J. Noordhuis

Microstructure and Mechanical Properties of a Laser Treated Al Alloy

Abstract An AlCuMg alloy, Al 2024-T3, was exposed to laser treatments at various scan velocities. In this paper the microstructural features and mechanical properties are reported. As far as the mechanical property is concerned a striking observation is a minimum in the hardness value at a laser scan velocity of 1 2 cm/s . Usually an increasing hardness with increasing laser scan velocities is reported in the literature. This remarkable property could be explained based on the microstructural features observed by transmission electron microscopy. It turned out that depending on the laser scan velocity nucleation and growth of precipitates could either be enhanced or suppressed. In addition at low scan velocities helical dislocations are observed which may contribute to an increase of the hardness. After subsequent shot peening, in all cases the formation of precipitates was observed, independent of the laser scan velocities originally applied. This phenomenon of precipitation, induced by shot peening afterwards is most striking at a high concentration of alloying elements in solid solution.

TEMPERING OF STEEL DURING LASER TREATMENT

This study concentrates on the annealing effects of a laser-treated area by a successive laser pass in a plain carbon, a chromium, and a tungsten steel. Transmission electron microscopy (TEM), optical microscopy (OM), and wear and hardness measurements revealed substantial effects on the properties and microstructure of the plain carbon and tungsten steel which have a partly martensitic cellular structure after the first laser treatment. Close to the melt zone, martensite was again formed, whereas further away, nucleation of carbides was observed. The hardness increased in the former area and decreased in the latter. An analytical model sustained that the softening is due to carbon diffusion and martensite tempering. The softer zone in the tungsten steel exhibited a decrease of wear rate relative to the not-annealed zone in contrast to the plain carbon steel. Besides some diffusion, no annealing effects were found in the chromium steel which possessed an austenitic cellular structure after the first laser treatment.

Surface Modification by Means of Laser Melting Combined with Shot Peening: A Novel Approach

Abstract Among the available laser applications laser surface melting has turned out to be a powerful technique for the production of wear-resistant layers. Despite the advantages of this process, laser surface melting results in tensile stresses which may assist crack propagation. In this paper it will be shown that shot peening can overcome this drawback effectively. It turned out that a preceding laser treatment of an eutectic aluminium-silicon alloy is able to amplify considerably the effectiveness of the shot peening treatment. In particular the maximum attainable hardness and compressive stress increase upon increasing the quench rate, i.e. upon increasing the laser scan velocity. The high concentration of silicon in solid solution turned out to be the main reason for the enhanced mechanical performance, not only directly through solid solution hardening, but also by precipitation hardening and by a higher dislocation density. The latter contribution is affected indirectly by a changed cross slip behavior.

Ne implantation induced transformation in stainless steel

Abstract This paper reports a microstructural investigation of the changes induced by Ne-implantation in stainless steel of the austenitic type. At a critical dose of 2.3·1017/cm2 a martensitic phase transformation was observed. In particular, attention has been paid to the effect of the stress field of neon bubbles on the nucleation of martensite. It is found that the critical size of the nucleus as well as the activation energy is drastically lowered. No definite orientation relationship between martensite and austenite has been detected. This is probably caused by the small size of martensite particles and the deformation in the vicinity of the bubbles.

Defect profiling of neon-implanted and laser-melted steel by positron annihilation

Abstract Positron annihilation measurements have been performed on laser-annealed 0.22 wt.% C steel implanted with 50 keV Ne to a dose of 2 × 10 17 cm -2 . In implanted but not laser-treated steel damage is observed at depths up to 10 times the mean implantation range, which has been explained as being due to channelling of neon to greater depths in (poly)crystalline material. Both the neon profile and the defect “tail” have been examined after several annealing steps.

The influence of noble gas bubbles on mechanical properties of steel

Indentation tests and wear measurements have been performed on a laser treated carbon steel and an austenitic steel to evaluate the effects of noble gas ions on micro-hardness, cracking and wear behavior. The following are the principal conclusions: - In all cases studied implantation leads to an increased hardness. - The implantation induced compressive stress leads to a decreasing cracking probability. - The formation of bubbles leads to a decreased wear resistance.

Fundamental and Applied Aspects of Noble Gas Bubbles in Steel

This paper reports a study aimed at improving the wear performance of common steels by noble gas ion implantation and laser melting. It turns out that the shear stress field of a noble gas bubble in a metastable austenitic steel (SS 304) induces a martensitic transformation, provided that bubble size and pressure are large enough. The wear resistance decreases with increasing dose. In laser melted stable austenitic steel (RCC) subsequently implanted with noble gas ions, transmission electron micrographs has provided clear evidence of the formation of a large number of bubbles, Orowan looping and pinning of dislocations by these bubbles. The wear rate decreases with increasing dose. However, this is certainly not a general observation of laser melted-ion implanted steels. The low carbon steel CK22 for instance shows a martensitic structure after laser melting with a high dislocation density. In contrast to RCC, the wear rate increases with increasing dose suggesting that moving dislocations are interacting with forest dislocations rather than with the noble gas bubbles.

Microstructure and mechanical properties of laser treated aluminium alloys

Al-Cu alloys and an Al-Cu-Mg alloy, A1 2024-T3, were exposed to laser treatments at various scan velocities. In this paper the microstructural features and mechanical properties are reported. As far as the mechanical property of the Al-Cu-Mg alloy is concerned a striking observation is a minimum in the hardness value at a laser scan velocity of 1/2 cm/s. Usually an increasing hardness with increasing laser scan velocities is reported in the literature. This remarkable property could be explained based on the microstructural features observed by transmission electron microscopy.

Martensitic Transformation in 304 Stainless Steel after Implantation with Neon

X-ray diffraction on neon implanted AISI 304 austenitic stainless steel revealed both a tensile strain in the implanted layer which increases with implantation dose and which is responsible for a martensitic transformation at a critical dose of 2.3*1017Ne+/cm2, and a three times smaller compressive strain in the underlying substrate which can be explained by the existence of neon bubbles at high pressure.