Surface topography and cross-section analysis of K340 cold work tool steel double-tracks deposited by directed energy deposition technique

Abstract

Tool steels are widely used as dies and tools due to their exceptional properties. However, cracking and wear can significantly impact the performance and longevity of the tools. The possibility of repairing the damaged parts by Directed Energy Deposition (DED) results in an extended tool life and improved environmental impact compared to traditional techniques. To this purpose, the current study aims at investigating the processability of K340 cold work steel by DED. To study the processing window, based on a face-centered composite design, double-tracks with different sets of nozzle travel speed ($v_N$), laser power (P), and carrier gas flow rate ($V_{\mathrm{cg}}$) were deposited. Deposits were characterized by scanning electron microscopy (SEM), coherence scanning interferometry (CSI), and micro-instrumented indentation, and quantitative analysis of influence factors was performed by ANOVA and response surface methodology. The results showed that increasing P and decreasing $v_N$ resulted in an improved surface roughness. However, surface features describing extreme values, such as Sp and Sz, showed the presence of not fully incorporated particles. Additionally, $V_{\mathrm{cg}}$ showed no significant effect on parameters describing the average roughness, such as Sa, Sq and Sdq. Cross-section analysis revealed that almost defect-free deposits can be made with a porosity fraction area of as low as 0.04 %. However, the deposited material showed a different microstructure than that of the substrate, and heat affected zones were also observed. By increasing P, the dilution tended to rise from 5 % to 36 %. The laser power was shown to be the main parameter affecting the dilution. The process optimization to have the best possible combination of high geometrical accuracy and low surface roughness and low defect level was implemented and validated by experimental results. By investigating the processing window and optimizing the process, this study facilitates the application of DED as a repairing process of K340 tools and contributes to sustainable manufacturing.