The status of rapid solidification is discussed in terms of recent progress in modelling methods of achieving solidification at high cooling rates and its effects on alloy constitution and microstructure. Applications currently in view are reviewed under the following headings: high-strength structural materials; tool and bearing materials; high-temperature materials, corrosion-resistant, catalytic and storage materials; and electrical and magnetic materials. It is concluded that the future of rapid solidification is not identifiable with any one process, activity or application and that many unresolved questions remain to challenge both the scientific and business communities.

The status of rapid solidification of alloys in research and application

Short-duration high-intensity pulsed electron beams have been used to anneal ion-implantation damage in silicon and to electrically activate the dopant species. Lattice regrowth and dopant activation were determined using He(+)-4 backscattering, SEM, TEM, and device performance characteristics as diagnostic techniques. The annealing mechanism is believed to be liquid-phase epitaxial regrowth initiating from the substrate. The high-temperature transient pulse produced by the electron beam causes the dopant to diffuse rapidly in the region where the liquid state is achieved.

Pulsed-electron-beam annealing of ion-implantation damage

An analytical heat flow model is used to identify dimensionless parameter groups which determine the temperature field produced in a material by a scanning laser beam. The groups are used to plot experimental data for metallic alloys on a processing diagram for a range of continuous CO 2 laser treatments. Practical operating regions for each type of treatment are thus identified, which coincide with those predicted using the heat flow model. The model is extended in order to construct more detailed diagrams for transformation hardening, surface melting and keyhole welding, which quantify the depth of treatment. By using realistic estimates of certain poorly-known process variables, good agreement is observed between measured and predicted data. Methods for optimising processing parameters with respect to various criteria are presented for transformation hardening. The diagrams, which are constructed on a personal computer, are a useful tool for summarising current data, optimising practical processing parameters, and assessing the potential of novel laser treatments on new materials.

Diagrams for laser materials processing

In this article, the laser-engineered net shaping (LENS) process is implemented to fabricate net-shaped Fe-based Fe-B-Cr-C-Mn-Mo-W-Zr metallic glass (MG) components. The glass-forming ability (GFA), glass transition, crystallization behavior, and mechanical properties of the glassy alloy are analyzed to provide fundamental insights into the underlying physical mechanisms. The microstructures of various LENS-processed component geometries are characterized via scanning electron microscopy (SEM), X-ray diffraction (XRD), differential scanning calorimetry (DSC), and transmission electron microscopy (TEM). The results reveal that the as-processed microstructure consists of nanocrystalline α-Fe particles embedded in an amorphous matrix. An amorphous microstructure is observed in deposited layers that are located near the substrate. From a microstructure standpoint, the fraction of crystalline phases increases with the increasing number of deposited layers, effectively resulting in the formation of a functionally graded microstructure with in-situ-precipitated particles in an MG matrix. The microhardness of LENS-processed Fe-based MG components has a high value of 9.52 GPa.

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Processing and Behavior of Fe-Based Metallic Glass Components via Laser-Engineered Net Shaping

Surface modification of austenitic stainless steels S31603. S30400 and super duplex stainless steel S32760 was achieved by laser surface melting using 2 kW continuous wave Nd—YAG laser. The cavitation erosion and corrosion characteristics of laser surface melted stainless steels in 3.5% NaCl solution at 23 °C were studied by means of a 20 kHz ultrasonic vibrator at a peak-to-peak amplitude of 30 mm and a potentiostat respectively. In a series of 4 h cavitation tests, the cavitation erosion resistance of laser-melted S31603 with 50% overlapping of melted tracks was found to be improved by 22% at 23 °C as compared with the as-received specimen and very close to that of the as-received S30400. No improvement on the cavitation erosion resistance of S30400 and S32760 was found. The pitting potential of laser-melted S31603 increased from 359 to 452 mV at 23 °C. The deformation mechanism by cavitation and the pitting morphology of the laser-melted stainless steels were discussed in terms of microstructure and residual stress.

Cavitation erosion and pitting corrosion of laser surface melted stainless steels

A scanning laser beam was used to melt and normalize the surface layer of sensitized 304 stainless steel. Subsequent Strauss tests indicated a complete resistance to intergranular corrosion. Mechanical testing at strains less than 15% also showed laser surface melting to indefinitely extend specimen life in a stress corrosion environment. At strains greater than 15%, the laser‐scanned protective layer was breached by cracks. A maximum critical laser‐scanning velocity compatible with normalization of the surface layer is calculated. Similarly, a minimum critical laser‐scanning velocity required to avoid resensitization is determined. The stress distribution in a 304 stainless‐steel specimen with a laser‐melted and self‐quenched surface layer is estimated and shown to be compatible with the observed appearance of martensite in the melted surface layer.

T. R. Anthony

Papers

Surface normalization of sensitized stainless steel by laser surface melting