Papers by Keyword: Precipitation Hardening

Abstract: One of the main functions of semisolid metal alloy forming processes, notably rheocasting and thixoforming, is the manufacture of parts, casings and frames for mechanical assemblies and systems. These parts not only must have the required minimum mechanical properties in terms of yield strength and elongation, but also must be able to withstand cyclic tensile and compressive forces. However, there is little fatigue strength data for the materials used for these parts. The present work seeks to fill this gap by determining the fatigue limit at 10⁷ cycles of Al-6.0wt%Si-2.5wt%Cu alloy, or simply Al6Si2.5Cu, thixoformed in a pneumatic press at 585 °C (the temperature corresponding to 40 % solid fraction) with isothermal treatment times of 30 and 60 seconds. The parts were also subjected to T6 heat treatment, for which they were solution heat treated at a temperature of 520 °C for 4 hours followed by aging at 180 °C for 10 hours. For all the conditions tested, the microstructures were characterized to determine the grain size, appearance and shape of the silicon particles, in addition to the residual porosity. For the best conditions observed, 30 s holding time and T6 heat treatment, the grain size varied between 100 µm and 130 µm; the shape factor was around 0.60, indicating an excellent degree of roundness; and there was low residual porosity of around 0.3 %, resulting in a yield strength of up to 240 MPa with 4.5 % elongation. The average fatigue strength was estimated by the staircase method and was between 95 MPa and 98 MPa for 10⁷ cycles.

Abstract: Warm deep-drawing of pre-aged (under-aged) blanks of 7xxx series aluminum alloys (Al-Zn-Mg) at moderate temperatures of roughly 120–230°C is a promising route for producing parts with considerable geometrical complexity, good paint bake hardening response, and, thus, excellent final mechanical properties. Furthermore, oil-based lubricants can be used, eliminating the need for elaborate cleaning routines. However, finite element (FE) simulation of the process is challenging: time-temperature regimes during coupon testing for material cards should closely follow the real conditions in the press because the material undergoes significant changes at warm-forming temperatures, such as recovery and precipitation/coarsening/reversion of hardening phases. When convective heating is used for Nakajima or tensile testing, heating rates are usually too low to adequately represent real process conditions (where inductive or contact heating may be used). Here we present a method for establishing FE material cards and calibrating the GISSMO damage model using miniaturized tensile specimens for a dilatometer with inductive heating. The simulations are compared with warm deep-drawing experiments of pre-aged 7xxx and good agreement of minimum draw temperature for two alloys is achieved. The findings are discussed with regards to transmission electron microscopy investigations and final mechanical properties published earlier. It was found that warm-forming is suitable to produce complex 7xxx parts with high final strength. Conditions in the press can be represented by using miniaturized tensile specimens and inductive heating for calibration of material cards/damage models.

Abstract: This paper deals with three types of triplex steel, where containing 25 to 28 wt.% manganese, 0.8 to 0.89 wt.% Carbon, 9.9 to 11.11 wt.% Aluminum, and with different Nickel content. Two types contain Ni in range of 0.9 to 2 wt.% and third type doesn’t contain Ni. The precipitation of Nano-size kappa carbides is the most proper technique used for this objective. It is expected that inter-metallic strengthening mechanism should act more effective in promoting the strength of Triplex steel with ductility. From this point of view, this research was designed to study the effect of inter-metallic inductive alloying element as Nickel on promoting of the strength and ductility of the high aluminum containing high manganese steel. Optical microscope, scanning electron microscope (SEM) and X-ray diffraction (XRD) were used to detect of inter-metallic precipitates through steel investigated ranged in Nickel from 0 to 2 wt.%. Mechanical and strain hardening properties were determined in the steel investigated after different regimes of heat treatment. It was found that Ni₃Al inter-metallic compound provides the austenite matrix with good strength and ductility, depending on the ageing time. Further deterioration was obviously observed in the steel investigated as increasing the ageing time, attributing to coarse structure occurrence.

Abstract: Maraging steels are interesting for research after heat treatment, from which name is derived "maraging" – martensite-aging. After solution annealing and precipitation hardening the X3NiMoCoTi 18-9-5 alloy has excellent mechanical properties (tensile strength reaches up to 2000 MPa and hardness is 50-55 HRC), it is ductile and well weldable. The advantage of these materials is the possibility to be manufactured not only by conventional methods but also by modern additive manufacturing (AM) methods. One of which is selective laser melting (SLM). In this paper, the influence of heat treatment on the final microstructure and mechanical properties of the 3D-printed X3NiMoCoTi 18-9-5 maraging steel is investigated.

Abstract: The solitary distinguished thermal treatment to aluminium alloy matrix of nonferrous group with uniformly dispersed reinforcement particles is precipitation or age hardening with or without mechanical working. Al7075 ternary alloy composites belongs to Al-Zn-Mg group and bulk properties can be enhanced by the formation of the immensely small consistently spread out precipitates of solute rich second phase within the matrix phase. The treatment comprises solutionising (550°C) supported by controlled aging (100 and 180°C) below the solvus temperature of the given alloy. Aging expedites the diffusion of solute atoms to form secondary particles (intermetallics) from the room temperature super saturated solid solution. This process is supported by deformation known as rolling as post activity, develops strain hardening in matrix and reinforcement. The present work shows the distinction in the aging phenomenon on alloy and alloy matrix composites (reinforced with 3 and 6 wt% of eutectoid steel powder, 10-30 micron size) upon undergoing cold rolling prior to aging i.e., after solution treatment. The variation in the hardness distribution with aging kinetics for both aging temperatures with and without cold deformation (10 and 20%) in between the consecutive stages like, solution treatment and aging were investigated and peak hardness values were noted in each aging temperatures and results were analysed. In every hardness test 10 trials were performed and the average of 6 consistent readings are taken as the outcome. 20 to 40% improvement in peak hardness is observed with intentional deformation over without deformation. The composite may be used for light duty cold working dies where bulk hardness and frictional characteristics are very important.

Abstract: In this work, to clarify the effect of carbide precipitation state on strength and toughness, Ti,V alloyed precipitation hardened ferrite single phase steel sheets with different carbide size were investigated. In order to change the precipitated particle size, cooling conditions after hot rolling were changed. Under condition A, steel sheets were cooled to 873K by water spray (for fine precipitation). Under condition B, steel sheets were air-cooled from 1053K for 20sec, then cooled by water spray to 873K (for coarse precipitation). The experimental results were following. The balance of tensile strength and Charpy absorbed energy was better in condition B. (Ti,V)C were observed in both conditions, but the size of (Ti,V)C were larger in condition B. From the above, it was suggested that as the carbide size become larger, the decrease in toughness per strengthening amount becomes smaller.

Abstract: A textured Mg-Al-Zn alloy rolled plate was solution treated and aged at 320 oC for 2 h and 116 h, respectively. Afterwards, the three conditions were compressed at room temperature along the transverse direction to activate {110} twinning. Ageing treatments were observed to strengthen the alloy in relation to the solution-treated condition. This has been mainly attributed to the restricted lateral growth of twins in the presence of particles and thus to the additional stress required for twin growth. Accordingly, a slightly reduced twin volume fraction, but an increased number of smaller twins was observed after compression in the aged samples.

Abstract: In recent years, steels microalloyed with high Ti levels have received increasing attention due to the interesting combination of high strength and formability because of the dispersion of nanometric sized titanium carbides that can be formed within the matrix. However, one of the problems related to these compositions is that their performance can be highly sensitive to variations in the processing route. To study this, in this work coiling simulations were performed by dilatometry tests with a reference Nb microalloyed steel (0.03%Nb) and a high-Ti steel (0.03%Nb-0.1%Ti) using temperatures from 550 to 675oC. The mechanical behaviour of the samples was characterized using Vickers hardness. A large hardness increase was observed in the high-Ti steel samples, resulting in estimated yield strength increases between 69 and 214 MPa. This improvement in the mechanical behaviour was very dependent on the coiling temperature; the maximum hardness was observed at 625oC-650oC, while this decreased drastically for temperatures from 550oC to 600oC and at 675oC. EBSD and TEM analysis has been performed to study the contribution of microstructural constituents and precipitation to the observed mechanical behaviour.

Abstract: Fe-20Al-5Ti (at.%) single crystals composed of the bcc Fe-Al matrix and the Fe₂AlTi precipitates with the L2₁ structure was examined. In the single crystals furnace-cooled (FC) from 1373 K to room temperature, coarse Fe₂AlTi phase about 300 nm in diameter were precipitated in the bcc matrix. A misfit strain and a dissolution temperature of the L2₁ precipitates are +0.59% and 1151 K, respectively. The single crystals exhibited high yield stress above 600 MPa up to 973 K while further increase in temperature resulted in a decrease in yield stress due to the dissolution of the precipitates. In the FC crystals, 1/2<111> dislocations in the bcc matrix bypassed the coarse L2₁ precipitates due to their large misfit strain, resulting in high strength. In contrast, the fine L2₁ precipitates about 30 nm in diameter were observed in the crystals after solutionization and annealing at 873 K. The crystals with the fine L2₁ precipitates demonstrated high yield stress above 1100 MPa at and below 773 K. Uncoupled or paired 1/2<111> dislocations cut the fine L2₁ precipitates, leaving an anti-phase boundary (APB) inside the precipitates. The APB inside the precipitates was considered to be responsible for strong precipitation hardening.

Abstract: Al 7xxx alloy is a heat treatable Al alloy with superior strength. Solution treatment in precipitation hardening sequence of the alloy has an important role to dissolve second phases and bring vacancies out to form precipitates in the ageing process. Another strengthening can be done by Ti addition as grain refiner. As cast alloy by squeeze casting was homogenized at 400 °C for 4 h. Solution treatment was conducted at 220, 420, and 490 °C, followed by rapid quenching. Subsequent ageing was conducted at 130 °C for 48 h. Characterization was performed by optical microscope, SEM-EDS (Scanning Electron Microscopy – Energy Dispersive Spectroscopy), Rockwell hardness testing, XRD (X-Ray Diffraction), and STA (Simultaneous Thermal Analysis). Ti added alloy showed rounder grains, lower hardness, and more reduction in second phase volume fraction along with increasing solution treatment temperature than those in alloys without Ti addition. Otherwise, the alloy final hardness was increasing and higher after the ageing process due to higher second phase dissolution which leads to more precipitates formed.