Research Papers

Local Microstructure and Hardness Variation After Pulsed Laser Micromelting on S7 Tool Steel

[+] Author and Article Information
Justin D. Morrow

Department of Mechanical Engineering,
University of Wisconsin-Madison,
1513 University Avenue,
Madison, WI 53706
e-mail: jdmorrow@wisc.edu

Frank E. Pfefferkorn

Department of Mechanical Engineering,
University of Wisconsin-Madison,
1513 University Avenue,
Madison, WI 53706
e-mail: frank.pfefferkorn@wisc.edu

Contributed by the Manufacturing Engineering Division of ASME for publication in the JOURNAL OF MICRO- AND NANO-MANUFACTURING. Manuscript received November 9, 2015; final manuscript received June 13, 2016; published online July 11, 2016. Assoc. Editor: Stefan Dimov.

J. Micro Nano-Manuf 4(3), 031006 (Jul 11, 2016) (10 pages) Paper No: JMNM-15-1078; doi: 10.1115/1.4033924 History: Received November 09, 2015; Revised June 13, 2016

Laser surface melting is being increasingly used as a method of surface polishing steels and other alloys, but understanding the effect of this process on the microstructure and properties is still incomplete. This work experimentally explores several basic questions about how the surface microstructure and properties of S7 tool steel change during a pulsed laser micromelting (PLμM) process. Evaluations of the microstructure and hardness suggest that diffusion-controlled processes such as melt homogenization and surface back-tempering are relevant during rapid microscale laser melting and that the laser parameters and process planning contribute to determining the final surface hardness. The results also suggest that some influence can be exerted over the final hardness obtained from laser surface melting by changing the processing parameters.

Copyright © 2016 by ASME
Topics: Lasers , Melting , Tool steel
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Fig. 1

Diagrams of (a) unidirectional and zig-zag scan paths and (b) the laser spot overlapping typical in creating PLμM lines and areas

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Fig. 2

(a) A typical load versus displacement (P versus h) curve, (b) SEM micrograph of a single indent, and (c) diagram of the hardness (H) mapping method on a mock dataset where each data point represents an individual indent

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Fig. 3

Scanning electron micrographs of (a) a PLμM spot, (b) line, and (c) area. The left side image shows the resulting surface topography and the right side shows the underlying microstructure. Inset images show (d) the edge of a laser spot and ((e) and (f)) carbide formation in an overlap region of a PLμM area scan.

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Fig. 4

The hardness as a function of indentation depth plots for the (a) laser spots, (b) laser lines, and (c) laser areas. Indents that had a depth exceeding the calibrated range (i.e., hc > 230 nm) are included in gray. Each figure has an inset histogram showing the distribution: The measured hardness of the laser lines and area showed a unimodal distribution while the laser spots showed a trimodal distribution in the hardness, as shown in the inset histogram. SEM evaluation (d) of eight laser spots showed that the indentation placement on laser spots was often near the melt pool edge and occasionally missed the spots entirely.

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Fig. 5

Secondary electron micrographs of etched S7 steel: (a) as-received (annealed) and (b) furnace hardened

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Fig. 6

Microstructure and hardness of condition showing the correlation between local back-tempering and hardness change: (a) A, (b) B then A, (c) A then B, and (d) B. The tempered overlap regions are indicated with arrows in (b). Hardness ranges (mean ± 1 standard deviation) of the furnace hardened and annealed surfaces are also included as gray boxes for reference.




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