Microstructure and hardness of 319-type Al-Si-Cu-Mg alloys

ALUMINUM-SILICON ALLOYS

SOLUTION HEAT TREATMENT AND AGING

The bars were prepared for each alloy composition and divided into sixteen sets; one set was kept in the as-cast condition; one set was solution heat-treated at 495°C for 8 hours, then quenched in warm water at 65°C and maintained in solution heat-treated conditions; seven sets were solution heat-treated at 495°C for 8 hours, then quenched in warm water at 65°C followed by artificial aging at 180°C for 2, 4, 6, 8, 12, 24, and 48 hours, respectively; the remaining seven sets were solution heat-treated at 495°C for 8 hours, then quenched in warm water at 65°C followed by artificial aging at 220°C for 2, 4, 6, 8, 12, 24, and 48 hours, respectively. The solution and aging heat-treatments were carried out in a forced-air Blue M Electric Furnace equipped with a programmable temperature controller, accurate to ± 2°C. The aging delay was less than 10 s. For each individual heat treatment, five test bars were used. The above procedures were applied in the heat treatment of the hardness and impact test samples.

MECHANICAL TESTING

The mechanical properties of the alloys examined in this study were evaluated through their hardness and impact properties. A description of the castings prepared for these tests and the details relating to the test samples sectioned from the respective castings have already been provided in subsection 3.3, above.

Hardness Testing

Hardness test bars measuring 10 mm x 10 mm x 55mm were cut from the casting, as shown in Figures 3.4 and 3.5. All test samples were heat-treated in the same way as the impact samples, while the specimen surfaces were polished with fine sandpaper to remove any machining marks. The hardness measurements were carried out on the as-cast and heattreated samples using a Brinell hardness tester, applying a steel ball of 10 mm diameter and a load of 500 kgf for 30 seconds. Figure 3.4 shows the Brinell hardness tester used for these measurements. An average of four readmgs obtained from two perpendicular surfaces was taken to represent the hardness value in each case.

Impact Testing

As mentioned earlier in subsection 3.3, each impact mold-casting provided ten impact test bars and each L-shaped mold provided twelve bars. The samples were sectioned from the casting, and machined according to the dimensions shown in the diagram in Figure 3.5. The specimen surfaces were polished with fine sandpaper to remove any machining marks; it should also be noted that the impact tests were performed on unnotched samples.
A computer-aided instrumented SATEC SI-1 Universal Impact Testing Machine (SATEC Systems Inc., Model SI-1D3), as shown in Figure 3.6, was used to carry out the impact tests. This machine is equipped with bolt-on weights in addition to the pendulum.
The pendulum is capable of being latched in two separate modes, known as “high latch” and “low latch,” providing a total of four operating capacities, namely, a capacity of 25 ftlbs (33.9 J) on low latch and 60 ft-lbs (81.35 J) on high latch without the bolt-on weights attached, and a capacity of 50 ft-lbs (67.8 J) on low latch and 120 ft-lbs (162.7 J) on high latch with the additional weights attached. A data acquisition system connected to the impact machine monitored the dynamic behavior of the test specimen and measured the load and energy values as a function of time.
The total absorbed energy (Et) during impact testing was determined, together with a number of specific parameters such as crack initiation (Ej) and crack propagation (Ep) energies, total time, and the maximum load required to break the specimens. The loaddeflection curves and energies absorbed were obtained using a Dynatup IPM/PC Impact Testing System. The average values of the energies obtained from the five samples tested for each alloy condition were taken as the representative values for that particular condition.

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Table des matières

RÉSUMÉ 
ABSTRACT 
ACKNOWLEDGMENTS
TABLE OF CONTENTS 
LIST OF FIGURES 
LIST OF TABLES 
CHAPTER 1
DEFINING THE PROBLEM
1.1 INTRODUCTION
1.2 OBJECTIVES
CHAPTER 2
REVIEW OF THE LITERATURE 
2.1 ALUMINUM-SILICON ALLOYS
2.1.1 Al-Si-Cu Alloy System
2.2 SOLIDIFICATION OF Al-Si-Cu 319-TYPE ALLOYS
2.3 ROLE OF MODIFICATION
2.4 ROLE OF MAGNESIUM
2.5 INTERMETALLIC PHASES IN Al-Si-Cu ALLOYS
2.5.1 Copper Intermetallic Phases
2.5.2 Iron Intermetallic Phases
2.6 HEA T TREATMEN T O F Al-Si-Cu ALLOY S
2.6.1 Solution Heat Treatment
2.6.2 Quenching
2.6.3 Precipitation Heat Treatment (Aging)
2.6.4 Incipient Melting
2.7 POROSITY
2.7.1 Theory of Porosity Formation
2.8 MECHANICAL PROPERTIES OF Al-Si ALLOYS
2.8.1 Hardiness Testing
2.8.2 Impact Testing
2.8.2.1 Effect of Sample Configuration
CHAPTER 3
EXPERIMENTAL PROCEDURES
3.1 INTRODUCTION
3.2 ALLOY PREPARATION AND MELTING PROCEDURE
3.3 MELTING AND CASTING PROCEDURES
3.4 SOLUTION HEAT TREATMENT AND AGING
3.5 MECHANICAL TESTING
3.5.1 Hardness Testing
3.5.2 Impact Testing
3.6 METALLOGRAPHY AND MICROSTRUCTURAL EXAMINATION
CHAPTER 4
MICROSTRUCTURE AND HARDNESS OF 319-TYPE Al-Si-Cu-Mg ALLOYS
4.1 EFFECT OF MODIFICATION
4.2 EFFECT OF MAGNESIUM
4.3 COMBINED EFFECTS OF MAGNESIUM WITH STRONTIUM
4.4 EFFECT OF COOLING RATE
4.5 EFFECT OF HEAT TREATMENT
4.6 EFFECT OF INTERMETALLIC PHASES
4.7 RESULTS AND DISCUSSION
4.7.1 Microstructural Analysis
4.7.2 Porosity Characteristics
4.7.3 Hardness
CHAPTER 5
IMPACT TOUGHNESS AND FRACTOGRAPHY OF 319-TYPE Al-Si-Cu-Mg ALLOYS
5.1 EFFECTS OF SAMPLE CONFIGURATION
5.2 EFFECTS OF MODIFICATION
5.3 EFFECTS OF MAGNESIUM
5.4 COMBINED EFFECTS OF MAGNESIUM WITH STRONTIUM
5.5 EFFECTS OF COOLING RATES
5.6 EFFECTS OF HEAT TREATMENT
5.7 EFFECTS OF INTERMETALLIC PHASES
5.8 RESULTS AND DISCUSSION
5.8.1 Impact Toughness
5.9 PRECIPITATION PROCESSES
5.10 FRACTOGRAPHY RESULTS
5.10.1 Secondary Electron Beam Imaging (SEM Fractography)
5.10.2 Backscattered Imaging
CHAPTER 6
CONCLUSIONS 
RECOMMENDATIONS FOR FUTURE WORK 
REFERENCES

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