples [2].

3.3 Aging Heat Treatment

An aging heat treatment was also considered due to the possible positive impact on post-braze grain structure resulting in enhanced mechanical properties. The aging heat treatment consisted of holding the samples at ~288°C for 8 hours. The intention was to assess if the 2nd phase precipitation in this operation could affect the post-braze microstructure and hence, mechanical properties. Figure 11 shows the aging cycle that was used. There is evidence in the literature from Røyset and Ryum that suggests this treatment will allow the Al3Sc phase to precipitate and provide desirable results [7].

Figure 11: Aging heat treatment data at 288°C for 8 hours.

The aging heat treatment was applied to AlSc samples in two approaches. The first approach was to apply the 8-hour, 288°C aging cycle and then apply a 600°C simulated braze. This was done to assess if the Al3(Sc1-x Zrx) dispersoid would impact the post-braze microstructure by preventing recrystallization and limiting grain growth.

The second approach was to apply a 600°C simulated braze and then apply the 8-hour,

288°C aging cycle. This approach was used to determine if there was enough solutionized Sc and Zr remaining to form Al3(Sc1-x Zrx) dispersoids that would significantly strengthen the alloy. The results of both approaches are presented in the burst testing results (Section 5.4). The second approach was also evaluated using tensile testing (Section 5.3). 25

CHAPTER 4: MECHANICAL TESTING AND PROCEDURES

In order to meet the objectives of this research, microstructure analysis, tensile tests, and burst tests were performed. Tensile tests and burst tests allowed for evaluation and comparison of the mechanical properties of the different alloys and process histories.

Microstructure analysis allowed for the correlation of properties to grain structure. The basic procedures for mechanical testing are consistent with those found in the literature

[4]. Much of the procedure is highlighted below.

4.1 Sample Preparation

The samples that were used in this research were provided by Alcan. Samples were cut into sections that were approximately one meter long. There were three separate bundles that were labeled as: AA 3102, AlSc 2, and AlSc 3. The 2 and 3 after the AlSc signify the billet from which they were extruded.

Prior to cutting the samples to size, a portion of each bundle was cold rolled as explained in Section 3.1. The samples were prepared using the following steps:

1. Each sample was cut with a jewelers saw to ~203mm (~8 inches) in length.

2. A line was drawn ~38 mm (~1.5 inches) from each end to mark where the jaws

should align for the tensile test. This provided ~127mm (~5 inches) between the

tensile test jaws. 26

3. Each sample’s mass, length, initial thickness, and reduced thickness were

recorded as seen in Table 4.

Table 4: Measurements taken during sample preparation Initial Reduced Mass Length Sample Alloy Thickness Thickness (g) (mm) (mm) (mm) 21-B5 2AlSc 5.94 202.13 1.34 1.34

21-B15 2AlSc 5.81 201.68 1.34 1.27

4. From these measurements the cross sectional area (A) of each sample was

calculated with Equation 4.1. This was needed in order to calculate the tensile

yield stress and ultimate tensile stress. The cross sectional area varied depending

on the percent reduction due to cold rolling. In Equation 4.1, is the mass, is

the length, and is the density. The density of aluminum was taken as 2.7×10-3

g/mm3.

( ) ( )

The samples chosen for heat treatment were processed in the tube furnace as described in Sections 3.2 and 3.3.

4.2 Tensile Testing

A Tinius Olsen 1000 tensile test machine was used to perform the test (Figure

12). A USB data acquisition module was added to collect the data from the machine and the extensometer. An adjustable power supply was used with the extensometer for calibration. The graphical interface of the data acquisition software is shown in Figure

13. Force, extensometer and head displacement data were taken for each test at a sampling rate of 4 Hz.

Figure 12: Tinius Olsen 1000 tensile test machine with extensometer [4].

Figure 13: Personal DaqView data acquisition software [4].

The samples were prepared using the steps described in Section 4.1. Each sample was loaded in the jaws of the tensile test machine with the end of each jaw lined-up with the marking that was ~38 mm (~1.5 inches) from its respective end. This left ~127mm

(~5 inches) between the jaws.

The strain rate to which standard tensile tests are performed is 10-2 to 10-4 sec-1

[11]. The corresponding crosshead speed needed is calculated with Equation 4.2.

̇ ( )

In Equation 4.2, ̇ is the strain rate, ν is the crosshead speed, and is the gauge length which was essentially the distance between the jaws for these tests. The distance between jaws is ~127mm (~5 inches) and an initial strain rate of 10-2 sec-1 was chosen.

This resulted in a cross-head velocity of 1.27mm/sec (0.05 in/sec). 29

The following steps were performed for each tensile test to ensure a consistent set of data were acquired:

1. A prepared sample was loaded into the top jaws, lining up the marking that

was ~38 mm (~1.5 inches) from the end.

2. The extensometer was attached to the center of the sample.

3. The extensometer was calibrated by setting the power supply output to 6.00

volts. This setting was recommended by the manufacturer.

4. The pin was removed from the extensometer.

5. The bottom of the sample was loaded into the bottom jaws, just like the top.

6. The gauges were all zeroed except for the force gauge. The force gauge on the

tensile test machine showed a compressive force as a result of loading the

sample into the jaws.

7. The test was started.

8. Once the extensometer reached a 3 mm elongation, the test was paused and

the extensometer removed so as to prevent damage.

9. The test was resumed and allowed to run until a failure occurred.

10. Once the sample failed, the test was stopped, and the sample was removed.

The data acquired during tensile testing was extensometer length, the crosshead length, and the force. With these data the yield strength and the ultimate tensile strength were determined. 30

The ultimate tensile strength is the maximum engineering stress that occurred during the tensile test. This was determined by dividing the maximum test force by the initial sample cross sectional area.

The yield strength is determined using the following steps (Figure 14):

1. The force data were graphed as a function of the extensometer data.

2. A 0.2% strain offset line was placed on the graph. This was given the

same slope as the initial part of the graph. The bottom was placed at the

0.2% offset point (0.002 Strain). The 0.2% yield force was determined by

observing where the line crosses the graph.

3. The yield strength was calculated using the 0.2% yield force and dividing

it by the initial cross sectional area of the sample.

Figure 14: Graph of Force (N) vs. Strain (mm/mm) for tensile test on sample 21_B1, AlSc with no reduction in thickness and brazed at 500°C.

4.3 Burst Testing

The burst testing apparatus (Figure 15) and room temperature testing procedure are consistent with those found in the literature [4, 12]. The apparatus was constructed with components that were rated to 10,000 psi (69 MPa). The apparatus is connected to a computer and tests were controlled through software. The following steps were performed for each burst test:

1. The sample was loaded into the collets and locked into place. 32

2. The charge tank was filled with water and then pressurized to fill the lines

with water. The air was then allowed to bleed out of both ends of the system

to ensure only water remained in the lines.

3. The MTS software was activated and the following steps were performed by

the controller:

a. Step 1 – The data file destination folder was chosen and data

acquisition began.

b. Step 2 – The ram applied force at a constant rate.

c. Step 3 – The internal pressure was monitored and once it dropped by

10% (sample failure) the ram was stopped.

d. Step 4 – The ram was returned to its starting position.

4. The sample was removed and the process was started over for the next

sample.

Figure 15: Burst Testing Apparatus using MTS Machine [4].

The data collected during the test consist of internal pressure, volume of water displaced, and ram force, sampled at a rate of 10 Hz. The graph in Figure 16 is an example of the pressure curve that occurred during testing. When a failure occurs there is a sharp instantaneous drop in pressure. In Figure 16, this is seen around the 6000 psi

(41.3 MPa) mark.

Figure 16: Sample graph of burst test pressure over time.

Burst testing was completed at two different phases of the research. The first group of testing was completed on samples that fall into the following categories:

 0 % rolling reduction and no simulated brazing cycle

 0% rolling reduction and 600°C simulated brazing cycle

 5% rolling reduction and no simulated brazing cycle

 5% rolling reduction and 600°C simulated brazing cycle.

The second group of testing was completed on samples prepared as follows:

 0% rolling reduction and 288°C aging cycle prior to a 600°C simulated

brazing cycle 35

 0% rolling reduction and 600°C simulated brazing cycle prior to a 288°C

aging cycle.

The results of these tests are explained in the results section.

4.4 Metallography

Metallography was performed on a select number of samples and this technique was used to evaluate the microstructure and to compare it with mechanical properties.

The grains’ size, shape, and consistency were of primary interest. Standard mounting and polishing procedures were followed and an electrolytic etch was applied. The procedure consisted of the following steps:

1. The sample was trimmed to fit into a mold.

2. The sample was positioned in the mold to allow the area of interest to be

investigated once mounted.

3. The mold was filled with epoxy and then placed in a vacuum to remove the

air. The epoxy filled mold was then left overnight to harden.

4. The mounted sample was removed from the mold and engraved with a label to

make it uniquely identifiable (Figure 17).

5. The mounted sample was polished using the following steps. Each step was

done until the surface appeared to be uniform and all scratches from the

previous step were removed: 36

a. The face of the sample was ground down using a 180-grit SiC disc. This

was done until the area of interest was reached.

b. The grinding continued with a 320-grit SiC disc.

c. The last grinding step was done with a 600-grit SiC disc.

d. The sample was polished starting with a 9µm diamond suspension sprayed

onto a cloth disc.

e. The cloth disc was changed and polishing continued with a 1µm diamond

suspension.

f. The final step of polishing was completed with another cloth disc and a

0.06µm colloidal silica suspension.

6. The sample was given an electrolytic etching using 5 ml HBF4 and 200 ml

H2O with a stainless steel anode for 30 seconds. The etching was repeated as

needed to obtain the desired grain definition.

(a) (b) Figure 17: Mounted and polished samples of AlSc multi-channel tube. (a) shows a top view and (b) shows a side view, dimensions are in cm.

Once the samples were etched, micrographs were taken using a microscope with a polarizing filter. Figure 18 shows micrographs of as-extruded multi-channel tube that were mounted in different orientations. Figure 18a is perpendicular to extrusion and 19b is a longitudinal section through an external wall [2]. Further analysis and discussion of microstructure is presented in the following chapter.

(a) (b) Figure 18: Microstructure of as-extruded AlSc multi-channel tube. The tube shown was from the billet that underwent the 400°C, eight-hour, homogenization cycle; (a) is a section perpendicular to extrusion; (b) is a longitudinal section through an external wall [2].

CHAPTER 5: RESULTS AND ANALYSIS

5.1 Extrusion

The extrusion results for each billet are summarized in Table 5. With the addition of Sc and Zr, the ram (specific) pressure increased by ~44% compared with AA 3102

(Figure 19). The different homogenization cycles on billets 2 and 3 had negligible impact on extrusion. Post extrusion microstructure is analyzed in Section 5.2.

Table 5: Summary of extrusion results. The specific pressures were calculated from the measured hydraulic pressures [2]. Billet Exit Temperature Maximum Specific Billet Temperature 12″ after platen Pressure (°C) (°C) 1. AA3102 64,545 psi (445.02 MPa) 495 529 2. AlSc#2 94,373 psi (650.68 MPa) 503 504 3. AlSc#3 94,746 psi (653.25 MPa) 503 517 4. AA3102 66,740 psi (460.16 MPa) 495 499

mm 0.0 25.4 50.8 76.2 101.6 127.0 152.4 177.8 110000 759

100000 690

90000 621 ) i 80000 552 s p (

e 70000 483 r u s

s 60000 414 e a r P P 50000 345 M c

i f i

c 40000 276 e

p Billet 1 - 3102 S 30000 Billet 2 - AlSc 207 Billet 3 - AlSc 20000 138 Billet 4 - 3102 10000 69

0 0 0 1 2 3 4 5 6 7 Ram Position (inches) Figure 19: Detailed extrusion results showing ~44% increase of ram (specific) pressure throughout extrusion for AlSc when compared to AA 3102 [2].

5.2 Microstructure Analysis

There were significant differences in microstructure between the as-extruded

AlSc samples and the AA 3102 samples. Figure 20 shows the fine, recrystallized, equiaxed grains that formed during extrusion of AA 3102. The AlSc microstructure, shown in Figure 21, was mostly unrecrystallized, heavily distorted grains that did not recrystallize during or subsequent to extrusion. This resistance to recrystallization was caused by the Sc and Zr additions, which are reported to form Al3(Sc1-x Zrx) and Al3Sc dispersoids [7]. There are also some areas along the edge where the recrystallization of very fine grains has occurred. 40

(a) (b) Figure 20: Microstructure of as-extruded AA 3102; (a) is a section perpendicular to extrusion; (b) is a longitudinal section through an external wall [2].

(a) (b) Figure 21: Microstructure of as-extruded AlSc multi-channel tube. The tube shown was from the billet that underwent the 400°C, eight-hour, homogenization cycle; (a) is a section perpendicular to extrusion; (b) is a longitudinal section through an external wall [2].

Figure 22 shows the difference between the microstructures of AlSc samples with brazing cycles at 575°C and 600°C. Figure 22a shows a microstructure much like the as- extruded microstructure while Figure 22b shows a partially recrystallized structure with 41

the formation of some rather large grains at 600°C. The difference in microstructure shows that the recrystallization temperature with large amounts of strain is between

575°C and 600°C. This may be due to much of the Sc-Zr intermetalic particles being dissolved into the Al matrix at that temperature. According to the solvus of the Al-Sc phase diagram (Figure 5b), this should occur at 595°C. The dissolution of Sc and Zr may render the aluminum susceptible to recrystallization and grain growth, as it was no longer in fine, intermetalic form.

During extrusion, the AlSc alloy microstructure did not recrystallize like the AA

3102 alloy did. However, some large grains formed in the AlSc alloy at 600°C. The heavily deformed AlSc alloy microstructure was due to the Sc and Zr additions raising the recrystallization temperature above the extrusion temperature of 500-520°C.

Correlations with the 600°C recrystallization temperature can be seen in the results of tensile testing in the Section 5.3.

(a) (b) Figure 22: AlSc alloy longitudinal sections. (a) After 575°C brazing cycle and (b) after 600°C brazing cycle. Image (a) resisted recrystallization while much of (b) shows recrystallization [2].

Figure 23 shows the difference in microstructure of AA 3102 alloy samples with

4% reduction in thickness that are in the pre-brazed and post-brazed conditions. The recrystallization and large grain formation can be attributed to the strain that occurred within the internal walls during roll-sizing. During the simulated brazing cycle, the stored energy is released and causes the internal walls to recrystallize and experience grain growth. This grain growth weakens the internal wall because grain boundary strengthening is reduced in these areas.

(a) (b) Figure 23: Microstructure of AA 3102 multi-channel tube, roll-sized four percent with (a) no heat treatment, and (b) after a simulated brazing cycle. Recrystallization and large grains formed in the internal walls, whereas much of the outer wall did not recrystallize [2].

5.3 Tensile Testing

Figure 24 shows tensile test results for AlSc samples as a function of thermal cycle temperature. The AlSc samples included the two homogenization cycles and samples that were given 0% and 5% post-extrusion rolling reduction to simulate a 43

straightening and sizing operation. The data show that the addition of a small amount of cold work had negligible impact on the yield and tensile stress after a simulated brazing thermal cycle. The data also show that the different homogenization cycles performed on the billets prior to extrusion have negligible effect on the post braze yield and tensile stresses.

Figure 24: Tensile test results for experimental AlSc tube samples in the post-braze condition [2]. 45

To further assess the influence of cold work, samples with and without a simulated brazing cycle at 600°C were tested with various amounts of rolling reduction.

Figure 25 shows that cold work being induced prior to brazing has negligible effect on post braze mechanical properties. This is due to the heavily strained (retained), post- extrusion microstructure that was discussed in Section 5.2. The strain imposed by the rolling reduction did not cause a significant change in microstructure.

Figure 25: Tensile test results for experimental AlSc tube samples in the post-braze and cold rolled condition.

Figure 26 shows the yield and tensile stresses for AA 3102 as a function of thermal cycle temperature for samples that were given a 5% reduction in thickness and with no reduction. The samples without a reduction in thickness have a small, linear reduction in strength as a function of increasing temperature during the thermal cycle.

This is due to the microstructure being similar throughout the different temperatures. For samples with a 5% reduction in thickness, the graph in Figure 26 shows a sigmoidal reduction in strength. This is because the microstructure experiences increasing levels of partial recrystallization and large grain formation with increasing temperatures. The microstructure change can be seen in Figure 23 in Section 5.2.

Figure 26: Tensile test results for AA 3102 alloy multi-channel tube samples in the post-braze condition [2]. 48

The mechanical performance of the post-braze AlSc alloy has a comparable decline to that of post-braze, reduced AA 3102 due to both alloys experiencing partial recrystallization and grain growth during the 600°C simulated brazing cycle making the samples weaker.

An aging cycle of 288°C for 8 hours was added after the brazing cycle. This was done to determine if the Sc and Zr would significantly increase strength by producing

Al3(Sc1-x Zrx) dispersoids causing precipitate hardening. Figure 27 shows the results of post braze AlSc samples compared to brazed then aged AlSc samples. The brazed then aged AlSc demonstrated an increase in yield strength of ~156% (154 MPa vs.60 MPa) over brazed AlSc and an increase of ~295% (154 MPa vs.39 MPa) in yield strength over brazed AA 3102 (Figure 27).

Figure 27: Tensile test results for AA 3102 and AlSc that displays the difference in yield and tensile stress. YS is yield strength and TS is tensile strength.

5.4 Burst Testing

The results of the burst testing are presented in Table 6. The differences in room temperature burst pressures of the non-aged, brazed AlSc samples and brazed AA 3102 samples, with and without thickness reductions, are negligible (<8% change). The non- brazed results of AlSc are more desirable when compared with those of AA 3102. When comparing the samples with 0% reductions in thickness, AlSc demonstrated an ~18-30% increase in burst pressure over AA 3102. For samples with 5% reductions in thickness,

AlSc demonstrated a ~25-30% increase in burst pressure over AA 3102. . 50

Table 6: Results of burst tested samples. Results are the average of two samples tested for each category with an uncertainty of less than ± 5 % unless otherwise noted. The uncertainty was found by using average deviation for each category. Units are MPa (psi). Burst Pressure MPa (psi) AA 3102 2-AlSc 3-AlSc 0% reduction, no braze 38 (5500) 50 (7300) 50 (6600)* 0% reduction, 600°C braze 39 (5600) 39 (5700) 39 (5500) 5% reduction, no braze 48 (7000) 63 (9100) 60 (8700) 5% reduction, 600°C braze 39 (5700) 42 (6100) 42 (6100) 5% reduction, 288°C aging then 600°C 41 (5900) braze 5% reduction, 600°C braze then 288°C 69 (10000)** aging *uncertainty ± 7.2 % **reached highest testing pressure without failure

Results from samples that were aged at 288°C for 8 hours prior to brazing simulation at 600°C are also presented in Table 6. The aging cycle had no impact on post-braze mechanical performance. The lack of a change can be attributed to the 600°C simulated brazing causing the dissolution of Sc and Zr as discussed in Section 5.2. The dispersoids created during the aging cycle did not sufficiently cause the AlSc to resist recrystallization.

The 288°C aging heat treatment applied subsequent to a 600°C simulated brazing was also investigated. These samples demonstrated a significant increase in burst pressure. The test reached the maximum pressure of the apparatus prior to the samples failing. Values presented in Table 6 for these samples are the maximum recorded 51

internal pressure since the failure pressure was beyond the limitation of the testing apparatus. The samples with a 5% thickness reduction that received a 600°C simulated brazing and then a 288°C aging heat treatment demonstrated a greater than 65% increase over samples with a 5% thickness reduction that received a 600°C simulated brazing and no aging cycle. When comparing the 5% reduced, brazed, then aged AlSc to the 5% reduced and brazed AA 3102, there was a ~80% increase in burst pressure. This increase in strength can be attributed to the precipitates that formed during the aging heat treatment.

CHAPTER 6: CONCLUSIONS

With respect to the objectives of this research the following conclusions can be made:

1. The post extrusion grain structure of the experimental AlScZr alloy was

heavily deformed and mostly non-recrystallized. The extrusion temperature

was 500°C.

2. Partial recrystallization and severe grain coarsening of the AlScZr alloy in this

research occurred between 575°C and 600°C, when held at those brazing

temperatures for 2 minutes.

3. Additional cold work performed on the AlScZr alloy after extrusion had

negligible effect on the post-braze and non-brazed mechanical properties.

4. An aging cycle after brazing significantly increased the yield strength, tensile

strength, and burst strength of the AlSc alloy due to precipitation

strengthening. Burst test failure pressure increased by at least 66% compared

to samples brazed at 600°C with a 5% reduction in thickness. Burst test

failure pressure increased by 80% compared to AA 3102 alloy samples brazed

at 600°C with a 5% reduction in thickness.

5. Post-braze (non-aged) yield and tensile stresses for the AlScZr alloy were

within 8% of those of AA 3102 when brazed at ~600°C. 53

6. The ram pressure was 44% greater for the AlScZr alloy compared to AA

3102.

CHAPTER 7: FUTURE WORK

This research was done with a specific AlScZr alloy. Future work can consider the extrusion and mechanical properties of AlSc multi-channel tube with different amounts of Sc such as those with more and less Sc. This would demonstrate the relationship between extrusion properties and mechanical performance as a function of wt. % Sc. Once more complete histories are created, mechanical properties could be targeted and the trade-off of material and extrusion expense could be better estimated.

REFERENCES

[1] Rebinger, D. C., 2004, "Safety concept proposal for R744-A/C-systems in passenger,"

VDA Alternate Refrigerant Wintermeeting, Saalfelden, Austria.

[2] Kraft, F. F., and Williams, C. R., 2008, "The Effect of Sc and Zr Additions on Al

Extrusion and Post-Braze Grain Structure," Proceedings of the 9th International

Aluminum Extrusion Technology Seminar and Exposition (ET '08), ET Foundation.

[3] Gowreesan, V., 2004, "Process-Structure-Property Relationship of Micro-Channel

Tube for CO2 Climate Control Systems," MSME Thesis Ohio University.

[4] Miller, H., 2006, "Instability and Failure in Aluminum Multi-Channel Tubing,"

MSME Thesis Ohio University.

[5] Willey, L. A., 1971, "Aluminum Scandium Alloy," US Patent (3,619,181).

[6] Røyset, J., August 27, 2007, "Scandium in Aluminum Alloys,"

Http://www.Home.no/al-Sc/, 2007(September 10).

[7] Røyset, J., and Ryum, N., 2005, "Scandium in Aluminum Alloys," International

Materials Review, pp. 19-44.

[8] Tsivoulas, D., and Robson, J. D., 2006, "Coherency Loss of Al3Sc Precipitates during

Ageing of Dilute Al-Sc Alloys," Materials Science Forum 519-521, pp. 473-478.

[9] Russel, A., and Lee, K., 2005, "Structure-Property Relations in Nonferrous Metals,"

John Wiley and Sons., p. 385. 56

[10] Ocenasek, V., and Slamova, M., "Resistance to Recrystallization due to Sc and Zr

Addition to Al-mg Alloys," Materials Characterization, (47) pp. 157-162.

[11] Kalpakjian, S., and Schmid, S.R., 2003, "Manufacturing Processes for Engineering

Materials," Pearson Education, Inc., p. 39.

[12] Kraft, F. F., Huffman, L., and Gowreesan, V., 2003, "Mechanical Testing and

Analysis of Micro-Channel Tube for CO2 Climate Control Systems," Proceedings of the

2003 Vehicle Thermal Management Systems Conference (VTMS 6), pp. 335-342. ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! !

Thesis and Dissertation Services ! !