Manfred Wollmann

Enhanced Fatigue Strength of Commercially Pure Ti Processed by Rotary Swaging

Fully reversed bending fatigue tests were performed on polished hour-glass specimens of commercially pure titanium grade 1 with three different grain sizes, that were produced by severe plastic deformation (rotary swaging) and subheat treatments, in order to examine the effect of grain size on fatigue. An improvement in fatigue strength was observed, as the polycrystal grain size was refined. The endurance limit stress was shown to depend on the inverse square root of the grain size as described empirically by a type of Hall-Petch relation. The effect of refining grain size on fatigue crack growth is to increase the number of microstructural barriers to the advancing crack and to reduce the slip length ahead of the crack tip, and thereby lower the crack growth rate. It was found that postdeformation annealing above recrystallization temperature could additionally enhance the work-hardening capability and the ductility of the swaged material, which led to a marked reduction in the fatigue notch sensitivity. At the same time, this reduction was accompanied with a pronounced loss in strength. The high cycle fatigue performance was discussed in detail based on microstructure and mechanical properties.

Fatigue behaviour of commercially pure aluminium processed by rotary swaging

Fatigue strength of ultrafine-grained commercially pure aluminium (Al 1050) produced by severe plastic deformation (rotary swaging) was investigated. Fatigue tests were carried out on smooth and notched specimens. Results show improved static and fatigue strength of the rotary swaging processed material. However, the processed material was highly notch sensitive due to low work hardening capability, low ductility as well as low uniform strain. It was found that post-deformation annealing above recrystallization temperature can additionally enhance the work hardening capability and the ductility of the swaged material, which led to marked reduction in fatigue notch sensitivity. At the same time, this reduction is accompanied with pronounced loss in strength. Fatigue notch sensitivity of commercially pure aluminium can be good correlated to the work hardening capability and ductility. This behaviour was discussed in details based on microstructure and mechanical properties study.

Evaluating the effects of hydroxyapatite coating on the corrosion behavior of severely deformed 316Ti SS for surgical implants

The present work investigates the effects of severe plastic deformation by cold rolling on the microstructure, the mechanical properties and the corrosion behavior of austenitic stainless steel (SS) 316Ti. Hydroxyapatite coating (HA) was applied on the deformed material to improve their corrosion resistance. The martensitic transformation due to cold rolling was recorded by X-ray diffraction spectra. The effects of cold rolling on the corrosion behavior were studied using potentiodynamic polarization. The electrochemical tests were carried out in Ringers solution at 37±1 °C. Cold rolling markedly enhanced the mechanical properties while the electrochemical tests referred to a lower corrosion resistance of the deformed material. The best combination of both high strength and good corrosion resistance was achieved after applying hydroxyapatite coating.

Ball-Burnishing and Roller-Burnishing to Improve Fatigue Performance of Structural Alloys

The HCF response to burnishing of a number of structural materials is compared and contrasted. It is shown that alloys which exhibit marked work-hardening during burnishing respond very beneficially with regard to HCF performance while others which show little work-hardening may even react with losses in HCF strength. Possible explanations for such behavior are outlined in terms of mean stress and environmental sensitivities of the fatigue strengths of the various materials and microstructures.

Effects of shot peening on the fatigue performance of ultrafine-grained CuNi3Si1Mg

ABSTRACT This study evaluated the effects of shot peening (SP) for modifying the fatigue performance of ultrafine-grained CuNi3Si1Mg. Fatigue damage is possibly caused when electromechanical connectors are subjected to cyclic loading or high numbers of repeated plugging. The fatigue performance of the precipitation-hardened ultrafine-grained CuNi3Si1Mg was significantly enhanced from 325 MPa in the electropolished condition to 400 MPa after SP at an Almen intensity of 0.25 mmA. In the precipitation-hardened ultrafine-grained CuNi3Si1Mg, SP induced a high degree of work hardening and maximum compressive residual stress values on the surface layers, which resulted in a better fatigue performance than the electropolished counterpart. After SP, the dissolution of precipitates in the surface layers also improved fatigue performance by reduction in the negative effect of the surface precipitates.

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.

Höhere Lebensdauer für Leichtmetalle

Das Dauerschwingverhalten von Leichtmetallen auf Basis von Mg, Al und Ti lasst sich durch Kugelstrahlen entscheidend verbessern, das belegen neueste Untersuchungen. Dazu mussen die Strahlparameter entsprechend angepasst werden. Der folgende Beitrag informiert uber die werkstoffspezifischen Besonderheiten bei der Auswahl geeigneter Strahlmittel und Strahlintensitaten.