Laser-based powder bed fusion of non-weldable low-alloy steels

Abstract

This study focuses on the processability of four low-alloy steels (AISI 4130, 4140, 4340 and 8620) via laser-based powder bed fusion (LB-PBF). In the as-built condition, the alloys consisted of tempered martensite that was the result of an intrinsic heat treatment (IHT) during LB-PBF. In terms of defects, a distinct transition in porosity was observed that correlated to the volumetric energy density (VED). At low VED, specimens contained a lack of fusion porosity, while at high VED, they contained keyhole porosity. Additionally, cold cracking was observed in 4140 and 4340 specimens produced at low/intermediate VEDs. This cracking could be mitigated by increasing the VED or laser power, as both enhance the IHT. This enhanced IHT lowered the material hardness below specific thresholds (<500HV 4340 and <460 4140), increasing ductility and allowing the specimens to avoid cracking. From these findings, crack-free, high-density (>99.8%) low-alloy steel specimens were produced without the requirement of build plate preheating.

Influences of laser remelting on mechanical performances of AISI4140 steel

“…The microhardness of the dark zones, on the other hand, is measured to be around 450 to 550 HV 0.1 , and can be considered as tempered martensite, which is in agreement with the findings in Ref. 18 The fresh martensite zones have lower plasticity compared to the tempered martensite, because of high concentration of dislocations and supersaturated carbon. 19 The high density of dislocations in the fresh martensite is also responsible for its high microhardness, making it more susceptible to cracking than the tempered zones in the presence of tensile residual stresses.…”

Development of powder bed fusion – laser beam process for AISI 4140, 4340 and 8620 low-alloy steel

“…These cracks primarily formed at the side surface and grew inwards perpendicular to the building direction. This type of defect is related to a cold cracking phenomenon that has been described previously in works by Hearn et al [17,21]. The mentioned cracking was not observed for the AISI 8620 alloy as this alloy had a much lower carbon content (0.22 wt.%), making it more ductile and thus less susceptible to form these defects.…”

“…Optical microscopy (OM) revealed a martensitic microstructure within the specimens, see Figure 10. Further hardness measurements found that this martensite was noticeably tempered, see Table 4, as the hardness was similar to conventionally quenched and tempered lowalloy steel [17]. SEM revealed a collection of fine martensite laths, that became more prominent as the carbon content of the alloys increased, see Figure 10.…”

“…OM imaging was also used to estimate the melt pool depth, which was done by measuring the depth of the top layer in etched specimen cross-sections. These measurements are described in more detail during previous work by the authors [17].…”

On the Recycling of Water Atomized Powder and the Effects on Properties of L-PBF Processed 4130 Low-Alloy Steel

“…Low-alloy steels are also potentially attractive due to their low cost and tunable mechanical properties especially after the hardening treatment of quench and tempering (Q & T). Even though low-alloy steels are widely used for structural applications in several industrial sectors, a fairly low number of investigations have been carried out so far on their properties after L-PBF processing [ 4 , 5 , 6 , 7 , 8 , 9 , 10 , 11 , 12 , 13 , 14 ]. The available information showed that the L-PBF processed steels in as-built condition provide outstanding mechanical properties.…”