Vitor Luiz Sordi

A general study of commercially pure Ti subjected to severe plastic deformation: microstructure, strength and corrosion resistance

Bars of Titanium Grade 2 were subjected to deformation by Equal Channel Angular Pressing, both at room temperature and at 300°C. Additionally, some specimens were cold rolled up to 70% reduction. From tensile tests, data such as yield, maximum strength, elongation and area reduction were obtained. Results show that the best strength - ductility combination is produced by four passes followed by cold rolling. Finally, the corrosion behavior was assessed following ASTM F2129 and no noticeable difference between the starting material and the ECAP-deformed was detected.

The effects of severe plastic deformation on some properties relevant to Ti implants

In some types of surgical implants, such as bone screws and plates, Grade 2 Ti is seriously considered as a replacement for the Ti-6Al-4V alloy. Advantages are lower cost and the absence of Al and V, which have been identified as potentially harmful to human health. The present paper shows that the lower strength of the commercially pure metal can be enhanced by Severe Plastic Deformation followed by conventional cold rolling, so as to reach a strength level higher than the technical requirements applicable to the alloy. This was ascertained by tensile and Vickers hardness tests from which it was concluded that the best combination of properties are obtained by submitting the metal to Equal Channel Angular Pressing (four passes at 300 °C) followed by a 70% thickness reduction by cold rolling. Although the present results are valid for the material only, and not for the product considered, that is, bone screws, it appears that this solution is a step towards the replacement of the Ti6-4 alloy by Grade 2 Ti, at least for some types of metallic medical implants.

Severe Plastic Deformation by Equal Channel Angular Pressing: Product Quality and Operational Details

As a technique, Equal Channel Angular Pressing (ECAP) is simple and inexpensive. However, if die manufacture and operational details are not carefully planned and implemented, difficulties arise, which can interfere with the product characteristics and the pressing operation itself. The present paper offers guidelines on die design and manufacture, emphasizing geometry, material and heat treatment. Further, operational parameters such as lubrication, pressing temperature, deformation routes, die closure procedure and the influence of channel cross section on maximum acceptable load are described. Additionally, the effects of those variables on the product characteristics (deformation level and homogeneity) and integrity, plus process control and safety, are discussed.

Microstructural development and tensile strength of an ECAP: deformed Al-4 wt. (%) Cu alloy

The tensile strength of metals and alloys can be considerably increased by severe plastic deformation, a consequence of the extreme grain refinement thus achieved. In the present work the deformation was performed by Equal Channel Angular Pressing and the material was an Al-4% Cu alloy. Characterization included tensile tests, and microstructural observation by optical and transmission electron microscopy. After four passes, the yield strength showed an increase of 310% over the undeformed alloy, a figure reduced to 160% by a 250 °C / 1 hour post - deformation anneal. Additionally, the alloy displayed a very low work hardening capacity, especially after the annealing heat treatment. After four deformation passes the grain size of the annealed specimens was estimated to be close to 500 nm. The tensile behaviour and the absence of work hardening were discussed in terms of the fine microstructure.

Constant load creep data in air and vacuum on 2.25Cr-1Mo steel from 600 °C to 700 °C

Creep results on 2.25Cr-1Mo were obtained at 600 °C, 650 °C and 700 °C at five stress levels, under constant load, in air and vacuum. Two chambers were specially developed for carrying out creep testing in controlled environment, with the possibility of accommodating inside them the load train, the extensometry system and the thermocouples. The creep machines used in this project present the advantage of allowing the performance of both constant load or constant stress creep testing, using interchangeable profiles. The preliminary set of constant load data reported here was analyzed according to the conventional methodology of creep data analysis, with the identification of parameters of the Norton, Arrhenius, Monkman-Grant and some extrapolation relations involving stress, temperature, minimum creep rate and rupture time. The results indicate a strong effect of the oxidation phenomenon on the creep behavior of this steel. The data were consistent and meaningful so that the developed chambers can be considered to present good performance in the generation of creep data in vacuum.

The effect of equal channel angular pressing on the tensile properties and microstructure of two medical implant materials: ASTM F-138 austenitic steel and Grade 2 titanium

Titanium and F-138 stainless steel are employed in bone replacement and repair. The former material was ECAP-deformed at room temperature and at 300°C, followed in some cases by cold rolling. The steel was ECAP-deformed at room temperature only. Work-hardening behavior was studied by making use of the Kocks-mecking plots and microstructural evolution was followed by TEM. Conclusions show that for Ti, ECAP combined with cold rolling gives the best strength-ductility combination, whilst room temperature ECAP increases the tensile strength of the steel but caused substantial ductility loss.

The Machinability of Ultrafine-grained Grade 2 Ti Processed by Equal Channel Angular Pressing

All processes of severe plastic deformation (SPD) ar e known to improve the mechanical strength of metals and alloys through microstructural re Þ nement. Although the literature contains a large number of investigation s on the effect of Þ ne-grained microstructures on mechanical behaviour, data on the machinability is almost non-existent. This lack of information motivated the present work, in which the machinability of severely-deformed Grade 2 Ti is assessed in terms of cutting forces and the resulting product surface roughness. The SPD process here employed is Equal Channel Angular Pressing (ECAP), and the results are compared with those obtained on Ti and Ti-6% aluminum-4% vanadium (Ti6-4) alloy, both in the annealed condition. It was observed that the machining of ultraÞ ne-grained Ti in the as-deformed state, requires larger cutting forces than the necessary for the annealed material, whilst for the alloy, the forces are of the same order. Due to continuous chip generation taking place in commercially pure Ti but not in Ti6-4, the passive component of the cutting force and the average surface roughness of the Þ ne grained material are higher. Finally, whilst both annealed and Þ ne grained Ti wear the tool by an attrition mechanism, titanium alloy machining promotes only adhesion over the tool edge.

The Influence of ECAP Die Channel Geometry on Shear Strain and Deformation Uniformity

The present work was performed in order to analyze the influence of the outer corner radius (R) of ECAP die channels on the strain field of billets subjected to ECAP deformation in a Φ = 120o die, employing three different methods: (i) physical simulation, consisting of the direct measurement of deformations of a grid inscribed in longitudinally cut mid-planes of ECAPed billets; (ii) numerical simulation employing an explicit finite element code for large displacements and large plastic deformations, and (iii) calculation by the Iwahashi formula. Materials employed were Al-4%Cu and an eutectic Pb-62Sn alloy, and the dependence of shear strain with R was satisfactorily described using the three methods. The experimental method showed a small deviation from the other two, which was explained making use of the corner die formation concept. Similarly, this concept helped to understand the increase of strain heterogeneity with R. Also, it was shown that large corner radii decrease ECAP pressing loads, facilitating deformation of high strength materials. Finally, the data show that the deformation characteristics of the materials here studied do not exert a measurable influence on the shear strain magnitude and distribution.

Precipitation Studies of Undeformed and ECAP -Deformed Al-4%Cu Alloys : Effects on Microstructure and Tensile Properties

The present study is an assessment of the effects of precipitation heat treatments on tensile behaviour, work hardening (WH) characteristics and microstructural evolution of an Al-4%Cu alloy deformed by equal channel angular pressing (ECAP). Two ageing temperatures were employed (170 and 100oC) and their effect on strength and WH behaviour was compared with that exerted on the same alloy, but in two different initial conditions: quenched from solution temperature and slowly cooled before anneal. Grain and precipitate sizes of samples deformed by one and four ECAP passes and heat treated as described were measured employing transmission electron microscopy (TEM). It was concluded that the lower ageing temperature gives the best combination of strength and ductility, a high WH rate and, possibly, the smaller grain and precipitate sizes. The relative participation of the various hardening mechanisms to total strength was estimated from tensile tests and hardness measurements.

Deformation analysis on F138 austenitic stainless steel: ECAE and rolling

Twinning is an alternative mechanism to achieve ultra-fine grain structures through severe plastic deformation. The properties induced in a plastically deformed material are highly dependent on the degree of deformation, accumulated deformation energy and details on grain sizes and microstructure, which are on the scale of some tens of nanometers; therefore it is very important to understand misorientation distributions and dislocation arrays developed in the samples. In this work an F138 austenitic stainless steel was solution heat treated, deformed by Equal Channel Angular Extrusion (ECAE) at room temperature up to four passes, and rolled up to 70% thickness reduction at room temperature. The microstructure evolution was analyzed by x-ray diffraction and domain sizes calculated by Convolutional Multiple Whole Profile (CMWP) model, the misorientation boundaries were measured by electron backscattered diffraction (EBSD), and transmission electron microscopy. Mechanical behavior was tested by tensile tests.