W. Wallace

A TEM study of microstructural changes during retrogression and reaging in 7075 aluminum

Microstructural changes occurring during retrogression, and during retrogression plus reaging in 7075-T6 aluminum alloy have been investigated by means of transmission electron microscopy, and related to mechanical properties. TEM results indicate that the drop in strength during the initial stage of retrogression was due to the partial dissolution of G.P. zones while the growth of the semi-coherentη ′ was responsible for the rapid recovery of strength. It is suggested that the retrogression and reaging treatment resulted in the increase in volume fraction of G.P. zones and especially η′ precipitates over both the T6 and retrogressed conditions, therefore significantly improving the strength of the alloy.

Microstructural study of a high-strength stress-corrosion resistant 7075 aluminium alloy

A heat-treatment procedure providing for enhanced stress-corrosion cracking resistance without any sacrifice of yield strength in 7075 aluminium alloy is investigated using transmission electron microscopy. It is suggested that the heat treatment (known as retrogression and re-ageing) provides for large grain-boundary precipitates and coherent matrix precipitates. The latter provides for the high strength levels while the grainboundary precipitates provide for enhanced stress-corrosion cracking resistance. A hydrogen embrittlement mechanism of stress-corrosion cracking is assigned to this alloy system.

Research and Progress in Laser Welding of Wrought Aluminum Alloys. II. Metallurgical Microstructures, Defects, and Mechanical Properties

With the wide application of aluminum alloys in automotive, aerospace, and other industries, laser welding has become a critical joining technique for aluminum alloys. In this review, the research and progress in laser welding of wrought aluminum alloys are critically discussed from different perspectives. The primary objective of the review is to understand the influence of welding processes on joint quality and to build up the science base of laser welding for the reliable production of aluminum alloy joints. Two main types of industrial lasers, carbon dioxide (CO2), and neodymium-doped yttrium aluminum garnet (Nd:YAG), are currently applied but special attention is paid to Nd:YAG laser welding of 5000 and 6000 series alloys in the keyhole (deep penetration) mode. In the preceding article of this review (part I), the laser welding processing parameters, including the laser-, process-, and material-related variables and their effects on welding quality, have been examined. In this part of the review, the metallurgical microstructures and main defects encountered in laser welding of aluminum alloys such as porosity, cracking, oxide inclusions, and loss of alloying elements are discussed from the point of view of mechanism of their formation, main influencing factors, and remedy measures. The main mechanical properties such as hardness, tensile and fatigue strength, and formability are also evaluated.

Research and Progress in Laser Welding of Wrought Aluminum Alloys. I. Laser Welding Processes

With the wide application of Al alloys in automotive, aerospace and other industries, laser welding has become a critical joining technique for aluminum alloys. In this review, the research and progress in laser welding of wrought Al alloys have been critically discussed from different perspectives. The primary objective of this review is to understand the influence of welding processes on joint quality and to build up the science base of laser welding for the reliable production of Al alloy joints. Two main types of industrial lasers, carbon dioxide (CO2) and neodymium-doped yttrium aluminum garnet (Nd:YAG), are currently applied but special attention is paid to Nd:YAG laser welding of 5000 and 6000 series alloys in the keyhole (deep penetration) mode. In this part of the review, the main laser welding processing parameters including the laser-, process-, and material-related variables and their effects on weld quality are examined. In part II of this article in this journal, the metallurgical microstructures and main defects encountered in laser welding of Al alloys such as porosity, cracking, oxide inclusions, and loss of alloying elements are discussed from the point of view of mechanism of their formation, main influencing factors, and remedy measures. In part II, the main mechanical properties such as hardness, tensile, and fatigue strength and formability are also discussed.

Heat treatment of hot isostatically processed IN-738 investment castings

Abstract An examination has been carried out of the microstructures of hot isostatically processed IN-738 turbine blades. Areas of void closure were found to be atypical of the bulk microstructure and showed recrystallization, twinning, residual gas micropores, and denudation of primary γ′. The effect of heat treatment following hot isostatic processing and solution treatment at 1200°C was investigated. Heat treatments involving controlled cooling at 140°C/hr from 1200°C to the primary aging temperature, followed by secondary aging at 845°C gave rise to the longest rupture lives in tests at 760°C and 586 MPa. Such heat treatments gave rise to finely serrated grain boundaries and a mixture of cuboidal primary γ′ and spheroidal secondary γ′. Appreciably faster or slower cooling rates gave rise to shorter rupture lives, and either planar grain boundaries, or serrated grain boundaries with overaged γ′.

Laser Gas Nitriding of Ti-6Al-4V Part 1: Optimization of the Process

A multi-variable test method, known as the Orthogonal Array, was used to optimize the parameters for the laser gas nitriding process (LGNP) to avoid surface cracking. Based on the fundamental requirement of a crack-free condition, the processing parameters were further optimized to improve the surface finish and to obtain a reasonable hardened depth. The effects of processing parameters have also been investigated with respect to the characteristics of the laser nitrided layer. Two types of lasers, i.e., CO2 and Nd:YAG lasers were used. The CO2 laser was operated in both the continuous as well as the pulse mode, while the Nd:YAG laser was used only in the pulse mode. A Nd:YAG laser in the pulse mode provided a better surface finish and lower cracking severity.

Laser Gas Nitriding of Ti-6Al-4V Part 2: Characteristics of Nitrided Layers

The characteristics of laser nitrided layers formed on Ti-6Al-4V are presented in this investigation. It has been determined that titanium nitride (TiN) is formed, which significantly increases the hardness of the nitrided surfaces. The amount of titanium nitride produced depends on the processing parameters such as laser pulse energy and nitrogen concentration. Nitrided layers are much smoother along the laser pass direction than perpendicular to this direction. The shrinkage effect in the laser melt zone produces surface residual tensile stresses in Ti-6Al-4V samples regardless of whether the processing environment is Ar, N2, or a mixture of these gases. Pre-heating or stress relieving after laser nitriding significantly reduces the residual tensile stress level.

The brittle-ductile transition in HIP consolidated near γ-TiAl + W and TiAl + Cr powder alloys

Abstract The properties, deformation microstructures and fracture characteristics resulting from tensile tests between 20 °C and 850 °C are compared for Ti-48Al-2W and Ti-47.5Al-3Cr. TiAl + Cr exhibits a brittle-ductile transition with an elongation at 20 °C of 2.2% increasing to 36% at 850 °C. A much less pronounced brittle-ductile transition exists for TiAl + W, with an increase in elongation from 1.3% at 20 °C to only 4.5% at 850 °C. In both alloys fracture occurs predominantly by transgranular cleavage at low temperatures, changing to intergranular and then prior particle boundary failure with increasing temperature. High densities of deformation twins and 1 2 〈110] dislocations form in TiAl + Cr deformed at ⩾ 700 °C. For TiAl + W much less deformation twinning occurs, even during tensile deformation at 850 °C. The inability of TiAl + W to accommodate strain by deformation twinning eliminates the brittle-ductile transition that occurs for most near γ-TiAl compositions. TiAl + Cr attains higher fracture strength at 20 °C (563 MPa) than TiAl + W (505 MPa). This improved strength is related to the substructure developed during HIP consolidation. The increased 20 °C ductility of TiAl + Cr is associated with a more homogeneous as-HIPed microstructure.

Microstructural Changes during Isothermal Forging of a Co-Cr-Mo Alloy

Interest has evolved recently in thermomechanical processing of the cast Co-Cr-Mo surgical implant alloys such as Vitallium and Vinertia. Work has shown that the wrought forms of these alloys exhibit much improved properties over their as-cast counterparts. In this paper, the response of as-cast Vinertia to isothermal forging is examined by means of isothermal and isostrain-rate compression testing. The effects of temperature, strain rate, and strain on the breakdown of the as-cast micro-structure are examined in detail. The effects of prior heat treatment on plastic flow and microstructure achieved are also considered. It is shown that the interaction between the carbide phase and the recrystallization induced during hot working governs the degree of homogeneity that can be achieved in the forged product. Control of carbide volume fraction, size, and distribution by appropriate prior processing can lead to a fine grain equiaxed structure with uniformly distributed carbides. The potential offered by isothermal forging for control of the microstructure in this type of alloy is discussed, as well as the limits imposed on the process by the starting material and by the strain gradients expected during the forging of implants.

The isothermal compression response of a near γ-TiAl + W intermetallic

Abstract Hot isostatically pressed Ti-48%Al-2%W powder compacts were isothermally compressed between 900 °C and 1200 °C, and at constant strain rate in the range 10 −1 to 10 −4 s −1 . The deformation processes occurring are determined by correlation of the stress-strain response, strain rate sensitivity exponent m and activation energy, with the deformed microstructures. The resulting “deformation map” indicates that at low temperatures and high strain rates, mechanical twinning and dislocation generation cause a high flow stress, low m and high activation energy. Increasing compression temperature causes dynamic recrystallization, flow softening and increased m . Concurrent dynamic recrystallization and superplasticity can occur, with m as high as 0.44. At high temperatures and strain rates −3 s −1 , superplastic deformation is accompanied by grain coarsening. A “forging map” indicates that cracking after 120% engineering strain can be avoided if appropriate compression parameters are applied.