Cost Analysis of T6 Induction Heat Treatment for the Aluminum-Copper Powder Metal Compacts

: This work compares an energy cost and an energy consumption results of the 4 wt.% cupper mixed aluminum based powder metal (PM) compacts processing under induction or furnace heating. Total power and energy consumptions and total energy costs per kilogram and compact have been analyzed. T6 precipitation heat treatment applications have been applied with two different methods, one with 2.8 kW, 900 kHz ultra-high frequency induction heating system (UHFIHS), other with 2 kW chamber furnace. In the first method, Al-Cu PM compacts have been heated by induction at 580 °C in one minute and then cooled down by water. Afterwards, the samples have been heated 170, 180, 190 and 200 °C respectively for artificial ageing and cooled naturally. In the second treatment, unlike the first study, Al-Cu PM compacts are heated by chamber furnace at 540 °C in 5 hours and cooled by water. Then PM compacts are artificially aged at 190 °C in 10 hours with same furnace. During both processes, energy and power consumptions for each defined process have been measured. Optimum heat treatment of the induction is determined. The cheaper energy cost is obtained by the induction heat treatment.


Introduction
The powder metallurgy is the method of procuring semi-finished and finished products with or without using (P/M) [1,2] blended or alloy powder additives. P/M compacts are produced by mixing pure or alloy powders, being compressed within a mould and being heated in order to allow the metal powders to form a metallurgical bond [3]. Sintering [4,5] is the method of producing components, of which density can be kept under control, and of which resistance & integrity are formed through the implementation of thermal energy to the metal powders in a temperature under the fusion temperature. When thermal energy is applied on compressed metal powder, the component condensates, and the average particle size level increases; this condensation and the increase in the particle size is named as sintering [6].
PM based compacts are sintered by medium frequency induction system [7][8][9] or ultra high frequency induction system [10,11]. Induction system could be using different processes like; forging [12], heat treatment [13], welding [14][15][16] or casting. Among the aluminum alloys, the copper [17,18] component is the component that has the highest level of 2000 series [19,20] alloys, and this increases the hardness and resistance of the alloy. As the most significant factor for increasing the mechanical characteristics of aluminum alloys [21] by optimizing them, T6 heat treatment [22,23] comprises of the solution treated and artificial ageing processes. AL 2000 alloys are used in many areas like aircraft and computer parts, gears, shafts, hinge pins, valves and brake system components. T6 heat treatment provides more endurance and cracking resistance compared to T4 and T351 heat treatments [24].
Analyzed the impact of AL 2024 T351 and T6 temper alloys-containing 4 % of Cumicrostructure heterogeneity effect on the stretching characteristics in the friction stir welding on their studies [25]. They pointed out that the AL 2024 T6 material is stronger than the AL 2024 T351 alloy, and it has lower ductility, as well. In study [26] 4,1 % Cu, 12 % Cu [27,28] and [29,30] carried out studies on Al alloys containing 4,8% Cu. In study [31] acquired the current and voltage data of a three phase induction motor by using the LabVIEW based measuring system, which is also used by [32] in order to receive the power data of arc furnace that is used for metal melting processes in the industrial areas both on their own studies and on this study.
The objective of this study is to produce 2000 series Al-Cu composition as cylindrical powder metal composition, which is one of the most frequently used block shapes, as well as carrying out the T6 heat treatment via conventional method and a competitor heat treatment method. The analyses concerning the total cost of manufacturer during the heat treatment of PM materials via both of the above stated methods were carried out, and the differences in the production times were compared between each other. The optimal temperatures of Al PM samples were identified during the artificial ageing through induction.

Materials and Methods
The sizes of the aluminum and copper powder are 45-106 μm. In the study, Al PM samples containing 4 % of Cu in weight were sintered in batch type oven. Then, T6 heat treatments were carried out using two different heating units either in induction or oven. The first groups of samples were applied with T6 heat treatment via induction system. The image of the induction and Power Controller system can be seen on Fig. 1. The second groups of samples were applied with T6 heat treatment in the same oven.
In order to procure a homogeneous mixture, the A1 based powder containing 4 % of Cu in weight was mixed for half an hour with V-Type mixer in 20 d/m, and then it was coldpressed on 300 Mpa with uniaxial single lap "Hidropar" branded press machine. The sizes of procured PM compacts are as follows: diameter: 18 mm, height: 2 mm. All the samples were sintered in open atmosphere oven for 60 minutes on 600 °C temperature using 2 kW "Proterm" branded Chamber Furnace. You can see the image of oven-sintered Al-Cu PM sample on Fig. 3a. The induction system, which is used for the T6 heat treatment of the first group of samples has the frequency level of 900 kHz, and 2.8 kW power. The heating temperature is carried out and stabilized with the infrared thermometer connected to the system. The solution treated (one minute) was carried out on 580 °C in order to perform T6 heat treatment of Al based PM compacts, using UHFIS. Then, they were aged for 8 minutes in 4 different temperatures ranging between 170, 180, 190 and 200 °C for artificial ageing.
The image of the sample during the implementation of heat treatment with induction can be seen on Fig. 4. The image of the sample following the quenching process can be seen on Fig.  3b, while the image for the form of being aged in 190 o C with induction can be seen on Fig.  3c. The solution treated was applied on the second group of samples for 5 hours on 540 °C, with the help of the oven used for sintering, and quick-quenching was applied. Then, the T6 heat treatment was completed after being aged for 10 hours on 190 °C in order for artificial ageing. The whole of the treatment process for this study can be found on Fig. 2.    Densities of the sintered and heat treated samples are measured according to Archimedes principle. For the density calculations, the specific gravity of aluminum was taken as 2,71 g/cm 3 , while the specific gravity of copper was taken as 8,94 g/cm 3 . Then, the result was calculated as 2,959 g/cm 3 by taking 96 % of the aluminum specific gravity, and 4 % of copper specific gravity. On Fig. 5, you can see the structure of the LabVIEW TM based measurement and data acquisition system. Certain values like power consumption and power quality were calculated using the power data received from this data acquisition system [30,31]. Such electrical data as current, voltage and power coefficients for experimental studies were recorded with LabVIEW TM 8.5 graphic interface via National Instruments data collection card through the current and voltage sensors in accordance with IEC standards. The power calculations were made using the power parameters received from the data collection system, as required [10,13].

Results and Discussion
On Fig. 6, you can see the time varying current graphic for induction sintering of Al-Cu PM sample on 580 °C. Since the sample reaches the set temperature approximately 5 seconds after the sintering process is started, it can clearly be seen that the current value decreases, as well. 17 A of current was drawn from the system for about 5 seconds, until the sample reached to the set temperature. After the system reached to the set temperature, it kept operating with a current value of approximately 5A.   Tab. II Power consumption values and power costs for solution treated and artificial ageing of Al-Cu PM samples with induction system and conventional methods.
The hardness and density values of the PM aluminum materials after the process is fully completed can be seen on Table I. The hardness value (37 HB) and the density value (2,367 g/cm 3 ) of the PM compacts after sintering increased as a result of precipitation hardening and artificial ageing. While applying heat treatment with induction, 580 o C of temperature value was used instead of 540 o C, which we used for solution treated, in order to procure fully homogeneous solid melt.
The solution treated time was taken as 1 minute in order to let the grain boundaries any liquid phase on the material, thus preventing any possible excessive brittleness. Analyzing the density and hardness values, the hardness and density values of the Al PM samples were almost the same, after quenching following the solution treated with induction and traditional method. The values procured via the traditional method for 5 hours (300 The same time-saving was achieved on the artificial ageing with induction. The hardness and density values of the samples aged for 8 minutes on 190 o C with induction were achieved within 10 hours (600 minutes) on 190 o C via the conventional method. Comparing both of these heat treatment methods, the duration of artificial ageing was 75 times less with the induction heating, and also the duration of precipitation hardening heat treatment was 100 times less in total.
The Al-Cu materials used in such industries as aviation, space science and automotive -which require high strength and hardness values will be able to be produced within much less period of time using induction precipitation hardening heat treatment.
On Table II, you can see the power consumption and cost values can be seen for the solution treated and ageing processes of Al-Cu PM samples with induction and conventional method. For the solution treated process performed for 1 minute on 580 o C with induction, the energy consumption value was 1.925 kWh.kg -1 , while the cost is 0.1885 $.kg -1 . However, for the solution treated process performed for 5 hours on 540 o C with conventional method, the energy consumption value was 5.361 kWh.kg -1 , while the cost was 0.5249 $.kg -1 . For the artificial ageing process performed for 10 hours on 190 o C with induction, the energy consumption value was 2.769 kWh.kg -1 , while the cost was 0.2712 $.kg -1 ; on the other hand, for the artificial ageing process performed for 10 hours on 190 o C temperature with the conventional method, the energy consumption value was 3.317 kWh.kg -1 , while the cost was 0.3248 $.kg -1 .

Conclusion
On this study, T6 heat treatment was applied on 3 wt.% Cu + Al PM cylindrical sample by using UHFIS and conventional method. The procured results are as follows: • It was confirmed that the temperature increase on induction artificial ageing process increases the production cost. • The optimum temperature for induction artificial ageing process was identified to be 190 o C. • When the dwell tiems of conventional method and induction were compared, the duration of the heat treatment was 300 times less for solution treated -quenching on Al based PM samples using induction; 75 times less for artificial ageing, and 100 times less in total for precipitation hardening. Comparing the hardness and density values, the procured values were almost the same. • Comparing the solution treated processes with induction and conventional method, the induction was confirmed to be 2.8 times less. When the costs for artificial ageing were compared, 1.2 times of decrease was confirmed on the costs.