In Situ Measurements of Melt-Pool Length and Cooling Rate During 3D Builds of the Metal AM-Bench Artifacts

Abstract

This work provides results and analysis of the in situ thermal measurement acquired during the 3D builds performed for the 2018 additive manufacturing benchmark tests. The objective is to provide context for post-process characterization of distortion, residual strain, and microstructure, which are reported elsewhere in this Journal, and to provide validation data for thermal models of the build process. Four bridge artifacts (75 mm long, 5 mm wide, 12.5 mm tall) are created in each of three builds using a commercial laser powder bed fusion system. The builds are performed using nickel super alloy 625 (IN625). High-speed infrared thermography performed during four of the builds is used to measure the melt-pool length and cooling rate within a select region. The temperature of the substrate and build volume is measured during the fifth build to provide data to establish the boundary conditions for thermal models.

Appendix

Thermocouple Measurement Method

An additional IN625 build was performed several months after the original AM-Bench experiments with thermocouples located at various locations on the substrate and in the build chamber. This additional build was performed to provide greater insight into the temperature at locations on the substrate and build volume to enable modelers to adequately define the boundary conditions. Note that these thermocouple measurements were not available at the time of the AM-Bench competition. The only differences between this build and the earlier builds are that the laser was replaced and calibrated by the manufacturer, and the build completed 16 min sooner.

A total of 14 thermocouples were included in the build volume, as depicted in Fig. 18. Each thermocouple (Omega GG-K-30 type K) has a measurement uncertainty equal to the larger of 2.2 °C or 0.75%. The signals are acquired using a National Instruments NI 9213 module at a rate of 1 Hz. The thermocouple wires are welded to the steel plate and IN625 substrate using a spot welder. Each wire is stress-relieved using aluminum tape, which is evident in the images in Fig. 18.

Fig. 18

The thermocouple measurement setup. a Location of 12 of 13 thermocouples in the build volume. b Powder packed around the IN625 substrate shortly before the build begins. c The viewport on the door with TC 0 and the wire pass-through. d The completed build with powder removed

Table 3 presents the approximate locations of each thermocouple (excluding TC 0) relative to the top front left corner of the IN625 substrate. One thermocouple (labeled TC 0) is attached to the custom door and hangs just above the viewport. This thermocouple is used to measure the temperature of the build chamber environment. Eight thermocouples (TC 1 though TC 8) are welded to the upper and lower edges of the sides of the IN625 substrate. The thermocouples were welded to the top and bottom edges at the mid-point of each side. This placement prevented the thermocouples from interfering with the recoating blade or the larger (250 mm by 250 mm) steel build plate on which the IN625 substrate is bolted. Four thermocouples (TC 9 through TC 12) are welded to the steel build plate, on a line parallel to the X axis that bisects the IN625 substrate. TC 13 is adhered to the frame around the build volume using aluminum tape. This thermocouple, along with those welded to the build plate (TC 9 through TC 12) and those welded to the left and right sides of the substrate (TC 1, 2, 7, and 8), is coplanar.

Table 3 Locations of each thermocouple relative to the substrate origin

Substrate Temperature Results

Figure 19 presents an overview of the measurements acquired by all thermocouples. When the build begins, the ambient gas in the build chamber is approximately 32 °C, the frame around the build volume is 49.8 °C, the steel plate is on average 73.9 °C (four thermocouples measuring between 73.1 and 74.5 °C), and the IN625 substrate is on average 73.5 °C (eight thermocouples measuring between 72.2 and 74.6 °C). Although the build plate temperature was set to 80 °C for the build, the steel plate never reached this temperature, and it was not until after the build started and the laser began providing heat to the substrate did a portion of it reach 80 °C. In fact, a majority of the thermocouples remained below the programmed build plate temperature. During the build, the ambient thermocouple (TC 0) increased to a maximum of 38 °C during the build, while the thermocouple attached to the frame around the build volume actually decreased slightly in temperature during the build to 48.5 °C. This decrease in temperature is likely due to the fact that as the build progresses, and the part grows in height, the build platform lowers, thus increasing the distance between it and the location of TC 13.

Fig. 19

Overview of the thermocouple measurements. TC 1 through TC 8 are shown in greater detail in Fig. 20

Figure 20 presents the temperature history measured by the thermocouples welded to the IN625 substrate (TC 1 through TC 8). The greater temperature increases occur on the right side (positive X) of the substrate. This likely occurs because the bridge artifacts are positioned closer to this side than any other. There is a clear inflection point at the transitions between the legs and overhangs (Layer 350 at approximately 3.5 h) and between the overhangs and the bridge (Layer 450 at approximately 5 h). During the deposition of the overhang, it appears the temperature increases more rapidly for thermocouples TC 1 through TC 6. Once the bridge begins to build around the fifth hour, the temperature increase is much more gradual and practically reaches a steady state in some instances.

Fig. 20

Measurements from thermocouple welded to the IN625 substrate