Igor Altenberger

An in situ transmission electron microscope study of the thermal stability of near-surface microstructures induced by deep rolling and laser-shock peening

Abstract We investigate the thermal stability of near-surface microstructures induced by deep rolling and laser-shock peening in AISI 304 stainless steel (AISI 304) and Ti–6Al–4V using in situ transmission electron microscopy. The improvements in fatigue resistance at elevated temperature are related to the high-temperature stability of the work-hardened near-surface microstructure.

Material properties of high–strength beryllium–free copper alloys

High strength copper alloys can be produced either by generating very fine grained low alloyed single phased or precipitation hardened copper alloys or by highly alloyed precipitation hardened copper alloys. The latter process requires special processing methods such as spray forming in order to achieve a sufficiently homogeneous microstructure. Systematic investigations on the aging behaviour of the highly alloyed nickel-manganese bronze CuNi20Mn20 demonstrate that fully crystalline copper alloys with hardness exceeding 500 HV can be produced. In addition to age hardening, swaging or severe plastic surface deformation can be used for additional grain refinement and strain hardening before precipitation hardening. In contrast to CuMn20Ni20, the low-alloyed precipitation hardened copper alloy CuNi3Si1Mg exhibits excellent thermal and electrical conductivity while maintaining acceptable strength after swaging and precipitation hardening. Finally, a systematic comparison between spray-formed or precipitation high strength hardened copper alloys and classical well-known materials such as steels or aluminium alloys was carried out by using material property charts (Ashby-maps) and highlighting the fields of application and unique property combinations of copper alloys.

Enhancement of Surface Property by Low-Energy Laser Peening without Protective Coating

Laser peening without protective coating (LPwC) has been applied to metallic materials using low energy pulses of a Q-switched and frequency-doubled Nd:YAG laser. Compressive residual stresses of several hundred megapascals were imparted on the surface of the materials. Redistribution of the residual stress in the top surface due to thermal loading was evaluated non-destructively by synchrotron radiation of SPring-8. Accelerating stress corrosion cracking (SCC) tests showed that LPwC prohibited SCC of sensitized materials. LPwC largely prolonged the fatigue lives of titanium alloys, aluminum alloys and austenitic stainless steels.

Effects of Mechanical Surface Treatment on Fatigue Failure in Ti-6Al-4V: Role of Residual Stresses and Foreign-Object Damage

Mechanical surface treatments can play a major role in the enhancement of fatigue life in metallic materials. Deep rolling and laser shock peering processes, for example, can induce several favorable effects, including deep cases of compressive residual stresses, work hardening and low surface roughness. Since the fatigue life of smooth specimens is invariably dominated by crack-initiation processes, the presence of high near-surface dislocation densities can improve fatigue lifetimes substantially, even if residual (macro) stresses are not stable during fatigue. Conversely, in engineering applications such as turbine engine airfoils, the surfaces are very often notched or damaged, for instance by foreign-object impacts, which severely reduces the crack-initiation phase. In such cases, the fatigue life is dominated by crack propagation and a stable compressive residual stress state is crucial for enhancing fatigue life. In the present work the effect of mechanical surface treatments on the fatigue behavior and residual stress state of smooth and on foreign-object damaged specimens of the Ti-alloy Ti-6Al-4V is investigated. The results show that deep rolling can substantially improve the fatigue life of smooth specimens at elevated temperature (450°C) due to a higher resistance against fatigue crack initiation. For laser shock peened and foreign-object damaged specimens, information on the residual stress state in and around the foreign-object impacts yields a deeper insight into the nature of local fatigue damage and microcrack propagation. The results indicate that laser shock peening process parameters have to be very carefully chosen in order to diminish the effects of foreign object damage.

Recent Developments in Mechanical Surface Optimization

Modern mechanical surface optimization processes today go beyond simple empirical techniques Typical examples are presented, demonstrating the possibilities to increase lifetime and strength of components by mechanical surface treatments. A focal point is the relation between materials properties and microstructures created by near surface plastic deformations and their consequences on cyclic deformation, crack initiation and crack propagation during cyclic loading. In this context, also the stability of microstructures and residual stresses in surface layers is discussed.

Alternative Mechanical Surface Treatments for Fatigue Strength Enhancement

In this paper, The effects of laser-shock peening and high temperature deep rolling on nearsurface microstructures, residual stress states and fatigue behavior of various metallic materials are investigated and discussed. Similar to warm peening (shot peening at elevated temperatures), high temperature deep rolling may induce several favourable effects, especially in ferritic steels, where dynamic strain aging by carbon atoms can be exploited as a major strengthening mechanism. But also in materials without ‚classical‘ strain aging high temperature deep rolling is effective in improving the fatigue behaviour by inducing favourable, e.g. precipitation-hardened, nearsurface microstructures. As a consequence, these modified near-surface microstructures directly alter the thermal and mechanical relaxation behaviour of residual stresses. Laser-shock peening is already used in the aircraft industry (as a mechanical surface treatment for fan-blades) and owes its benefial effects to deep layers of compressive residual stress and work hardening and a relatively smooth surface roughness. Characteristic examples of microstructures and residual stress profiles as generated by laser-shock peening are presented. Moreover, the impact on the fatigue behavior of steels and a titanium alloy is outlined and discussed.

Enhancement of Surface Properties by Laser Peening Without Coating

Laser peening without coating (LPwC) has been applied to water-immersed materials using a water-penetrable light of a Q-switched and frequency-doubled Nd:YAG laser. Compressive residual stress of several hundred MPa was introduced at the surface of the materials. High-cycle fatigue (HCF) properties were evaluated through rotating-bending or push-pull type testing for an austenitic stainless steel (SUS316L), a titanium alloy (Ti-6Al-4V) and a cast aluminum alloy (AC4CH). LPwC prolonged the fatigue lives significantly, in spite of the increase in surface roughness ascribed to the ablative interaction of laser pulses with the materials.Copyright

Spray Forming of Copper Alloys

Spray forming as a process innovation in the copper industry has opened the door to several cutting-edge technologies for copper alloys. Indeed, spray-formed copper alloys have become “mature” materials within the last two decades and have seen industrial applications in several major key production technologies of the twenty first century. In several fields they have become competitors to classical engineering materials such as steel due to their homogeneous and tailored microstructure allowing production of complex alloy systems with a good combination of strength, ductility, workability and physical properties.

Ultrafine-Grained High Strength Cu-Ni-Si Alloys

Ultrafine-grained (UFG) pure copper has been in the focus of materials scientists over the last two decades, however ultrafine-grained high-strength copper alloys have scarcely been processed or characterized so far industrially.In this contribution, UFG copper alloys, especially Cu-Ni-Si alloys, being well known as ideal materials for electromechanical connectors, springs and leadframes, are presented. Precipitation hardened Cu-Ni-Si alloys are a well established and technologically important class of materials for a wide range of applications where high strength and good conductivity are required. Yield strength and fatigue properties of metallic alloys can be significantly enhanced by severe plastic deformation methods. In contrast to other strengthening methods such as solid solution hardening, severe plastic deformation leads to a weaker decrease of electrical conductivity and is therefore a means of enhancing strength while maintaining acceptable conductivity for current bearing parts and components. Characterization of these materials after severe plastic deformation by swaging, wire drawing and subsequent aging was carried out using conductivity-, hardness-and tensile tests as well as highly-resolved microstructural characterization methods.The results reveal that UFG low alloyed copper alloys exhibit impressive combinations of properties such as strength, conductivity, high ductility as well as acceptable thermal stability at low and medium temperatures. By a subsequent aging treatment the severely plastically deformed microstructure of Cu-Ni-Si alloys can be further enhanced and thermal stability can profit from grain-boundary pinning by precipitated nanoscale nickel silicides.