Optimisation of selective laser melting parameters for the Ni-based superalloy IN-738 LC using Doehlert’s design

Abstract

Purpose The Ni-based superalloy IN-738 LC is known to be susceptible to porosity and different types of cracking during the build-up process and, thus, challenging to manufacture using selective laser melting (SLM). Determining a feasible set of operating parameters for SLM of nickel-based superalloys involves new approach to experimental design based on the Doehlert method that assists in determining an optimal (feasible) set of operating parameters for SLM of IN-738 LC powder alloy. Design/methodology/approach The SLM parameters are evaluated in terms of their effectiveness in obtaining the microstructure with a porosity content of <0.5 per cent and without micro-cracking. The experimental approach is exemplified with the Doehlert matrix response variable, relative density, by comparing Archimedes method with microstructural assessments of pores and cracks from image analysis. The effect of heat treatment (HT) and hot isostatic pressing (HIP) on the microstructure of the SLMed IN-738 LC powder alloy has been examined and the consequential tensile response characterised. Findings By using optimised process parameters (low heat input, medium scanning speed and small hatching distance) which provides medium energy density, samples of IN-738 LC with a macroscopic porosity <0.5 per cent and free of micro-cracks can be manufactured by SLM. The results indicate that HIP of SLMed material did not lead to a noticeable effect on mechanical properties compared to HT of SLMed material suggesting that the level of both porosity and crack density might be already below the detection limit for the mere heat-treated material. Originality/value SLM processing parameters (power, scan speed, hatching distance) for IN-738 LC were successfully optimised after only 14 experiments using Doehlert design. Two independent methods, Archimedes method and image analysis, were used in this study to assess relative density of SLM-produced samples with sets of processing parameters showing coherency in prediction with predicted response by Doehlert design.

The Effects of Selective Laser Melting Process Parameters on Relative Density of the AlSi10Mg Parts and Suitable Procedures of the Archimedes Method

“…When hatching distance is low, the high relative density parts are easily to obtain. This can also be found in other studies [9,19,20,22]. The SLMed AlSi10Mg alloy samples with the combination of higher laser power, higher scan speed, and lower hatching distance also have higher ultimate tensile strength (UTS) and yield strength [18].…”

“…The two combinations of parameters are both near to the energy density, which corresponds to the highest relative density in Figure 2. In Figure 3, the micro-pores can be observed in the samples built with large hatching distance and high scan speed-this phenomenon was also observed in other alloys [10,22].…”

“…The relative density of parts has a close relationship with the existence of internal pores [21]. Nataliya Perevoshchikova et al [22] used the Archimedes method and image analysis to assess the relative density of SLM-produced samples of Ni-basedsuperalloyIN-738LCwith sets of processing parameters.…”

“…The surface methodology was then applied to represent these effects. The Doehlert design was applied to optimize the SLM processing parameters of Ni-based superalloy [22], however, it was not applied to the optimization of Al-Si-Mg alloy in past reports.…”

Microstructure and Mechanical Anisotropy of Ni–Mo–Cr-Based Alloy Manufactured by Laser Metal Deposition

“…In the case of laser metal deposition (by powder feeding), which uses a high-power laser beam to directly spray and deposit metal powder, it has the advantage of greater manufacturing speed, capability of producing relatively larger parts and application to 3D-shaped base metals compared to powder bed deposition, and these advantages were utilized in attempts to manufacture bulk and coating layers with Nibased super alloys. 1116) Previous studies have reported microstructural interpretation according to variables concerning the manufacturing process, 11,15) defect formation such as pores and cracks. 11,13) Most studies present the basic mechanical properties of Ni-based materials manufactured with additive manufacturing.…”