Soutenance mémoire
METALLOGRAPHIC MICROSTRUCTURE EXAMINATION
EXPERIMENTAL PROCEDURES:
Artificial and Natural Aging:
Samples prepared for aging treatment investigations were first solution treated at 530 °C for 5 h followed by water quenching. They were subsequently grouped and aged at room temperature, 175, 195 and 215 °C, respectively, for different holding times ranging from 1 h to 5 weeks. After aging, the samples were air cooled to room temperature.
Metallographic Microstructure Examination:
For metallographic observations, 10×10 mm samples were cut from the as-cast and solution-treated alloy samples and mounted in bakélite using a Struers Labopress-3 Mounting Press shown in Figure 3.4. The samples were then ground and polished to the desired fine finish by means of a Struers Tegrapol-35 Grinder-Polisher, see Figure 3.5. The grinding was carried out using successively #120, # 240, # 320, # 500, # 800 grit SiC papers. The polishing was carried out by applying polycrystalline diamond powder suspended in water with particles of 6 (im and 3 jim for rough and fine polishing, respectively. Struers oil is used in both polishing stages as a cooling and lubricating medium. Finally, to produce a mirror-like surface of the samples, the ultimate polishing stage was completed using 1 |im colloidal silica suspension with water as a coolant and lubricant. After polishing, the mounted samples were washed with a mix of soap and alcohol, and then dried using compressed air. The samples used for identification of i phase were etched for 5 s at ambient temperature in etchant solution with chemical composition of 1 ml HF(40%), 3 ml HC1(38%), 5 ml HNO3(70%) and 191 ml distilled water.
Hardness Test:
The hardness measurements were conducted on all the samples using a QualiRock Rockwell Hardness Tester shown in Figure 3.9. The tester is equipped with a steel ball indenter with a diameter of 1/16 in. The indenter was first manually pressed into the sample to a certain degree by a minor load of 10 kgf and the zero datum was established. A major load of 50 kgf was then applied for a dwell time of 3 s. The resulting Rockwell number, scale F, represents the difference in depth from the zero datum position as a result of the application of the major load.
Electrical Conductivity Measurement:
Eddy current testing technique was used for measuring the electrical conductivity of the AA2618 alloy in all heat-treated conditions. Electrical conductivity was measured on ground and polished samples in accordance with ASTM standard E 1004-94 using portable FISCHER SIGMASCOPE®SMP 10 Electrical Conductivity Measurement System shown in Figure 3.12. An electrical conductivity measurement probe with a diameter of 11 mm was used for all measurements. The change in the electrical conductivity was measured at room temperature in terms of percentage of international annealed copper standard (%IACS). Before conducting electrical conductivity test, the equipment was calibrated on standards of known conductivity. The measured values were considered to be accurate within ±0.5 %IACS. Six readings were taken for each sample and then the average value was calculated for plotting the graph of electrical conductivity verses aging time.
Quench Sensitivity Test:
AA2618 alloy plates were machined into cylindrical bars which would be used for the quench sensitivity test. The bars were 35 mm in diameter and 160 mm long. They were surrounded by insulating asbestos except the two ends in order to create a one dimensional heat transfer environment along the axial direction of the bar. By this way, the heat transfer along the sides of the cylinder would not occur and the radial variation of hardness could be avoided. Small holes were drilled in the bars at different distances measured from the quench ends to introduce up to seven type K thermocouples that were connected to a PC-based data logging system, as shown in Figure 3.13, to record temperature changes during heating and cooling.
The bars were solutionized for 5 h at 530 °C in the forced-air electric furnace shown in Figure 3.2. The furnace door was then opened and the test bar was grabbed using insulated gloved hands. The bar was placed on the Jominy end quench fixture as shown in Figure 3.14 and the water flow to the nozzle was initiated. The time elapsed between the opening of the furnace door and the initiation of water flow was approximately 5 s. Once the temperature of the whole bar dropped to below 50 °C, the bar was removed from the fixture and immediately placed into a room temperature water bath. After reaching room temperature, the bar was dried and then aged at 215 °C for 1 h.
Characterization of the As-cast Microstructure of the AA2618 Alloy:
The as-cast microstructure of the received AA2618 aluminum alloy was first investigated in order to identify the intermetallic phases contained as a basis for the following research work.
Metallographic Observations:
The optical micrograph shown in Figure 4.1 (a) reveals that the as-cast structure of the AA2618 alloy consists of uniform equiaxed a-Al grains along with a considerably high amount of coarse intermetallic phases distributed at the aluminum dendrite boundaries. At higher magnification (Figure 4.1 (b)), three different intermetallic phases, namely block-like AfeCu, branched AlçFeNi, and trace dark Mg2Si, can be clearly observed. The optical micrograph shown in Figure 4.2 was obtained from as-cast AA2618 alloy sample after etching, in which the AlçFeNi phase can be obviously identified in brown color. The AlçFeNi phase, which exists as block-like or branched particles, is the predominant intermetallic phase in the alloy microstructure.
DSC Analysis:
DSC analysis was used to determine the melting points of the intermetallic phases formed in the as-cast AA2618 alloy with the intension of optimizing the solution treatment of the alloy. Figure 4.5 shows the relevant DSC curve, which exhibits six endothermic peaks at 508.3, 539.9, 553.4, 575.5, 634.0 and 636.5 °C. The first peak represents the low melting point of the eutectic like AkCuMg phase precipitated by a quaternary eutectic reaction at the last stage of solidification. The second peak is attributed to the melting of block-like AI2CU phase. The following three peaks may be related to the melting of intermetallic phases, namely i, Al7Cu2(Fe,Ni) and i, respectively. The last large peak corresponds to the bulk melting of the primary a-Al.
Effects of Solution Treatment:
Metallographic Observations:
The BSE images shown in Figure 4.6 (a-d) display the micro structure of the AA2618 alloy after solution treatment for 5 h at different temperatures ranging from 480 °C to 545 °C. It can be observed that as the solution treatment temperature increases from 480 °C to 545 °C, the intermetallic phases undergo a pronounced change when compared to the as-cast micro structure shown in Figure 4.1 and 4.3. Solution treatment whether at 480 °C or 505 °C can partially dissolve the bright Cu-containing phases, namely A^Cu, A^CuMg and i, as shown in Figure 4.6 (a-b). After solution treatment at 530 °C, these phases seem to become almost completely dissolved in the solid solution. The gray A^FeNi phase was only subjected to partial fragmentation and spheroidization, particularly at 530 and 545 °C.
CONCLUSIONS:
An investigation was carried out on the effects of the heat treatment on the microstructure and mechanical properties of the AA2618 DC cast alloy proposed for the manufacturing of precision molding plates. Based on the results obtained, the following conclusions can be drawn:
(1) The microstructure of the as-cast AA2618 alloy is composed of AfeCu, AkCuMg, AMDuiNi, Al7Cu2(Fe,Ni), and AlçFeNi intermetallic phases, distributed at a-Al dendrite boundaries.
(2) The solution treatment carried out at 530 °C for 5 h was found to be the optimum solution treatment, by which the AkCuMg, AI2CU and AlyCityNi phases could be dissolved completely into the solid solution, whereas the Al7Cu2(Fe,Ni) and AlçFeNi phases were only subjected to fragmentation and spheroidization.
(3) The effect of solution soaking time on the microstructure and mechanical properties of the AA2618 alloy is not as significant as that of the solution temperature.
(4) AA2618 alloy is a quench sensitive aluminum alloy. With the same solution and aging conditions, the air-cooled samples yields lower hardness and strength values than water quenched samples. The Jominy end quench test results demonstrated that the hardness value of the AA2618 alloy declines with decreasing cooling rate.
(5) The peak-aged conditions of the AA2618 alloy were attained after aging for 36 h at 175 °C, 10 h at 195 °C and 1 h at 215 °C with surface hardness values of 100.0, 97.4 and 96.8 HRF, and center hardness values of 97.5, 95.5 and 94.8 HRF, respectively.
(6) The aging treatments applied to the AA2618 alloy result in different combinations of strength and ductility, which may satisfactorily fulfill the specifications of the large mold manufacturers.
(7) AA2618 alloy was found to be prone to the natural aging, displaying hardness values comparable to those obtained by the artificial aging at 195 and 215 °C.
(8) DSC analysis suggested that the main strengthening phase of the AA2618 alloy, under artificial aging, is S-AkCuMg phase, whereas the strengthening effect caused by the natural aging is probably attributed to the formation of Cu-Mg co-clusters and/or GPB zones.
(9) The electrical conductivity of the AA2618 alloy decreased during the first 4 hours of aging at 175 °C and during the whole natural aging process, due to the formation of GPB zones and increased over other artificial aging periods, due to the purification of the aluminum matrix.
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Table des matières
INTRODUCTION
1.2 OBJECTIVES
2 LITERATURE REVIEW
2.1 AL-CU-MG ALLOYS
2.2 AA2618 ALLOY
2.2.1 Effects of nickel and iron in 2618 alloy
2.2.2 Effects of combined additions of Ni and Fe
2.2.3 Effects of other additives on 2618 alloy
2.3 SOLUTION HEAT TREATMENT
2.4 QUENCHING
2.5 PRECIPITATION HARDENING ,
2.5.1 Precipitation sequence
2.5.2 Precipitation hardening of AA2618
2.5.3 Effects of precipitates on the electrical conductivity
3 EXPERIMENTAL PROCEDURES
3.1 MATERIALS
3.2 HEAT TREATMENTS
3.2.1 Solution Heat Treatment and Quenching
3.2.2 Artificial and Natural Aging
3.3 METALLOGRAPHIC MICROSTRUCTURE EXAMINATION
3.4 DSC ANALYSIS
3.5 HARDNESS TEST :
3.6 TENSILE TEST
3.7 ELECTRICAL CONDUCTIVITY MEASUREMENT
3.8 QUENCH SENSITIVITY TEST
4 RESULTS AND DISCUSSION
4.1 CHARACTERIZATION OF THE AS-CAST MICROSTRUCTURE OF THE AA2618 ALLOY
4.1.1 Metallographic Observations
4.1.2 DSC Analysis
4.2 EFFECTS OF SOLUTION TREATMENT
4.2.1 Metallographic Observations
4.2.2 DSC Analysis
4.2.3 Mechanical Properties
4.3 QUENCH SENSITIVITY OF THE AA2618 ALLOY
4.4 EFFECTS OF AGING TREATMENTS
4.4.1 Mechanical Properties
4.4.2 Electrical Conductivity
4.4.3 DSC Analysis
5 CONCLUSIONS
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