Effect of thermal history on microstructure evolution and mechanical properties in wire arc additive manufacturing of HSLA steel functionally graded components

In this study the microstructure evolution during wire arc additive manufacturing (WAAM) with high strength low-alloy (HSLA) steel was studied to employ this knowledge in the WAAM parts microstructure control, which was demonstrated by the functionally graded specimen manufacturing with only ER110S-G grade wire. The experiment design was based on the application of two WAAM modes: the cold metal transfer (CMT) and a conventional self-regulated gas metal arc welding (C-GMAW) mode. Material thermal history was estimated with the use of finite element modeling and the microstructure transformations were assessed using the calculated continuous cooling transformation (CCT) diagram. The microstructure was investigated with the use of optical and scanning electron microscopy. Mechanical properties were determined by tensile and microhardness tests, digital image correlation analysis was applied to detect the deformation zones during tensile tests of the functionally graded specimen. The results of the study showed a strong correlation between cooling rate, tempering time and microstructure and hence mechanical properties: depending on the thermal history the microhardness varied from 283 to 411 HV. Both C-GMAW and CMT modes resulted in the formation of two main zones: the no-diffusion upper zone and the diffusion lower zone. In the upper zone the microstructure is determined by cooling rate from 800 °C down to 500 °C. The upper zone consists of martensite, bainite, a mixture of martensite and bainite, martensite-austenite phase (MA) and retained austenite (RA). In the lower zone the microstructure is determined by the tempering time in the range 700–200 °C. The lower zone consists of a tempered martensite, troostite, sorbite, alpha ferrite, MA-phase and carbites. Both C-GMAW and CMT modes resulted in the relatively high ultimate tensile strength (UTS): 807 MPa for CMT and 759 MPa for C-GMAW. CMT and C-GMAW resulted in the different elongation in the horizontal and vertical directions, it ranged from 5 to 10%. Cooling rate and tempering time regulation allowed to manufacture the functionally graded specimen with the relatively high strength and with the predetermined fracture localization.