alluminium

TENSILE TEST ON ALUMINIUM 7017

Aluminium alloys are widely employed in the automobile and armour applications to enhance crashworthiness and reduce weight of the components. The understanding of the strain rate sensitivity parameters and mechanism of deformation of these alloys is indispensable for the successful design of components. Aluminium alloys have found potential applications due to high strength to density ratio. Al 7017 alloy has got uses in armour applications.

The present alloy under study is Al–4.5Zn–2.5Mg–0.3Si–0.40Fe, commercially named as 7017 aluminium alloy. Tensile testing is performed at various temperatures on the tensile specimen. High strain rate tensile tests are conducted on 7017 aluminium alloy under different strain rates ranging from 0.01, 500, 1000 and 1500s−1  and at temperatures of 25, 100, 200 and 300 °C. The fracture surface of the specimen tested is studied under SEM. The quasi static yield and ultimate tensile strengths of the alloy are 458 MPa and 508 MPa, respectively. Ductility measured as percentage elongation is 13%. The specimens are heated by the resistance-heated furnace. Thermocouple wire is connected on the specimen, which can measure the specimen temperature variation.

A decrease of stress beyond a maximum stress at higher strain rate has been observed. This can be attributed to dominance of thermal softening. The deformation of the material under high strain rate loading being an adiabatic process means heat generated during the deformation process has insufficient time to dissipate before the deformation process is complete. This leads to conversion of a significant portion of the plastic work into heat and localization.

Tensile flow stress increases considerably from the strain rate of 0.01–1500s−1. The maximum tensile flow stress of 7017 aluminium alloy enhances by 250MPa as the strain rate is increased from 0.01 to 1500s−1. In order to describe the mechanism of failures under different loading conditions, the failure modes are investigated. Decreasing temperature and increasing strain rate can result in the increase of flow stress. This is because the low strain rate provides long time for energy accumulation, and high forming temperature promotes the nucleation and growth of dynamically recrystallized grains and dislocation annihilation, and thus reduces the stress level. During the tensile tests, the strength of 7017 aluminium alloy is mainly dependent on deformation processes of strain hardening and thermal softening. When the material undergoes plastic deformation, the dislocation density increases and causes the strain hardening. The change in fracture mode is observed at different strain rate loading conditions. A shear mode of fracture is significant at the lower strain rates; where as a more cup- and cone-like surface representing dimple structure is found at the higher strain rates. The dimple structure demonstrates that the material shows ductile fracture. When the load increases, the small dimples collapse and form big dimples. The number of dimples is more at high strain rates than that at quasi-static and intermediate strain rates. The toughness of the material is observed to be more at high strain rates compared to that at low strain rates. The large energy absorption capacity of the 7017 aluminium is mainly due to its high level of work hardening and large fracture elongation. It is noticed that the fracture surface of 7017 aluminium alloy at low strain rate tensile tests at room temperature consists of the cleavage pattern. However, the fracture plane at high strain rate tests (1500s−1) at various temperatures is mostly composed of the dimple pattern, but the number of dimple enhances with rise in temperature from 25°C to 300°C. When the temperature increases to 200°C, the number of dimples rises and the dimple size of 7017 aluminium alloy are larger than that at lower temperature. Failure due to nucleation, growth, and coalescence of micro voids results in improvement of ductility. The “serrated” dimples become more prominent at lower deformation temperatures. The characteristic features of softening ratio, deeper dimples and necking in high strain rate tests represent that the necking development is delayed under dynamic loading due to inertia effect. The amount of necking and damage tend to increase with strain rate. It is also understood that an increased dislocation density is obvious in the dynamic loading condition. Therefore, the larger and deeper dimples also state clearly that dynamic loading leads to the enhancement of ductility of 7017 aluminium alloy. It seems that the plasticity under dynamic loading is enhanced and this phenomenon can be attributed to the dominant softening effect under dynamic deformation.

         

     

SEM micrographs of fracture surface

(a) 0.01/s, 20°C

(b) 500/s, 20°C

(c) 1500/s, 20°C

(d) 500/s, 100°C

(e) 1000/s, 100°C

(f) 1500/s, 100°C

(g) 500/s, 300°C

(h) 1000/s, 300°C

(i) 1500/s, 300°C.

Tensile test on Duralumin:

What is Duralumin and where is it used? Duralumin is a hard, light alloy of aluminium with copper and other elements. It is ductile and malleable and hence has many applications as sheets and plates in structural components for aerospace application and military equipment. It is also used in aircraft structures, rivets, hardware, truck wheels, screw machine products, and other structural applications.

What happens when Duralumin specimen is subjected to axial tensile stress?

In duralumin specimen, when subjected to axial tensile stress, there is a plastic deformation ahead of crack. This is known as necking which begins at the ultimate stress point. After necking, the duralumin specimen fails resulting in cup and cone fracture surface as shown below.

                      Duralumin cone                                                                 Duralumin cup

The fracture in Duralumin is initiated at microvoids, which can be seen in the microstructure given below (obtained using Scanning Electron Microscope). These coalesce to form an internal crack. Final failure occurs when the shear stress causes the remaining cross section to tear. The shear stress is greatest at 45 degrees to applied load and hence forms angled walls, resulting in distinctive cup and cone profile.

                                            SEM image of the fracture surface of duralumin