The presentcommunication concerns a detailed analysis of metal flowinto a porthole extrusion die to produce a thin-walled square magnesium tube by means of three-dimensional FE simulation in both the transient state and steady state. The research was aimed to get an insight into the longitudinal weld seam formation during extrusion through porthole dies and to evaluate the factors determining the quality of theweld seams. FE simulation revealed distinctive stages at the beginning of an extrusion cycle, corresponding to the changes in extrusion pressure during the process. It showed that the commonly observed defect at the extrudate head was due to entrapped air under the bridges in the upper part of the welding chamber. The dead metal zones existed at the corners between the container and die face and between the bottom and sidewall of the welding chamber. Because of the friction at the die bearing, the metal flow through the die bearing resembled laminar flow. Only the virgin metal from the interior of the billet flowed along the bridges and formed the welding seams. As ram speed increased, the mean stresses and temperatures on the welding plane in the welding chamber increased, which was reflected in the increases in extrusion pressure and extrudate temperature, being beneficial to the solid-state bonding at the weld seams. Tensile tests confirmed that extrusion at a higher ram speed led to enhanced transverse tensile strength and strain of the extruded square tube, as a result of improved bonding at the longitudinal weld seams.

3D computer simulations of extruding a wrought magnesium alloy AZ31 into a rectangular section at various ram speeds were performed and the results verified in extrusion experiments under identical conditions. It has been found that during upsetting and transient-state extrusion, the temperature in the billet is redistributed all over; it increases in the deformation zone close the die orifice and decreases in the rear part of the billet. The maximum temperature is located at the centre of the outer surface of the extruded section where more heat is generated and less heat lost. The minimum temperature occurs at the edge of the billet in contact with the container and the die face where the dead metal zone is situated. During upsetting and transient-state extrusion, the maximum temperature increases significantly, while during steady-state extrusion it only changes slightly. The amplitude of the extrudate temperature increase from the initial billet temperature during steady-state extrusion at a given ram displacement decreases as ram speed increases. Thus, the higher ram speed, the smaller the effect of ram speed on temperature increase. The increase of the extrudate temperature varies linearly with logarithmic ram speed.With this relationship established, the extrudate temperature at different ram speeds can conveniently be predicted.

At present, a fundamental knowledge of the thermal and mechanical interactions occurring during the extrusion of magnesium is lacking. This acts as a serious technological barrier to the cost-effective manufacturing of lightweight magnesium alloy profiles. In the present research, a three-dimensional finite element (FE) simulation of extrusion to produce a magnesium alloy profile with a cross shape was carried out as an efficient means to gain this understanding. It revealed the redistribution of temperatures in the billet throughout the process from the transient state to the steady state, the formation of the deformation zone and dead metal zone, and varying fields of effective stress, effective strain, effective strain rate, and temperature close to the die orifice. The predicted extrudate temperature and extrusion pressure were compared with experimental measurements. The key to controlling the extrudate temperature and extrusion process was found to lie in the capabilities of predicting the temperature evolution during transient extrusion, as affected by extrusion conditions. The relationship between ram speed and the extrudate temperature increase from the initial billet temperature was established and experimentally validated.

In tube hydroforming, the pressure for forming a small corner is far greater than other positions and thinning or cracking often occurs at the transition region between the corner and a straight side. To reveal the reasons of the local thinning and provide the clues to avoid the defect at the transition region, the stress distribution and deformation pattern of the corner are analyzed by mechanical analysis and numerical simulation. The effect of the friction coefficient on stress distribution is also considered. The material at the transition region much more easily satisfies plastic yielding conditions and produces compressive strain through the thickness. Therefore, severe thinning deformation easily occurs at the transition region. When the ratio of wall thickness to the corner radius is greater than 0.5, the difference of yielding conditions between the straight side and the corner is significant enough to induce serious deformation nonuniformity, even excessive thinning.

A two-dimensional finite element model of a typical tailor-welded tube (TWT) with two different thicknesses is built up, in which the weld is included into the model as the third segment of the tube blank. Hydroforming processes in a die with an equal inner diameter and in a die with two different inner diameters are simulated. The bulging and thinning behavior of the TWT and the effect of the thickness difference and the weld on the forming process are discussed. The weld displacement and possible initial cracking position are predicted. In a die with an equal diameter, the position of the weld plays an important role on the forming process. In the die with two diameters, the weld should be kept in the region with the smaller diameter, to avoid cracking in the weld or heat affected zone.With a tailor-welded tube, the weight of a tubular component can be reduced.

介绍了轻合金热介质内压成形基本原理，研究了温度对镁合金和铝合金管材热态内压成形性能的影响，进行了镁合金变截面件和铝合金变径管成形试验。所建立的热介质内高压成形装置最高加热温度315℃，压力100MPa，可实现高压热介质的传输和密封、成形件温度和加载曲线控制等。对镁合金和铝合金管材在不同温度下成形性能的测试分析表明，轻合金热成形需综合考虑总延伸率、均匀延伸率和n值等参数随温度的变化，选取合适的成形温度。

研究了两端非对称管件内高压成形改善壁厚均匀性的方法，采用数值模拟和试验研究对管件内高压成形进行了非对称加载路径和预成形的优化，获得了工件变形过程及厚度分布变化，并分析了加载路径对壁厚均匀性的影响。研究表明：采用优化的加载路径和预成形，补料和内压的良好匹配改善了两端非对称管件轴向补料的效果，形成有益的起皱，进而改善整形过程的应力应变状态，提高了壁厚均匀性和成形极限。