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Researchers at Delft University of Technology in the Netherlands studied the local control of the structure and mechanical properties of high-strength steel in arc additive manufacturing, and achieved local microstructure and performance control along the forming direction. Relevant research results were published in the Journal of Materials Research and Technology under the title "Local Control of Microstructure and Mechanical Properties of High-strength Steel in Electric Arc-based Additive Manufacturing".
Research highlights:
•Functional grading by regulating heat input and inter-channel temperature.
•Local microstructure and property control is achieved along the forming direction.
•Low heat input zones exhibit higher martensite fractions.
•Higher hardness and ultimate tensile strength are observed in LHI zone.
Additive manufacturing (AM), also known as 3D printing, is an advanced manufacturing technology that can produce near-net shape parts layer by layer and provides product design, manufacturing and repair for various industries such as aerospace, maritime and automotive. Revolutionary prospects. Compared to traditional manufacturing technologies, additive manufacturing stands out for its combination of high customization, manufacturing flexibility, and complex deposition features. The wire arc additive manufacturing (WAAM) process involves feeding material in the form of a wire to an area heated by an arc heat source and depositing the resulting molten material along a predetermined path, typically derived from a computer-aided design (CAD) file. Due to the high achievable deposition rates and manufacturing efficiencies, WAAM is suitable for manufacturing large-volume parts with dimensions exceeding one cubic meter. The arc welding processes used in additive manufacturing mainly include gas tungsten arc welding (GTAW), plasma arc welding (PAW) and gas metal arc welding (GMAW). Compared to GTAW and PAW-based AM, GMAW is often preferred in practice because the deposited material is supplied coaxially from the welding gun and does not require an external wire feeder, thus simplifying tool path planning. Moreover, its deposition rate is generally higher than GTAW and PAW. GTAW-based AM has high deposition rates (typically 1-10 kg hr -1), low material waste and high process efficiency (approximately 90%). It also offers great potential for local composition and microstructural control (also known as functional grading).
Additive manufacturing offers great potential for producing metal parts with distinct local microstructure and mechanical properties. While functional grading is typically accomplished through compositional changes or in situ thermomechanical processing, changes in process parameters during AM can provide a promising alternative approach. Taking the arc-based additive manufacturing process as the research object, high-strength steel (S690 grade) is functionally graded by adjusting the speed and inter-channel temperature. Combining thermal simulations of monolithic bead layer plate deposition with experimental measurements, it was shown that the microstructure and mechanical properties of the part can be controlled by rationally adjusting process parameters. To demonstrate functional grading, a rectangular block was made using a constant wire feed rate and varying movement speeds. Rectangular blocks are deposited from low heat input regions (LHI) sandwiched between high heat input regions (HHI).
Experimental setup
Monolithic bead layer deposition with a length of 120 mm was prepared on a substrate with dimensions of 140 × 50 × 10 mm3 at different speeds of 5 ~ 20 mm s - 1. Multiple sets of process parameters are then used to locally control the microstructure and mechanical properties to produce rectangular blocks with dimensions of 125 × 21 × 23 mm3, as shown in Figure 1. Functional grading of block performance is achieved by changing the movement speed, and process parameters are selected based on the eva luation results of single-piece bead laminates.
Figure 1: (a) Schematic diagram of the arc-based additive manufacturing process, often called wire arc additive manufacturing (WAAM). (b) Schematic illustration of high and low heat input regions of deposited rectangular blocks. The yellow arrow on the block indicates the bidirectional printing strategy.
The high and low temperature zones are composed of a mixture of polygonal ferrite, acicular ferrite and bainite, while the low and low temperature zones are mainly composed of martensite. Hardness and profile-based indentation plasticity measurements indicate that the LHI zone has higher hardness (32%) and strength (50%) but lower uniform elongation (80%) compared to the HHI zone. Current research shows that by adjusting the process parameters of arc additive manufacturing, it is possible to achieve functional grading, providing opportunities for customized properties of parts.
The purpose of this study is to locally control the microstructure and mechanical properties (i.e., functional grading) of a high-strength steel alloy (S690) in arc additive manufacturing by regulating heat input and inter-channel temperature. It can be assumed that as the heat input and inter-channel temperature decrease, higher martensitic phase fractions, hardness and material strength can be obtained. In this process, heat input is controlled by changing the speed of movement. A numerical simulation method based on the finite element method was established to predict the heat distribution in the WAAM process. A test cuboid was additively manufactured, and microstructural characterization and microhardness measurements were performed on samples extracted from the deposited block. Furthermore, in order to determine the local yield and ultimate tensile strength of the constructed components, measurements were performed using the profilometry-based indentation plasticity measurement (PIP) technique. The tissue evolution and corresponding mechanical properties in different regions are discussed in detail. The results of this study contribute to our understanding of the microstructure control and mechanical property customization of high-strength steel alloy WAAM.
Temperature-dependent material properties were used in the simulations, which were determined based on the material composition using JMatPro software, as shown in Figure 2.
Figure 2: Temperature-dependent material properties of high-strength steel used in thermal simulations.
The numerically predicted thermal distribution was compared with the experimentally measured temperatures at two points on the substrate at an experimental speed of 5 mm s−1, and the results are shown in Figure 3.
Figure 3: Comparison of numerically predicted and experimentally measured temperatures at two monitoring points located in the middle of the substrate, 8 mm and 16 mm from the centerline of the deposited bead. The data were collected from a single bead plate experiment with a movement speed of 5 mm s−1.
Figure 4: CCT (continuous cooling transition) diagram corresponding to the high-strength steel composition used in this study.
Optical micrographs of single beads deposited at 8 mm s−1 and 20 mm s−1 are shown in Figure 5. The spheres deposited at a low speed exhibit a mixed structure of polygonal ferrite, acicular ferrite, bainite and low carbon martensite, similar to the spheres deposited at a speed of 5 mm s−1.
Figure 5: Optical micrographs of beads deposited at different motion speeds: (a) 8 mm s−1 and (b) 20 mm s−1.
Figure 6 shows the average hardness, confirming the expected trend of increasing hardness with increasing stroke speed. The hardness of the sample deposited at 5 mm s−1 is approximately 260 HV0.1. The sample deposited at a temperature of 20 mm s−1 has the highest hardness, reaching 440 HV0.1, which is due to the rapid cooling of the deposited material and the higher martensitic phase fraction. This change in hardness as a function of speed demonstrates that by controlling the process parameters of high-strength steel, arc additive manufacturing can achieve graded microstructures and properties.
Figure 6: Vickers hardness of single bead plate coating changes with movement speed.
Macrostructure and grain structure
Figure 7 shows a cross-section of a functionally graded block showing a lack of porosity and a lack of fusion. Figure 7(b) is a high-magnification optical micrograph of the interface between the high-hi region (bottom) and the low-hi region (top). At the interface between the two regions, significant changes in microstructural characteristics are visible. There are more coarse grains in the high and low temperature areas, while there are finer grains in the low and low temperature areas.
Figure 7: (a) Cross-section of a functionally graded block and (b) the interface between low heat input (LHI) and high heat input (HHI) regions. Subfigure (b) depicts the magnification of the area specified by the black rectangle in subfigure (a).
Figure 8: Columnar and equiaxed grains are observed in the cross-section of the low heat input (LHI) region of the sample. The area between the yellow lines shows equiaxed grains.
Microstructure evolution in high heat input area
Optical and scanning electron microscopy of the two HHI regions are shown in Figure 9. Micrographs show the presence of similar microstructural components in both HHI regions. The microstructural components of the high heat input zone are mainly polygonal ferrite, acicular ferrite, granular bainite and trace martensite.
Figure 9: Optical micrograph showing (a) HHI bottom region and (b) HHI top region, and SEM micrograph showing (c) HHI bottom region and (d) HHI top region. A mixed microstructure of different ferrite morphologies and martensite was observed. Yellow arrows indicate martensite-austenite islands.
The XRD spectrum of the HHI region is shown in Figure 10.
Figure 10: (a) Macroscopic view of a functionally graded block showing the locations where X-ray diffraction (XRD) patterns were recorded. (b) and (d) XRD spectra obtained from the top high heat input area and the bottom high heat input area respectively. (c) and (e) Magnified views of the retained austenite peaks in the XRD spectra of the two HHI regions.
Figure 11 provides a higher magnification image of the HHI region. Martensite-austenite islands are located between bainite laths and distributed along austenite grain boundaries.
Microstructure evolution in low heat input area
The structure of the low heat input zone consists of low carbon martensite, grain boundary ferrite and Widmanstätten ferrite, as shown in Figure 12.
Figure 12: (a) Optical micrograph and (b) SEM micrograph of the low heat input (LHI) region. The yellow arrow indicates the presence of cementite particles or MA.
The SEM micrograph of the low heat input region (Fig. 13) shows the precipitation of cementite, indicating a tempered martensite microstructure.
Figure 13: Magnified view of the low heat input (LHI) region showing carbides in tempered martensite within the LHI region.
The XRD measurement in Figure 14 confirms that no residual austenite peak is detected. Similar to the HHI region, ferrite peaks are observed at {110}, {200} and {211}. Due to the increased cooling rate in the LHI region, the diffusion of carbon during austenite decomposition is limited.
Figure 14: (a) Macroscopic view of a functionally graded block showing the location of X-ray diffraction (XRD) measurements. (b) XRD pattern obtained from the low heat input (LHI) region, showing only ferrite peaks.
Correlation between Vickers hardness and microstructure
The hardness distribution of the sample along the build direction is shown in Figure 15.
Figure 15: Hardness measurement along the forming direction on a cross-section of a functionally graded block.
The lower hardness values observed in the low heat input region are due to the presence of a white band-like area that can easily be mistaken for the fusion line. The leucorrhea area and fusion line are shown in Figure 16(b).
Figure 16: (a) Hardness measurement on white tape to verify the occurrence of softening. (b) Indentation in the white band area. The area between the yellow lines represents the white band area. The red dashed line indicates the fusion line.
Indentation plasticity measurement method based on contour measurement
Figure 18 shows the stress-strain curve estimated from PIP measurements corresponding to the indentation in each area. The yield (50%) and ultimate tensile strength (34%) of the bottom and top HHI regions are both lower than those of the LHI region.
Figure 18: Stress-strain curves derived from profilometry-based indentation plasticity measurements (PIP) of (a) top high heat input (HHI) region, (b) middle low heat input (LHI) region, and (c) bottom high heat input region Measurement. The yellow dot indicates the location of the dent measured by PIP.
In conclusion
The use of arc additive manufacturing technology to control the microstructure and mechanical properties of high-strength steel S690 was studied. Unlike previous studies that relied on compositional changes or in situ thermomechanical methods to control properties, this study regulates energy input and produces spatially varying microstructures by adjusting process parameters. Based on the results of this study, the following conclusions are drawn.
•Using higher motion speeds in single bead experiments resulted in increased cooling rates between 800°C and 500°C, higher martensite phase fractions, and higher hardness. This demonstrates the possibility of functional grading by adjusting the movement speed in arc-based additive manufacturing.
•Through the hardness measurement of the cross-section in the construction direction of the rectangular block, the grading of its microstructure is confirmed. The hardness value in the low heat input area is higher and fluctuates greatly, while the hardness value in the high heat input area is lower and more uniform.
•Spatial changes in microstructural components in the rectangular block can be observed by adjusting the process parameters, indicating successful microstructure grading. During the deposition of high-strength steel, the use of lower heat inputs, combined with temperatures of 50°C, produces higher cooling rates, leading to the formation of martensite. In contrast, increasing heat input and inter-channel temperature reduced the cooling rate, producing a mixed structure of polygonal and acicular ferrite, bainite and martensite, which may be related to the hardness measurements.
•The fluctuations in the hardness profile in the low heat input zone are caused by the intercritical reheating heat affected zone, which has a mixed structure due to martensite (tempered and untempered), bainite and ferrite Lower hardness. Using lower stroke speeds to increase the cooling rate creates microstructural inhomogeneities, resulting in localized soft zones that are significantly less hard than the surrounding material.
•Plasticity measurements based on profilometry showed that the yield and tensile strength increased (approximately 150 MPa) and the elongation decreased (2.3%) in the low heat input region compared to the high heat input region. On the contrary, the high heat input regions showed higher elongation (10.2% and 12.4%) due to the presence of grain boundary ferrite, acicular ferrite and tempered martensite, indicating that these regions can accommodate larger deformation .
It is worth noting that this study is limited to a high-strength low-alloy steel, but the proposed method for obtaining spatially varying microstructure and properties can be applied to other metallic materials. When combined with numerical simulations, this approach can lead to the fabrication of parts with controlled local mechanical properties. Future studies should examine the impact of other parameter combinations, improve fabrication efficiency and performance eva luation to exploit the functional hierarchical potential of such customized microstructures. Awareness of achievable performance should also be improved during the design phase of the part to realize the full potential of WAAM functional grading.
Related paper links: https://doi.org/10.1016/j.jmrt.2023.07.262
