Northwestern University Robert R. McCormick School of Engineering and Applied Science

Advanced Manufacturing Processes Laboratory

Laser Shock Peening

Goals

To improve the fatigue life, edge sharpness retention and hardness of a variety of dental surgical tools by the process of laser shock peening.

Challenges

  1. Dental surgical tools are made of surgical grade hardened 400-series steels, which makes it difficult to induce surface residual stresses by laser shocks.

  2. Often the un-conventional and free-form geometries of these tools such as dental scalers is a challenge to be processed by any surface operation like shock peening.

Research Work

The research is aimed at developing a laser shock-peening setup using a high pulse energy 1064/532 nm laser (Continuum Surelite EX) with a flat-top beam profile, and an intervening transparent medium (distilled water), without the use of an ablative layer (tape, foil or vacuum grease), to peen the materials for dental surgical instruments. The incident laser pulse is focused by a spherical focusing lens within the distilled water, and causes optical breakdown leading to formation of plasma at the surface of the workpiece. The plasma then acts as an elastic medium to propagate shock waves inside the material surface, thereby creating compressive residual stress.


The strategy is to find the optimum set of processing conditions leading to the maximum residual stress at the material surface. The residual stresses are characterized by a high power X-Ray beam capable of penetrating to a depth of about 1 mm. Prior to testing on a dental instrument, 304L is used as a testing material for the process.

continuum surelite

Fig. 1 - Continuum Surelite EX laser: 532 nm 8 ns pulses @ 0.5 J/pulse (max)

 

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Fig. 2: (a) Shock peened spots on a 304L steel substrate; (b) Microscope image of a single crater; (c) Cross-section profile of the crater

 

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Fig. 3: Residual stress vs. depth below the surface, after shock-peening at two different pulse energy levels