Allan H. Clauer

Quantitative assessment of laser‐induced stress waves generated at confined surfaces

Laser‐induced stress waves in iron samples were analyzed by measuring the pressure environment at the back surface of various sample thicknesses. These results were compared with numerical calculations obtained from a one‐dimensional radiation hydrodynamics computer code. The experiments were conducted in an air environment under ambient conditions and the metal surfaces were confined by transparent overlays. Peak pressures exceeding 50 kbar were measured with quartz pressure transducers at a laser power density of about 109 W/cm2. Computer predictions agreed favorably with the experimental results and indicated that peak pressures exceeding 100 kbar could be generated by appropriate modifications in the laser environment and target overlay configuration.

Effects of Laser Induced Shock Waves on Metals

A high-energy, pulsed laser beam combined with suitable transparent overlays occn generate pressure pulses of up to 6 to 10 GPa on the surface of a metal. The propogation of these pressure pulses into the metal in the form of a shock wave produces changes in the materials micro structure and properties similar to those produced by shock waves caused in other ways. This paper reviews the mechanism of shock wave formation, calculations for predicting the pressure pulse shape and amplitude, in-depth microstructural changes and the property changes observed in metals. These property changes include increases in hardness, tensile strength and fatigue life. The increases in fatigue life appear to result from significant residual surface stresses introduced by the shock process.

Pulsed laser induced deformation in an Fe-3 Wt Pct Si alloy

The plastic deformation produced by laser induced stress waves was investigated on an Fe-3 wt pct Si alloy. The intensity and duration of the stress waves were varied by changing the intensity and pulse length of the high energy pulsed laser beam, and also by using different overlays on the surfaces of the specimens. The resulting differences in the distribution and intensity of the deformation caused by the stress waves within the samples were determined by sectioning the specimens and etching (etch pitting) the transverse sections. The magnitude of the laser shock induced deformation depended on the laser beam power density and the type of surface overlay. A combination transparent plus opaque overlay of fused quartz and lead generated the most plastic deformation. For both the quartz and the quartz plus lead overlays, intermediate laser power densities of about 5×108 w/cm2 caused the most deformation. The shock induced deformation became more uniform as the thickness of the material decreased, and uniform shock hardening, corresponding to about 1 pct tensile strain, was observed in the thinnest specimens (0.02 cm thick). 200 ns laser pulses caused some surface melting, which was not observed for 30 ns pulses, the pulse length used in most of the experiments. Deformation of the Fe-3 wt pct Si alloy occurred by both slip and twinning.

Laser shock hardening of weld zones in aluminum alloys

The feasibility of using a high energy, pulsed laser beam to shock-harden weld zones in 5086-H32 and 6061-T6 aluminum sheet was investigated. The tensile strength, hardness, and microstructure of samples 0.3 cm thick were studied before and after laser shocking. After laser shocking, the tensile yield strength of 5086-H32 was raised to the bulk level and the yield strength of 6061-T6 was raised midway between the welded and bulk levels. The increases in ultimate tensile strength and hardness were smaller than the increases in the yield strength. The microstructures after shocking showed heavy dislocation tangles typical of cold working.

Laser Shock Processing Increases the Fatigue Life of Metal Parts

Laser shock processing (LSP) produces a surface compressive residual stress in the metal part being treated that can significantly improve those properties which are affected by the initiation and propagation of surface cracks. The properties of greatest interest are fatigue life and fatigue strength. But the process also can reduce fretting fatigue and stress-corrosion cracking as well as strengthen thin sections. The potential advantages of LSP include the possibility of direct integration into manufacturing production lines with a high degree of automation, use on machined surfaces, increased quality assurance, treatment of localized fatigue critical areas without masking, and the ability to make design changes that would not be possible using alternative methods for increasing fatigue resistance. Among the applications that have been identified are the manufacture of blades, disks, and vanes for aircraft gas turbine engines; gears, connecting rods, and crankshafts for automotive engines; and medical implants.

Transient reflectivity behavior of pure aluminum at 10.6 μm

Time‐resolved measurements of specular reflectivity of polished pure aluminum surfaces subjected to intense 10.6‐μm pulsed‐laser radiation in vacuum are reported. A sharp decrease in reflectivity to an anomalously low value (35%) was observed midway in the pulse followed by nearly complete recovery of full reflectivity near the end of the pulse. The peak‐power‐density threshold for this phenomenon was found to agree with the intrinsic surface melt threshold as determined from numerical heat‐transfer calculations and microscopic examination of the samples.

COMPARISON OF THE SINGLE ROLLER AND DOUBLE ROLLER PROCESSES FOR CASTING LOW CARBON STEEL

Publisher Summary This chapter provides a comparison of the single-roller and double-roller processes for casting low-carbon steel. The direct casting of thin metal strip and sheet eliminates many process steps currently needed in conventional processes to produce sheet/strip. The elimination of these process steps reduces energy requirements of the process and enables manufacturers of sheet/strip to significantly lower their processing costs. There are three carbon steel sheet/strip processes, namely, ingot, continuous casting, and direct strip casting (DSC). The calculation for the direct strip casting process includes estimates for ladle holding and heating, tundish preheating, casting, and postcasting treatments such as pickling, rolling, and annealing. The chapter presents a study in which two direct strip-casting processes, namely, the double-roller (DR) process and the single-roller (SR) process, were compared. Using the results of these analyses, a generalized model was developed. Based on this model, a sensitivity analysis was made to determine the way in which each parameter affects the metal and heat-flow patterns. The sensitivity analysis shows that the process parameters can be approximately split into two categories that include those which significantly affect the strip thickness and those which only weakly affect thickness.