Semi-solid processing and tribological characteristics of Al-Cu Alloy

Materials Today: Proceedings 2 ( 2015 ) 1175 – 1182

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2214-7853 © 2015 Elsevier Ltd. All rights reserved. Selection and peer-review under responsibility of the conference committee members of the 4th International conference on Materials Processing and Characterization.doi: 10.1016/j.matpr.2015.07.029

4th International Conference on Materials Processing and Characterization

Semi-solid processing and tribological characteristics of Al-Cu Alloy

Ashutosh Sahu1*, Ajit Behera2 1Metallurgical Engineering Department

Indian Institute of Technology, BHU, Varanasi, 221005, INDIA 2Department of Metallurgical & Materials Engineering

Indian Institute of Technology, Kharagpur-721302, INDIA

Abstract

In this paper Al-10Cu alloy was casted into rectangular blocks by using metal mold casting technique followed by warm working. Then the warm worked samples were annealed in semi-solid temperature region at temperatures 560 °C, 580 °C and 600 °C for different holding times followed by water quenching. Grain size distribution, hardness test and tribological test were carried out in order to study the effect of Strain induced melt activation (SIMA) process on tribological properties of the above alloy. SEM analysis of the wear surface was done after pin-on-disc test to study the mode of wear. In this way the optimum condition was found out for obtaining the best tribological properties of the above alloy by changing the morphology from dendrite to globular by using SIMA process. It was found that annealing at higher temperature for higher time period affect the grain size and the hardness of the material. © 2014 The Authors. Elsevier Ltd. All rights reserved. Selection and peer-review under responsibility of the conference committee members of the 4th International conference on Materials Processing and Characterization.

Keywords: SIMA; dendritic structure; globular morphology; hardness; tribological property

1. Introduction

Aluminum and its alloys play an important role in various industries such as automobile and aircraft industries due to its high strength to weight ratio, better corrosion resistance, ductility, malleability, good machinability etc. [1]. Aluminum alloys are very easy to cast using both conventional (sand mold casting, metal mold casting etc.) and unconventional casting (centrifugal casting, spray casting, semi-solid processing etc.) techniques. The * Corresponding author. Tel.: +0-000-000-0000 ; fax: +0-000-000-0000 . E-mail address: [email protected]

© 2015 Elsevier Ltd. All rights reserved. Selection and peer-review under responsibility of the conference committee members of the 4th International conference on Materials Processing and Characterization.

1176 Ashutosh Sahu and Ajit Behera / Materials Today: Proceedings 2 ( 2015 ) 1175 – 1182

conventionally cast Aluminum alloys contain dendritic morphology of primary phase in the microstructure which gives rise to limited mechanical properties such as strength, hardness, wear resistance etc. So in order to improve the mechanical properties, it is necessary to change the dendritic morphology into globular [2]. Out of different techniques to change the dendritic morphology the most advanced technique is semi-solid metal processing technique. It was developed in MIT, US in 1970s by Spencer et al. [1, 2]. There are three main types of semisolid metal processing techniques, viz. Thixocasting, Rheocasting and Strain Induced Melt Activation (SIMA) process [3]. Among the three techniques, the most advanced technique is SIMA. In this process the conventionally cast material is deformed by different metal forming processes in order to break the dendritic network which results in induced strain energy with various types of defects such as voids, dislocations etc. Then the material is annealed within solidus and liquidus temperature region for certain holding time so that the broken dendrites get converted into globular shape by minimizing the free energy and equiaxed grains are formed in the microstructure [2-4]. The globular morphology in the microstructure improves some of the mechanical properties (strength, wear resistance etc.) [1].

In the present study Al-10Cu alloy was subjected to SIMA process in order to change the dendritic morphology into globular. Grain size distribution was done after studying the microstructure under optical microscope in order to optimize the time and temperature required for annealing in the semi-solid temperature region. Hardness test and tribological test were done and from the obtained data, graphs were plotted for comparison. SEM analysis was done to study the wear surface and find out the type of wear. In this way an optimum condition was found out to process the material using SIMA process to get the best tribological properties.

* Corresponding author. Tel.: +91 8170837457. E-mail address:[email protected]

2. Experimental Work

The Al-Cu alloy was melted in an induction furnace at 750 °C to cast in the shape of rectangular blocks of dimensions 150 mm x 80 mm x 24 mm in metal mold made up of mild steel. The composition of the alloy is shown in table-1. The rectangular blocks were then subjected to warm forging and warm rolling operations after annealing at 350 °C for 30 minutes in a resistance furnace. Thickness reductions of 25% and 40% were obtained by warm forging and 35% was obtained by warm rolling. Then samples of size 40 mm x 12 mm x 12 mm were cut from the warm worked material and subjected to annealing in the semi-solid temperature region at 560 ºC, 580 °C and 600 °C for different holding times in a muffle furnace and quenched in water. Sample preparation for optical microscopy was done using different grades of grinding papers and polished using cloth polishing and then etched in Kellar solution (solution of 2.5% HNO3, 1.5% HCl and 1% HF with 95% distilled water) for 10-15 seconds and carefully examined under Zeiss Axio Vision Image Analyser optical microscope. After getting the microstructures grain size distribution was done in order to study the effect of holding time on grain size at the above temperatures. The number of grains that lie in the range of 0-20, 20-40, 40-60, 60-80, 80-100, 100-120, 120-140, 140-160, 160-180, 180-200, 200-220, 220-240 μm size were calculated for the material under different conditions of deformation and annealing in semi-solid temperature region at the above temperatures and holding times. Five microstructures were used to calculate the number of grains that lie in each range for a certain condition of deformation and annealing. The percentages of grains that lie in those ranges were calculated and graphs were drawn. Then hardness test was carried out using ‘Leco LV 700 AT’ Vickers Hardness tester. Ten readings were taken for each sample. Then pins of size 30 mm length and 8 mm diameter were made and subjected to tribological test on ‘Ducom TR-25’ pin-on-disc tribo-tester. The disc used was made up of EN32 steel. Wear test was done without any lubrication. Data of variations of wear rate with increasing sliding distance and with increasing load were taken and graphs were plotted in order to compare the tribological characteristics. SEM analysis of the wear surface was done using ‘Zeiss Supra 55’ FE-SEM to study the mode of wear.


Table 1: Composition of Al-Cu Alloy.

Element Cu Mg Mn Si Fe Zn Pb Al

Weight % 10 0.6422 0.5108 <1.0000 0.317 0.2228 0.1869 Balance

3. Results and Discussion

3.1Microstructure examination

Figure 1(a) shows the optical microstructure of the alloy in as-cast condition. Here it reveals the presence of dendritic structure of primary phase [1, 3, 8]. The white portion in the figure shows the primary α-phase and the black portion shows the secondary phase. Figure 1(b) shows the microstructure in 25% warm forged condition. In this condition some of the dendrites are broken and the secondary phase region at the grain boundary is squeezed. Figure 1(c) shows the microstructure in 40% warm forged condition. In this condition all the dendrites are broken and there is no inter-connection between the grains. Figure 1(d) shows the microstructure in the 35% warm rolled condition. It is seen that the grains are broken and elongated in the direction of rolling [2, 8].

Figure 1: Microstructure of Al-10Cu alloy in (a) as cast Condition, (b) Warm forged 25% deformed condition (c)

Warm Forged 40% deformed condition and (d) Warm Rolled 35% deformed condition. Figure 2(a) shows the beginning of spheroidization of the fragmented grains after annealing the warm forged

25% deformed material at 560 °C for 15 min holding time. It can be observed that some of the grains are completely converted to globular shape. But most of them are in fragmented condition. That means spheroidization in the semi-solid state is not complete. After recrystallization when it enters the semi-solid temperature region the secondary phase melts. At this stage the fragmented grains are subjected to a liquid penetration and further fragmentation occurs. Then the fragmented grains get converted into globular ones provided sufficient time be given [2, 7, 8]. If the sample is taken out without giving sufficient time and quenched then they remain in the fragmented condition. Figure 2(b) shows the microstructure of the alloy in 35% warm rolled condition which was annealed at 560 ºC for 15 min holding time. It can be observed that equiaxed grains are formed throughout and the morphology has been completely changed to globular. Figure 2(c) shows the microstructure of the alloy in 40% warm forged condition


which was annealed at 560 ºC for 10 min holding time. It is seen that the primary grains have been converted into globular completely in lesser time and also the grain size is much smaller compared to the other two and there is uniform distribution of fine equiaxed grains [8]. The process of changing the morphology has become kinetically energetic at higher deformations because more strain energy is stored in the material in terms of various defects at higher deformations. When temperature is increased the stored strain energy acts as a driving force and the grains recrystallize by losing the strain energy at lower temperatures and in the semi-solid temperature region more fragmentation occurs due to liquid penetration and the fragmented grains change into globular shape by minimising the free energy at a faster rate [4, 6, 8]. Hence lower holding time is required for changing the morphology at the same semi-solid temperature.

Figure 2: Microstructures of the material in (a) warm forged 25% deformed condition and annealed at 560 ºC for

15 min holding time, (b) warm rolled 35% deformed condition and annealed at 560 ºC for 15 min holding time (c) warm forged 40% deformed condition and annealed at 560 ºC for 10 min holding time.

Figure 3(a), (b) and (c) show the microstructures of the alloy in the above deformations and semi-solid processed

at 600 ºC for higher time periods. It is seen that the grains have been distorted and liquid entrapment has occurred on those distorted grains. This is because with increasing temperature and holding time grain coarsening occurs and also the liquid content increases [5, 7]. When grains become larger, the liquid comes on the surface of the grains. At this stage when the sample is quenched the liquid cannot come out of the surface and gets trapped on it [9].

Figure 3: Microstructures of the material (a) in warm forged 25% deformed condition and heated at 600 ºC for 45

min holding time, (b) in warm rolled 35% deformed condition and heated at 600 ºC for 60 min holding time, (c) in warm forged 40% deformed condition and heated at 600 ºC for 45 min holding time.

3.2 Grain size distribution

Figure 4 shows the grain size distribution of the alloy under 40% warm forged condition and annealed for 10 minutes and 45 minutes holding times at 560 °C temperature. It can be seen that majority portion of the grains lie in 60-80 μm range when annealed for 10 minutes holding time at 560 ºC and when annealed for 45 minutes holding time at 560 °C the majority portion of grains lie in 100-120 μm range and the percentage of grains lying in the higher ranges is also increased compared to the previous one. Similar trends were found in case of 25% warm forged and 35% warm rolled condition and at all temperatures and holding times.


Figure 4: Grain size distribution of (a) warm forged (40% deformation) and annealed at 560 °C for 10 min and

(b) warm forged (40% deformation) and annealed at 560 °C for 45 min.

3.3 Hardness test

From the hardness test it was found that the alloy in 40% warm forged condition when annealed for 10 minutes holding time at 560 ºC gave better hardness compared to others. Table-2 shows the variation of hardness of the alloy in the as-cast and warm worked conditions. The hardness of the warm worked alloy is lower than the alloy in as cast condition. This is because when the alloy was heated, the strain energy got released and with increasing warm working the hardness increased again as more strain energy was induced with increasing deformation. Table-3 shows the variation of hardness of the alloy in 40% warm forged condition and annealed for different holding times at the above three temperatures. The hardness decreased with increasing holding time and annealing temperature in the semi-solid temperature region. This is because at higher temperatures and holding times grain coarsening occurs and the grains become strain free which leads to lower hardness. The same trend was observed in 25% and 35% warm worked conditions.

Table 2: Vickers hardness in as-cast and warm worked conditions. Condition As Cast Warm Forged (25%) Warm Forged (40%) Warm Rolled (35%)

Vickers Hardness 122.6 (± 5.1) 73.15 (± 4.2) 103.96 (± 3.9) 100.48 (± 4.1)

Table 3: Vickers hardness after SIMA process.

Warm Forged (40%): SIMA

560˚C 580˚C 600˚C

5 min 150.56 (± 5.6) 137.95 (± 2.9)

10 min 167.88 (± 3.9)

15 min 155.56 (± 4.1) 146.38 (± 2.6) 134.4 (± 3.8)

30 min 138.92 (± 3.2) 130.9 (± 3.9) 120.2 (± 4.3 )

45 min 128.85 (±5.1) 121.83 (± 2.9) 110.8 (± 3.4)


3.4 Tribological characteristics

Figure 5(a) compares the tribological properties of the alloy in the as-cast condition with the same alloy in warm forged conditions and semi-solid processed at the above three temperatures and for minimum holding times. It can be observed that the alloy in 40% warm forged condition and semi-solid processed at 560 ºC for 10 minutes holding time gave the best tribological property with increasing sliding distance at a fixed load (10 N). This is because in the above condition the alloy has the least average grain size with well distribution of fine equiaxed grains throughout the microstructure, globular morphology, the highest hardness and the uniformly distributed equiaxed grains support the wear load better than the dendrites that are present in the conventionally cast alloy [1]. Figure 5(b) compares the tribological characteristics of the warm forged (40%) alloy when semi-solid processed at 560 ºC for 10 and 45 minutes holding times. It can be seen that the wear loss was more with increased holding time. This is due to the formation of coarse grains with significant decrease in hardness. With increasing sliding distance at a constant load it is observed from the graph that there is a decreasing trend in the wear rate beyond 900 m. This is because there is a running-in period for a particular sliding distance at a certain speed [10]. After that weight loss is decreased because during running-in period material comes out of the specimen in the form of debris particles and adheres on the surface and forms a layer which prevents wear loss. Figure 5(c) compares the tribological characteristics of the alloy in as-cast condition and warm forged condition and semi-solid processed for minimum time periods at 560 ºC. It can be seen that with increasing load for the same sliding distance wear loss also increased in every case. And in this case also the alloy in 40% warm forged condition when annealed for 10 minutes holding time at 560 ºC gave the least wear loss.


Figure 5: Tribological characteristics of the alloy in (a) as cast and different warm worked conditions and semi-

solid processed for minimum time with increasing sliding distance, (b) 40% warm forged condition and annealed for 10 min and 45 minutes holding times in semi-solid temperature region with increasing sliding distance, (c) as cast and different warm worked conditions and semi-solid processed for optimum time with increasing load.

Figure 6(a) shows the SEM image of the wear surface of the alloy in as-cast condition. From the picture it is clear

that the material has undergone plastic deformation during wear. The mechanism of wear can be attributed to adhesion and delamination [10, 11]. Figure 6(b) shows the image which indicates the condition of the wear surface of the same alloy when subjected to warm forging (40% deformation) and annealing in the semi-solid temperature region for 10 minutes holding time at 560 ºC. It can be seen that the adhesive wear is reduced up to some extent as there is considerable decrease in delamination and fine scratches have also appeared. The adhesive wear that is seen in both the cases can be due to the absence of lubrication in the experiment. In the absence of lubrication there will be some rise in temperature due to which the material softens. As there is considerable improvement in hardness in the second case, hence delamination also decreases although there is rise in temperature [10]. It seems that the mode of wear is changing from adhesive wear to abrasive wear. This change is due to the improvement in hardness of the material because of higher deformation given and the optimum holding time and temperature in the semi-solid temperature region.

Figure 6: SEM images of the wear surfaces in (a) as-cast condition, (b) 40% warm forged condition and semi-

solid processed at 560 ºC for 10 minutes holding time


4. Conclusion

From the above analysis it can be concluded that: 1) The holding time and the temperature must be optimised in the semi-solid temperature region and higher deformation must be given to the material to get the best tribological properties out of the material by processing in SIMA process. 2) Annealing for higher time periods at higher temperatures in the semi-solid temperature region affect the grain size and the hardness of the material.

Acknowledgements

Here it is acknowledged that the work was fully supported by Prof. S N Ojha in the department of Metallurgical Engineering, IIT-BHU, Varanasi. The material for research was provided by Khandelwal foundry. Warm working was done in National Metallurgical Laboratory, Jamshedpur under the guidance of Dr. V C Srivastava. Tribological test was done in the Mechanical Engineering department (IIT-BHU, Varanasi) under the assistance of Prof. A P Harsha. SEM analysis was done in Institute Of Minerals And Materials Technology, Bhubaneswar under the guidance of Dr B B Jha.

References

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Semi-solid processing and tribological characteristics of Al-Cu Alloy