The Determination of Optimum Forging Conditions for the Production of High Strength-High Impact Toughness Automotive Parts

The influences of various reheating and forging temperatures as well as cooling rates on the microstructure and mechanical properties, particularly impact energy, during the forging of a Nb-V microalloyed steel to be used for automotive safety parts were investigated. Increasing the prior austenite grain size increased the volume percent of acicular ferrite and reduced pearlite content in the microstructure even for very low post-forging cooling rates, resulting in improved toughness and tensile strength values. Increasing the cooling rate enhanced the acicular ferrite content, thereby increasing the impact energy properties. At lower reheating temperatures the yield strength and impact energy levels are determined by the percentage of pearlite present in the microstructure; while as the cooling rate is increased the amount of acicular ferrite and retained austenite are increased, improving the toughness and tensile strength of the forged part. This effect is more pronounced for the parts solutionized at 1250°C and is related to the presence of very fine carbonitride precipitates under these conditions, which contributes to improved yield strength, particularly at higher cooling rates. An optimized forging process was determined and adapted to a 25 MN production forging press to validate the experimental results on semi-industrial production scale. By adequate control of the above parameters, high-strength, high-toughness parts (T.S. = 800 MPa, CVN = 35 J) were forged and optimum mechanical properties were achieved without the need for any additional heat treatment.     

Post-Weld Heat Treatment of Additively Manufactured Inconel 718 Welded to Forged Ni-Based Superalloy AD730 by Linear Friction Welding

A new post-weld heat treatment (PWHT) cycle was designed for novel dissimilar linear friction welding (LFW) of selective laser melted (SLM) Inconel 718 (IN718) to AD730 forged nickel-based superalloy. The microstructure and hardness of the joints after the PWHT are investigated and compared with those of as-linear friction welded samples. The precipitation of γ′ + γ″ is determined as the main mechanism to increase the mechanical properties of SLM IN718 alloy. These particles coarsened during heat treatment at 1253 K and double aging. The results show that the thermomechanical history of linear friction welded joints can affect the microstructure of IN718 alloy such as the morphology of δ phase after solution treatment (ST) from the platelike in the weld zone (WZ) to the needlelike in the base material (BM). It was found that in AD730, nanometric size γ′ particles reprecipitated close to the weld line during rapid cooling after welding. The presence of ultrafine γ′ particles and coarsening of the remaining particles in the microstructure of the alloy, during PWHT, can enhance the strength and hardness. The developed PWHT resulted in uniform hardness across the new dissimilar joint.

     

An investigation to the effect of deformation-heat treatment cycle on the eutectic morphology and mechanical properties of a Thixocast A356 alloy

The influences of deformation, heat treatment temperature and holding time on morphology and size distribution of Si containing eutectic phases of Thixocast Al-7Si-0.4 Mg alloy were investigated. The novel thermo-mechanical treatment consisted of initial cold working practice followed by a solution heat treatment at 540 °C for durations ranging from 2 min to 120 min followed by water quenching. Optical and scanning electron microscopes were used to study the influence of process parameters on microstructure evolution. Also, final mechanical properties were investigated using hardness test. The results indicate that, under appropriate conditions it is possible to achieve an ultrafine grain microstructure with the eutectic Si fibers fragmented and spheroidized in the entire microstructure. It was also found that, an agglomeration of sphrodized particles occurs and is governed by Ostwald ripening mechanism.     

Optimization of Processing Parameters During Laser Cladding of ZE41A-T5 Magnesium Alloy Castings Using Taguchi Method

A continuous wave 4 kW Nd:YAG laser welding system was employed to clad single beads on machined 6.2-mm thick ZE41A-T5 aerospace magnesium alloy sand castings using nominal 1.6-mm filler rods of the parent metal. Based on the quality criterion of minimum dilution ratio, the Taguchi experimental method was used to optimize different process parameters and to identify the dominating factors. It was found that, for the process window investigated in this work, the optimal levels were 5.0 m/min for wire feed rate, 2.25 kW for laser power, and 2.5 m/min for cladding speed, with a minimum dilution ratio of 0.155. With 95% confidence, the wire feed rate is the dominating factor with the most significant influence on the dilution ratio. The dilution ratio usually decreases with increased wire feed rate while it increases with increased laser power. By contrast, the cladding speed appears to have insignificant influence on the dilution ratio over the cladding speed range used in this study.     

On the Occurrence of Liquation During Linear Friction Welding of Ni-Based Superalloys

A combination of experimental and analytical methods was used to study the possible occurrence of liquation during LFW of the newly developed AD730^TM Ni-based superalloy. LFWed joints were produced using a semi-industrial size facility and the interfaces of the joints as well as the ejected flash were examined using optical and Field Emission Gun Scanning Electron Microscopy (FEG-SEM). Physical simulation of the LFW thermal cycle, using thermomechanical simulator Gleeble™ 3800, showed that incipient melting started from 1473 K (1200 °C). The analytical model, calibrated by experiments, predicted that the highest temperature of the interface was about 1523 K (1250 °C). The constitutive equations based on lattice and pipe diffusion models were developed to quantify the self-diffusivity of the elements and control the extent of liquation by considering the effect of LFW process parameters. Analytical results show that the application of compressive stresses during LFW results in 25 times increase in the diffusion of Ni atoms at the weld interface. Therefore, no presence of re-solidified phases, i.e., occurrence of liquation, was observed in the microstructure of the weld zone or the flash in the present study. Based on the obtained results, a methodology was developed for designing the optimum pressure above which no liquation, and hence cracking, will be observable.

     

Thermomechanical Characterization of Mg-9Al-1Zn Alloy Using Power Dissipation Maps

A processing map is developed on the basis of the Dynamic Material Model for Mg-9Al-1Zn. The model considers the work piece as a dissipator of power and power loss variation with temperature and strain rate constitutes the power dissipation map. To this end, the thermomechanical (i.e., hot compression) characteristics of a Mg-9Al-1Zn alloy was studied in the temperature range of 250-425 °C and strain rates of 0.001-1 s^?1. The strain rate sensitivity (m), power dissipation efficiency (η), and instability parameter (ξ) are computed based on the experimental hot compression data. The deformation mechanisms of different regions in the maps are analyzed and corresponding microstructures are investigated. The processing map of Mg-9Al-1Zn alloy exhibits five workability domains. Dynamic recrystallization (DRX) was observed in three of the domains, while in the two other domains grain boundary sliding, twining, and precipitation are the dominant mechanisms. The optimum hot working conditions of Mg-9Al-1Zn alloy are located in the two domains where DRX takes place. They correspond to 375 °C/0.001 s^?1 and 380 °C/1 s^?1 with peak efficiency of 42 and 36%, respectively.     

A New Approach in Optimizing the Induction Heating Process Using Flux Concentrators: Application to 4340 Steel Spur Gear

The beneficial effects of using flux concentrators during induction heat treatment process of spur gears made of 4340 high strength steel is demonstrated using 3D finite element model. The model is developed by coupling electromagnetic field and heat transfer equations and simulated by using Comsol software. Based on an adequate formulation and taking into account material properties and process parameters, the model allows calculating temperature distribution in the gear tooth. A new approach is proposed to reduce the electromagnetic edge effect in the gear teeth which allows achieving optimum hardness profile after induction heat treatment. In the proposed method, the principal gear is positioned in sandwich between two other gears having the same geometry that act as flux concentrators. The gap between the gear and the flux concentrators was optimized by studying temperature variation between the tip and root regions of gear teeth. Using the proposed model, it was possible identifying processing conditions that allow for quasi-uniform final temperature profile in the medium and high frequency conditions during induction hardening of spur gears.     

The influence of thermomechanical treatment on the microstructure and mechanical properties of aisi 4130 steel

Ultra high strength AISI 4130 Steel samples were submitted to various rolling schedules in order to find the rolling conditions leading to the optimum combination of strength and toughness. Reheating temperatures between 900 to 1230°C and reductions between 15 to 90% were employed. Rolling temperatures above Ar3 and also between 620 and 700°C were used and in some experiments annealing was applied to the samples. The influence of the above parameters on the microstructure and mechanical properties of the samples was studied by measuring the variation of their hardness, tensile and impact properties as well as using optical and scanning electron microscopy. The results show that by a judicious control of reheat temperature, percent deformation, interpass time, finish rolling and annealing temperatures it is possible to obtain fine ferrite and nodular colonies of pearlite with small interlammelar spacing. The results are discussed in the frame work of classical nucleation theory and the possible mechanisms which may be responsible for the observed behavior are proposed.     

Investigation of α platelet boundaries in a near-α titanium alloy

Transmission electron microscopy (TEM) of a bimodal near-α titanium alloy revealed the existence of retained β phase layers and silicide precipitates at the α platelet boundaries inside transformed β grains. The β to α phase transformation accompanied by the precipitation of silicide resulted in the formation of a large number of dislocations at α platelet boundaries. Orientation relationships between silicide, β phase and α phase were also identified. However high-resolution TEM (HRTEM) revealed crystal mismatches between these phases generating high strains at α platelet boundaries. The strengthening effects of the platelet boundaries are discussed in terms of dislocations slip across the boundaries. The mechanism that governs the β to α phase transformation is also discussed.