PSI - Issue 14

Tulsi Chouhan et al. / Procedia Structural Integrity 14 (2019) 883–890 Author name / Structural Integrity Procedia 00 (2018) 000–000

885

3

2. Experimental Procedures 2.1. Specimen preparation

In the presented study, a commercial grade of Al powder (Make: Lobe Chemie) having 98% purity is used. The density of the powder is 2700 kg/m 3 , and melting and boiling are 660.37  C and 760  C, respectively. For specimen preparation, metal powder was compacted in a hydraulic press at a force of 100 kN for minimum 240 seconds to get green Al metal compacts. Four different specimen types were fabricated from the Al powder. The first specimen category used the Al powder as supplied by the manufacturer. The second set of green compacts was developed after ball milling of raw Al powder for 24 hours in a ball mill with powder to ball weight ratio of 1:30. The balls used were of steel having a diameter of 18 mm and mass of individual ball was 23.25 gms. To avoid cold welding of Al powder 2% stearic acid was added to act as process control agent. The rotational speed of the drum was set to 40 rpm. The rotational speed was stopped every 2 hours for at least 5 minutes, to keep the system temperature under control. The motive of ball milling for such a long time was to decrease Al powder size. The third set of Al powder comprised of 33% rock salt by weight percent. This third set was ball milled for 10 hours at 1:10 powder to ball weight ratio. Sintering of the three different types of Al-based green compacted specimens was done at 600  C, in an argon atmosphere. The flow rate of argon gas was set to 6 lit/min and the sintering time was set to 180 minutes, respectively. After removal of a specimen from the furnace, half of the specimens containing rock salt were boiled in a water bath for one hour to dissolve the salt content of sintered Al specimen. This set of specimen served as the fourth set of Al powder based specimen for high strain rate testing. The dimensions of the specimen were set in accordance with the recommendations of the literature (Gama et al. 2014). The diameters of the Al specimen were 13 ± 0.2 mm and the thicknesses were in the range of 7 ± 0.2 mm, respectively. The densities of four different categories of specimen fabricated are presented in Table 1.

Table 1. Densities of different types of Aluminum specimens fabricated.

S. No.

Material

Process adopted

Density (kg/m 3 )

1 2 3 4

Aluminum Aluminum

As received

2650 ± 2% 2275 ± 2% 2225 ± 2% 2150 ± 2%

Ball milled 24 hours Ball milled 10 hours

66 % Al with 33% rock salt 66 % Al with 33% rock salt

(Type 3) Boiled in water for one hour

2.2. Experimental Setup

The requirement of carrying out the compressive high strain testing of materials is fulfilled by the split Hopkinson Pressure Bar (SHPB) set-up. The SHPB set-up essentially consists of a gas gun, incident bar, transmission bar, striker bar, speed sensor, momentum trap, signal conditioner-amplifier and data acquisition system and information obtaining framework is shown in Fig. 1 and the details of key components are exhibited in Table 2.

Fig. 1. Schematic arrangement of split Hopkinson Pressure bar apparatus (Chouhan et al. 2017).