Consideration of key process parameters for achieving robust and uniform cutting of Ti-6Al-4V sheet metal using fiber laser with nitrogen assisted gas

Abstract

In laser cutting, to work on sensitive and intricate materials, such as titanium, requires rigorous control of parameters to achieve the best cutting results due to high power throughput and rapid cutting speed. Titanium is known for severe reaction with oxygen and nitrogen at elevated temperatures, however the more inert argon assist gas is cost prohibitive in commercial application. To achieve cut quality using nitrogen as an assist gas in titanium cutting, careful control is needed to prevent the formation of hard and brittle martensite as well as nitride- and oxide- phases. To better understand the roles of industrial grade raw materials and assist gas in commercial laser cutting, phase transformation, heat affected zone (HAZ) cut quality and the relationships amongst parameters have been selected to mimic industrial setups. 2mm thick titanium sheet was cut by a fiber laser cutting process using nitrogen assist gas by varying three cutting parameters including (i) laser power (1000-3000 watts), (ii) cutting speed (4000-26000 mm/min), and (iii) assist gas pressure (1-20 bars). Cut quality assessment was interpreted through the scanning electron microscope (SEM) and optical observation to link with mechanical properties from bending and v-notch fracture tests. The results showed that increase in cutting speed will limit the thickness of heat-affected zone (HAZ) which varied from 20-500 um. Pressure of the assist gas should be above a threshold value of 8 bar to assure complete ejection of the liquid melt pool. Key observations were reported along with root causes, such as, (i) non-uniformed and thick HAZ layer, (ii) thick and brittle recast (RC) layer, as well as (iii) micro-tearing due to localized expansion of nitrogen rich phase.