压铸过程中模具表面温度变化研究

Surface temperature of tools during the high-pressure die casting of aluminium

高压铝压铸成型过程中模具表面温度的变化

abstract: The objective of this work was to determine the temperature experienced within a pressure die-casting tool during alunimium part production. It was important to determine the temperature profile of the production process so that an accurate thermal cycle could later be simulated. The research overcame several challenges of this aggressive environment to show that the surface temperature of a die could be obtained from an H13 steel tool running on an aluminium pressure die-casting machine. The results show that the surface of a typical aluminium pressure die-casting tool heats to 400-450C within approximately 1s and cools to 150-200C within approximately 20s。

摘要：此次工作的目标是为了测定在生产铝件产品时压铸模具内部温度的变化情况。重点是确定产品在生产过程中模具的温度变化曲线，并且描绘出精确的热量周期曲线。研究需要克服恶劣工作环境的挑战，从而得出在铝压铸机中应用H13钢材制作的模具浇注系统的情况下，压铸模具表面温度的变化情况。研究结果显示出普通的铝压铸模具温度达到400~450度时大约需要1s，而冷却到150~200度时大约需要20s的时间。

Keywords: surface temperature, tools, high-pressure die casting, aluminium

关键字：表面温度模具高压压铸铝

1 Introduction

In cold-chamber die casting, the molten material is forced into the die via a hydraulic plunger-piston in three controlled phases producing high-quality castings. The process can be used with zinc-, magnesium-,aluminium-,and copper-based alloys.

1、简介

对于冷室压铸成型，熔融的材料是通过冲头作用挤压进入到模具中，通过对三个阶段的控制能够生产出高质量的制件。这种成型方法能够使用的材料为以锌、镁、铝、铜为基体的合金。

Phase1 is termed take up and slowly pushes the aluminium towards the die with minimum turbulence.

阶段1是压铸开始并且缓慢将铝液推动进入模具，从而得到最小的铝液波动。

Phase2 is the injection phase (filling of the die cavity). The cold-chamber pressure die-casting process typically casts aluminium alloys which are injected at 700-750C depending on the die geometry. This phase has to be fast enough to prevent chilling while the alloy is filling the die. The speed of this phase is approximately 10 m/s and typically takes 0.05-0.1s; however, speeds

can be as high as 100m/s. During this phase, any gases are expelled via machined vents in the die and through the parting line.

阶段2是注射阶段（充填模具型腔）。冷室压铸成型普遍使用铝合金，其能够在温度为700~750度时充填入压铸模的型腔中，形成几何形状。这一过程必须足够快速，并且要防止合金充填模具时发生急剧的冷却。这一阶段的速度大约是10m/s，并且普遍的充填时间为0.05~0.1s；甚至速度可以提高到100m/s。在此阶段，型腔中的气体都要经由模具中的机械装臵或者是通过分型面排出型腔。

phase 3 is the compaction phase, as the alloy solidifies in the cavity it begins to shrink away from the surface of the die. The force applied to the alloy (50-70 N/mm2) reduces this effect and reduces the size of inclusions and porosity caused by air, trapped during injection.

The die is usually water cooled and the surface sprayed with water-based die lubricant, causing thermal shock.

阶段3是压实阶段。这一阶段中型腔中合金开始凝固，并且从模具型腔表面位臵处开始收缩。当对型腔中的铝液施加一定的压力时（50~70N/mm2），能够减小体积收缩，能够减小内部部件的尺寸变化，并且能够消除由于气体和收缩造成的内部孔洞。

模具通常采用水冷，并且表面采用水基润滑剂进行喷雾，这都造成模具的热量波动。

The most important properties required of materials for die-casting tools are resistance to thermal fatigue and resistance to softening at elevated temperatures. Resistance to softening is required to withstand the erosive action of molten metal under high injection pressures and speeds. The performance of die-casting dies is related to the casting temperature of the work metal, the thermal gradients within the dies, and the frequency of exposure to a high temperature. During the high-pressure die casting of alumunium the die has to withstand severe operating conditions such as high pressure and rapid temperature fluctuations and, over time, tool failure occurs [2,3]. In actual die casting, the dominant tool failure mechanism is thermal fatigue cracking [4]. Initially molten metal contacts the die and causes the surface temperature to increase above that of the interior of the die [5]. The die face starts to expand; however, the cooler underlying layer resists this expansion creating a temporary compressive stress layer [6,7]. When the casting is removed, the die surface starts to cool and, as it does, the surface shrinks or contracts. The surface cools more quickly than the interior of the die; this places the subsurface of the die into residual tensile stress, which is made worse by the application of die lubricant [8]. During further cycling, the die surface is subjected to alternating compressive and tensile stresses that result in plastic deformation [9]. Continued cycling reduces the yield strength of the tool, causing increased residual tensile stresses to develop and cracks to initiate. This type of cracking is more prevalent in aluminium and brass die casting because of the higher temperatures and resulting thermal shock by the molten metal.