Rapid Material Characterization of Deep-Alloyed Steels by Shock Wave-Based Indentation Technique and Deep Rolling
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Processing technologies such as laser deep alloying could enable the development of new construction materials based on micro-samples because of the rapid as well as flexible mixture of a base material with pre-deposited alloying elements. To maintain high-throughput material development, these new material compositions need to be characterized rapidly as well. High-throughput experimentation in material development is recognized as a novel and beneficial scientific way to generate knowledge and to identify new alloy compositions. This paper presents and compares different approaches for high-throughput characterization of different micro-samples produced by laser deep alloying. These approaches are based on plastic deformation by the mechanical indentation and reveal information about the material properties by rapidly measurable values called descriptors. The experiments indicate that the shock wave-based indentation technique and the deep rolling process are suitable methods to gain insights regarding the material properties of laser deep-alloyed micro-samples. Both processes show that the plastic deformation slightly increases with elevated laser power in laser deep-alloying process. Subsequent conventional measurements indicated that at higher laser power, lower hardness and lower amount of chromium result from the deep-alloying process. Moreover, local deviations of metallographic constituent, which results in larger hardness deviations, were predicted based on the descriptors.
KeywordsMeasurement Deformation Manufacturing process High-throughput
Conventional material developments are based on expensive experimental investigations of different material properties. Processing technologies such as laser deep alloying could enable the development of new construction materials because of the rapid as well as flexible mixture of a base material with pre-deposited alloying elements . To maintain high-throughput material development, these new material compositions need to be characterized rapidly as well. The objective of realizing a specific performance profile of a material in a short time pursues the demand of efficient identification of new compositions. Thus, high-throughput experimentation in materials science has been recognized as a new approach to generate novel and beneficial scientific knowledge . New high-throughput techniques must operate at least at the same accuracy level as comparable methods. The new techniques do not measure specific material properties directly . Instead, values (so-called descriptive values) are linked to the specific material properties to measure more rapidly. Moreover, receiving information on material properties for micro-samples is not always possible with conventional methods, e.g., tensile tests. One example for deducing information from adapted processes could be a new method of hardness measurement. The hardness offers a descriptive value for the resistance of a material against the indentation of another material. However, the conventional indentation and evaluation process usually takes several seconds. A new method was presented in  and is based on laser-induced shock waves. The laser-induced shock wave indentation hardness measurement is further referred to as LiSE-hardness measurement. When energy density of a CO2 laser pulse exceeds a threshold, a plasma is induced, which results in a shock wave .
Laser-induced shock wave-based processes are commonly used such as for surface hardening of turbines to improve fatigue performance  or forming and joining operations . Laser systems offer not only a high processing speed due to the fast expansion of the shock wave but also a high-throughput because laser scanners are highly dynamic.
The shock wave can also be used to push an indenter inside a material surface. Another well-known process is deep rolling, which causes plastic deformation of surface and sub-surface layers leading to an increase in compressive residual stress as well as a smoothing of the surface . As the resistance of highly stressed components is important, the plastic deformation induced by deep rolling is conventionally used to optimize component properties . Investigating the plastic deformation induced by deep rolling may reveal information about the material properties . This paper presents and compares two different approaches for high-throughput characterization of different micro-samples produced by laser deep alloying. These approaches are based on plastic deformation by the mechanical indentation and give information about the material properties by rapidly measurable descriptors. The deformations introduced by these processes are characterized and analyzed to allow conclusions to be drawn about the material properties. The microstructure of the samples is varied by different ratios of base material to master alloy because of the variation of laser power in laser deep alloying. The goal of this paper is to understand the connection between the resulting plastic deformation and the subsequently assessed mechanical properties of the material as well as the microstructure in terms of the metallographic constituent.
2 Processing of Laser Deep-Alloyed Materials
Chemical composition of the base material C15 and the master alloy X2CrNiMo17-12-2 in weight percent (wt%)
3 Testing Methods
3.1 Metallographic Analysis
The measured values for sample geometry, hardness and chemical composition were determined by means of metallographic cross-sections taken in the center of the samples. The arithmetic average of five samples was determined for the specification of the cross-sectional melt pool area. The analysis of the melt pool area is intended to draw conclusions about the melt volume because the samples are rotationally symmetric. Kalling etchant was applied on the cross sections for the analysis of the martensitic structures. Samples for Vickers micro-hardness testing were prepared by grinding and polishing the cross section. The hardness of the generated alloys was measured using a micro-hardness tester. An indentation load of 0.98 N was applied at a dwelling time of 15 s. The distance between two hardness impressions was 0.2 mm. The evaluation was carried out at 50 × magnification. The measurements were taken over the cross-sectional area at 0.2 mm from the surface and for comparison also directly on the surface. The number of indentations was between 34 and 40, depending on the cross-sectional size of the sample. The arithmetic average of the hardness impressions from three samples with standard deviation was taken. The chemical analysis was made using EDX with the scanning electron microscope. The measurements were taken over the cross-sectional area at 0.2 mm from the surface. The distance between two measurement points was 0.6 mm. The arithmetic average with standard deviation was taken.
3.2 LiSE-Hardness Measurement
3.3 Deep Rolling
4.1 Analysis of Laser Deep-Alloyed Samples
4.2 Analysis of Deep Rolled Samples
As the hardness was determined to decrease with increasing laser power, it was expected that both the track widths and the track depths in deep rolling would increase with lower material hardness. Instead, the track width increases slightly with higher material hardness. The different behavior of track depth and track width could be traced back to elastic behavior of the indenter. An elastic deformation of the tool ball during the deep rolling process could lead to the observed deviations.
4.3 Analysis of LiSE-Hardness Indentations
4.4 Comparison Between Methods
The laser power influences the mixing ratio of pre-deposited master alloy and soft base material during laser deep alloying. Figure 9 shows the different metallurgical cross sections. In the unprocessed base material, characteristic ferrite and perlite phases as well as different grain sizes are obtained. Instead, only martensitic structures are found in the laser deep-alloyed materials processed with different laser powers. Small and fine-dispersed grains are observed in these martensitic structures. By comparing the base- and laser-processed material, it is obvious that the base material results in higher coefficients of variation in Vickers hardness. Moreover, the 3 kW samples reveal a rather inhomogeneous martensitic structure compared to the other laser deep-alloyed samples. No visible differences are obtained between the 4 kW and 5 kW laser deep-alloyed martensitic phases. A further explanation offers Fig. 7. The laser power influences the mixing ratio of pre-deposited master alloy and soft base material during laser deep alloying. Larger deviations and amounts of chromium are found in the 3 kW processed material. In contrast, higher laser powers lead to lower amounts of chromium and nickel in laser deep-alloying processes. It can be derived from Figs. 6 and 7 that the melt area and, thus, the melt volume are congruent to the measured alloy content. Thus, the lower material hardness could be a result of a larger melt volume and, accordingly, of a lower amount of chromium in the alloyed sample. Larger deviations and amounts of chromium could lead to the observed Vickers hardness deviations because chromium is a strong carbide-forming element, which increases the hardness of a material. Like the conventional hardness deviations, the LiSE-hardness measurement process shows that it is sensitive to changes in metallographic constituent and local deviations as indicated in Fig. 13. These deviations observed by the conventional and LiSE-hardness measurement were not detected by deep rolling. This could be explained by higher pressures of 85 bar, which are applied during deep rolling. These pressures result in an average load of 217 N. The deep rolling process is more effected by imperfections and cracks underneath the surface compared to the hardness measurement where 0.98 N was applied. The highest descriptor deviations of the laser deep-alloyed samples were also observed during deep rolling on the 3 kW samples. Particularly, the deep rolling process with 85 bar shows high track deviations in the center region. Cracks are mainly found underneath the surface in the center region as shown in Fig. 5. Higher laser powers such as 5 kW and even 6 kW could cause a recoil pressure during the laser processing and, accordingly, to an ejection of molten material. This may explain the asperity in the center region of the melt pool area for higher laser powers. In this case, it was not possible to evaluate the deep rolled track (as shown in Fig. 11). Thus, samples produced with 3 kW and with 5 kW laser powers reveal higher Vickers hardness deviations. Instead, samples produced with 4 kW laser power offer the best results in terms of lowest hardness deviations. These findings could also be observed by LiSE-hardness measurement. Additionally, the results of the LiSE-hardness measurements revealed that the indentation diameter results in a lower coefficient of variation values compared to the depth values.
Thus, the experiments reveal that LiSE-hardness measurement and deep rolling process are suitable methods to characterize the material properties of laser deep-alloyed micro-samples even with minor gradation in metallurgical constituent. Both processes show that the plastic deformation slightly increases with elevated laser power in the laser deep-alloying process (Fig. 14). Local deviations in metallographic constituent, which results in larger hardness deviations, can also be obtained by LiSE-hardness measurement. Both processes complement one another because the LiSE-hardness measurement is sensitive to local changes in metallurgical constituent and deep rolling is more sensitive to imperfections and cracks underneath the material surface.
Even small gradation in metallurgical constituent, which result in small changes in hardness and imperfections underneath the surface, could be detected by both measurement processes, which makes them suitable as descriptors for further analysis and high-throughput material characterization.
The conducted LiSE-hardness measurements reveal that the diameter values are preferred over depth values to predict the material hardness and local deviations in metallographic constituent. For further experiments, the coefficient of variation of the diameter can be used.
The LiSE-hardness measurement and the deep rolling process are suitable high-throughput methods to identify correlations and to gain new insights regarding the material properties of metallic alloy compositions.
Financial support of the subprojects D02 “Laser induced hardness measurements,” U02 “Laser deep alloying for the realization of finely graded alloys” and U04 “Mechanical Treatment” funded by the Deutsche Forschungsgemeinschaft (DFG, German Research Foundation) – Project Number 276397488 – SFB 1232 is gratefully acknowledged.
- 1.Vetter K, Freiße H, Vollertsen F (2017) High-throughput material development using selective laser melting and high power laser. In: Proceedings of the 7th WGP-Jahreskongress, Aachen, Germany, pp 511–518Google Scholar
- 3.Mädler L (2014) Is high-throughput screening for structural material/metals possible?, 4th NanoMan, Bremen, GermanyGoogle Scholar
- 9.Schulze V (2006) Modern mechanical surface treatment: States, stability, effects. Wiley-VCH Verlag, WeinheimGoogle Scholar
- 10.Kämmler J, Guba N, Vetter K, Vollertsen F, Meyer D (2018) Material characterization by deep rolling of laser deep alloyed micro-samples. In: Proceedings of the euspen. International Conference and Exhibition, Venice, Italy, pp 335–336Google Scholar
- 11.Vetter K, Hohenäcker S, Freiße H, Vollertsen F (2017) Use of additive manufacturing for high-throughput material development. In: Laser Manufacturing Conference (LiM), Munich, GermanyGoogle Scholar
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