Is the path loss exponent $\alpha$ is always 2 in friis equation model?
Path loss exponent is not always equal to 2, only in the free space scenario is this true. In general the received power $P_r$ is proportional to $d^{-\alpha}$, where $\alpha$ is somewhere around 2-6 depending on the situation. The choice of the path loss exponent should match the scenario you're trying to simulate.

For example, for communication at a high altitude with little obstructions then $\alpha$ is probably close to 2 since the situation is approximately free space condition. 

For communication in a building, within the same floor, then $\alpha$ may be around 2-3. This is because walls other obstructions attenuate the signal power further than the free space condition. 

Things can get even worse if we again consider communication in a building but now if the transmitter is on the first floor and receiver is on the third floor, then $\alpha$ could be as high as 4-6 depending on construction of the building, etc.