The laser metal deposition process is characterised by the occurrence of mutually influencing phenomena. Particularly important for understanding particle deposition are the phenomena originated by the three-way interaction of the laser beam, the powder stream and the substrate, which in previous models have been consistently oversimplified or neglected. This work presents a numerical model useful for studying powder stream formation, powder heating and mass addition into the melt pool, as well as various interactions that occur during processing. Experimental work is performed to validate powder stream formation and heating, and particle addition to melt pool. Good agreement is found with the modelled results. It is revealed from this work that the role of the substrate is more significant than previously thought and that considering the beam-stream-substrate interrelations allows simulations closer to real processing conditions to be performed.