Experimental study of spring back of different sheet alloys by pre-load laser bending

3.1. Spring back before laser application

The experimental calculations of spring back using preload clamping are shown in the form of graphs in Fig. 2. Spring back happens when the bent material tries to gain back its original form to a certain angle. During fabrication, material is usually over bend to the bending degree which is over past the required bending angle in order to compensate the spring back. In this way, over bending permits to acquire the desired bending angle and shape when the pressure is released from the material. Predicting the efficient values of spring back is quite critical. There are so many factors which influence on spring back: (1) The tensile strength of material (2) Thickness of the material (3) Type of tools being used during fabrication process (4) Types of bending [10, 13]. Metal sheets thickness and types of materials are major factors affecting on spring back values but in these experiments thickness of every metal sheet is almost same so we can eliminate the effect of thickness during bending on spring back. Bending process here is simple air forming when the force or pressure is applied on the metal sheet, it starts to bend and the material slightly springs back when the force is released.

Fig. 2(a) showed the spring back of Aluminum alloy Al-5052M, at even low displacement there is large amount of spring back. Behavior of Al-alloys can be explained on the fact that these alloys are malleable, light weight, low hardness 47B and ultimate tensile strength of 228 MPa and their compression modulus is about 2 % greater than tensile modulus. When material is bent, the inner part of the bend is compressed condition while the outer part is in stretched condition so as a result of this; the density of molecules is greater on inside of the bend as compare to the outer surface. The compressive forces are less on the outside surface of the bend which causes the material to try to return to its original position [13]. These properties make Al-alloys easy to bend and return to its original shape while giving high values of spring back. The mechanical properties of understudy alloys are given in Table 2.

In case of low carbon steel in Fig. 2(b), rolling direction is perpendicular to the applied bending force which also has opposing effect on values of spring back. Due to opposite rolling direction it is hard for molecules to move during bending causing more tensile force application. These low carbon steels have ultimate tensile strength in range of 440 MPa and hardness 71 HRB which are more than Al-alloys. But due to low carbon contents in these steels they have good ductility which also contributes to spring back values. Fig. 2(c) showed the spring back values after releasing from mechanically clamping of stainless steel 304M. It is standard 18/8 stainless steel grade which has outstanding forming characteristics. These alloys have ultimate tensile strength of 515 MPa and high hardness (170 HRB).

Fig. 2(d) explained the spring back behavior of Titanium (Ti-6Al-4V) alloys. These alloys are not easy to deform at low temperature due to bad plasticity but when enough force is applied that the material approaches to its yield point then the deformation of the material occurs. Titanium alloys have high ultimate tensile strength of 993 MPa and very high hardness (334B) then Al-alloys. All these factors attribute to the values of spring back. It is clear from the figure as well that at low displacement amount of spring back is almost neglect able but after large amount of force application at high displacement, spring back values increased even at room temperature.

Fig. 2Spring back before laser: a) Al-alloys, b) low carbon steel, c) stainless steel, d) titanium alloys

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3.2. Spring back after laser application

The laser parameters were selected on the basis of material properties and the Gaussian distribution was used. To set the values of laser bending process, following steps calculation were used. Laser spot moving time by:

where $∆x$ – distance travelled, $v$ – laser scanning speed. The distribution of heat flux density is considered to have uniform values with laser spot moving time $t$ can be calculated by using following formula:

Here $A$ – absorption coefficient of metal sheet, $P$ – Laser output power. The energy lost during the heat transfer can be found by $q=\beta (T-T°)$ equation, where $\beta$ – heat exchange coefficient (radiation and convection), $T°$ – Room temperature at test time. In Fig. 3 spring back values are shown in form of graphs after laser scanning. It is obvious that spring back values are high than the preload condition for all kind of understudy work pieces after laser scanning process. There are so many factors in laser bending process which effect on the spring back values which are: 1) input power (P), 2) scanning speed (S), 3) laser spot diameter (D), 4) laser scanning path [14-16].

In present report, results showed that after laser bending spring back values increased as compared to the mechanically clamping. Heating effect in laser bending plays an important role. High temperature helps in the movement of molecules/ atoms of material causing more plastic deformation. Previous studies showed that the as the laser power (P) increases, tensile strength of material decreases. The bending of materials decreases with the increase of scanning speed (S). Fig. 3(a) showed the Al-alloys spring back value after laser scanning. As it is explained earlier that due to malleable nature of Al alloys, it is easy to bend them even at low displacement (low application of force) at room temperature, but after laser scanning value of spring back increases. The output power for Al-alloys during the experiment is 700 W which helps to decrease the tensile strength causing more deformation. The scanning speed 10 mm/s is low which may have contributed to the low bending of Al-alloys here.

Fig. 3Spring back after laser: a) Al-alloys, b) low carbon steel, c) stainless steel, d) titanium alloys

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For other understudy alloys in this experiment, the laser parameters laser power and scanning speed are same. Low carbon steel (perpendicular rolling direction) and stainless steel 304M alloys have shown high values of spring back in Figs. 3(b) and (c) respectively than spring back from preloaded clamping. In Fig. 3(d) there is a great improvement in spring back value of titanium alloys than room temperature values which showed that deformation increases while reducing the tensile strength and hardness of these alloys. And when laser power releases, elastic recovery improves.

So the laser scanning method is more beneficial for those alloys that are more difficult to deform at low temperature. Since Al-alloys, low carbon steels and stainless steel have lower tensile strength and hardness, they can easily be shaped for different requirements. But titanium alloys have low plasticity so it is tough to use them. But laser bending technique could provide an efficient bending method for titanium alloys for practical applications.