Qian Bai

Experimental investigation of forming limit curves and deformation features in warm forming of an aluminium alloy

The determination of forming limit curves and deformation features of AA5754 aluminium alloy are studied in this article. The robust and repeatable experiments were conducted at a warm forming temperature range of 200 °C–300 °C and at a forming speed range of 20–300 mm/s. The forming limit curves of AA5754 at elevated temperatures with different high forming speeds have been obtained. The effects of forming speed and temperature on limiting dome height, thickness variation and fracture location are discussed. The results show that higher temperatures and lower forming speeds are beneficial to increasing forming limits of AA5754; however, lower temperatures and higher forming speeds contribute to enhancing the thickness uniformity of formed specimens. The decreasing forming speed and increasing temperature result in the locations of fracture to move away from the apexes of formed specimens. It is found that the analysis of deformation features can provide a guidance to understand warm forming process of aluminium alloys.

Viscoplastic Damage Constitutive Modelling of High-Speed Railway Axle Steel 25CrMo4

Ductile damage inadvertently exists in the steel during hot tension. The ductile damage during hot forming directly influences the mechanical properties of 25CrMo4 steel for high-speed railway axle. To investigate the grain growth/refinement rule and damage features of 25CrMo4 in hot forming, grain growth test and grain refinement test were conducted using the thermal mechanical simulator Gleeble-1500. In the grain growth test, the specimens were compressed to ensure that the initial austenitic grain size was small enough, then held at the deformation temperatures (1223K, 1273K, 1323K and 1373K) for 0min, 10min, 20min and 30min, respectively, to study the grain growth rule. In the grain refinement test, the specimens were stretched to different strain level at three temperatures (1313K, 1373K and 1433K) with two strain rates of 1.0/s and 10.0/s to study the grain refinement rule. The micro-voids and micro-cavities were found in tensile specimens during grain refinement test. Based on damage evolution mechanisms, damage constitutive equations are formulated to model the evolution of micro-voids and micro-cavities for 25CrMo4 under hot forming conditions. Partial experiment data were used to determine the material constants in damage constitutive equations by using the Genetic Algorithm (GA) method. To validate the model, the experimental data and computed curves of effective stress and grain size were compared. Close agreements were found between the experimental and prediction results. The developed viscoplastic damage equations are able to characterize the deformation behaviour of 25CrMo4 in hot tension process.

A Novel Forming Process for Powder Metallurgy of Superalloys

Powder metallurgy (PM) of nickel-based superalloys has been used for a wide range of products owing to their excellent special properties in processing and applications. Typical processes for high performance PM superalloys include hot isostatic pressing, hot extrusion and hot isothermal forging. Hot isostatic pressing is normally conducted at a high temperature, by using a low pressure for a long time in a closed vessel, resulting in high cost and low product efficiency. In this paper a novel forming process, i.e. direct powder forging for powder metallurgy of superalloys has been proposed. In this process, the encapsulated and vacuumed powder is heated up to the forming temperature and forged directly to the final shape, by using a high forming load for a very short time. Direct powder forging is a low-cost and energy-saving process compared to conventional PM processes, and in addition, press machines of conventional forging can be used for direct powder forming process. In direct powder forging it is important to control the relative density of the deformed part since the existence of voids could reduce the mechanical strength and fatigue life. In this paper, feasibility tests of direct powder forging are presented. Microstructure, relative density and hardness of the formed specimen were studied.

An Investigation of Direct Powder Forging of Nickel Superalloy FGH96

Powder metallurgy (PM) nickel-based superalloy has been widely used in high temperature applications and is most commonly manufactured using hot isostatic pressing (HIP). However, HIP is an expensive process and takes a long time at high temperature which leads to the formation of networks of prior particle boundaries (PPBs). In this study, a recently developed processing method - direct powder forging (DPF) was employed to produce a PM nickel-based superalloy component, using a single acting hydraulic press under normal atmosphere. EBSD study has been conducted for its microstructure, grains size distribution, and grain boundary misorientation; and mechanical testing has been carried out for its hardness and tensile properties at room temperature and 650°C. It has been found that the DPFed material has reached full density in the whole component. Networks of PPBs have been broken in the direct powder forged FGH96 alloy. Compared with HIPed FGH96 alloy, the DPFed material has a substantially higher recystallisation nucleation degree and more recrystallised sub-grains. After heat treatment, the tensile properties of the direct powder forged FGH96 alloy match or surpass those of the material produced by HIP plus isothermal forging.

A Novel Experimental Design to Obtain Forming Limit Diagram of Aluminium Alloys for Solution Heat Treatment, Forming and In-Die Quenching Process

Solution heat treatment, forming and in-die quenching (HFQ) is a patented process to form complex shape metal components at a high efficiency and a low cost. Conventional experiment approaches to determine forming limit curves (FLCs) at different strain paths are not applicable for the HFQ forming process. A novel biaxial tensile test rig is designed to overcome the difficulties and determine the FLCs at high temperatures based on the commercial Gleeble machine. This test device employs the circle plate and connecting rod mechanism in order to achieve different strain states, such as uniaxial tension, plane strain and biaxial tension. Resistance heating and air cooling are adopted to obtain an isothermal environment and to control cooling rates in Gleeble respectively. The designs of the cruciform specimen for this test are also introduced in this paper.

The Effect of Process and Model Parameters in Temperature Prediction for Hot Stamping of Boron Steel

Finite element models of the hot stamping and cold die quenching process for boron steel sheet were developed using either rigid or elastic tools. The effect of tool elasticity and process parameters on workpiece temperature was investigated. Heat transfer coefficient between blank and tools was modelled as a function of gap and contact pressure. Temperature distribution and thermal history in the blank were predicted, and thickness distribution of the blank was obtained. Tests were carried out and the test results are used for the validation of numerical predictions. The effect of holding load and the size of cooling ducts on temperature distribution during the forming and the cool die quenching process was also studied by using two models. The results show that higher accuracy predictions of blank thickness and temperature distribution during deformation were obtained using the elastic tool model. However, temperature results obtained using the rigid tool model were close to those using the elastic tool model for a range of holding load.