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首页> 外文期刊>International Journal of Fatigue >Effect of the casting process on microstructure and lifetime of the Al-piston-alloy AlSi12Cu4Ni3 under thermo-mechanical fatigue with superimposed high-cycle fatigue loading
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Effect of the casting process on microstructure and lifetime of the Al-piston-alloy AlSi12Cu4Ni3 under thermo-mechanical fatigue with superimposed high-cycle fatigue loading

机译：铸造工艺对叠加高循环疲劳载荷下热力学疲劳的铝活塞合金AlSi12Cu4Ni3组织和寿命的影响

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摘要

For the present research work, the well-known Al-piston-alloy AlSi12Cu4Ni3 was manufactured in three different processes (gravity die casting - GDC, low pressure die casting - LPDC, and high pressure die casting - HPDC) and T5 heat treated. The microstructure of the material from each process was analyzed, specimens were tested in OP-TMF loading with and without superimposed high-frequency fatigue and lifetimes were compared. The microstructure of GDC specimens shows a homogeneous distribution of primary Si and intermetal-lic phases. The LPDC material also shows a homogeneous microstructure over the whole sample. However, the Al-mixed-crystal formed bigger dendrite arms compared to the GDC material. The HPDC material shows a gradient in the microstructure getting finer from the center to the outer shape. In the very fine microstructure in the outer regions of the specimen no primary Si was formed and the Al-mixed-crystal built a globular-dendritic structure, surrounded by a eutectically solidified melt. Porosity was higher in the LPDC and HPDC compared to the GDC material. To simulate the thermally induced loading at the "hot side" of a piston during start-stop, strain controlled out-of-phase thermal-mechanical fatigue (TMF) tests with superimposed high-cycle fatigue (HCF) loading were performed. The TMF-cycles were carried out with a minimum temperature T_(min) = 200 ℃ and a maximum temperature T_(max) =440 ℃. The mechanical strain amplitude ε_(a,t)~(me) of the TMF cycles was kept equal to 50% of the thermal strain amplitude ε_a~(th) and the amplitude of the superimposed HCF cycles ε_(a,t)~(HCF) was varied between 0.03% and 0.05%. The lifetime of the specimens produced in the GDC-process represents the current state of the art. Compared to that, the specimens manufactured in LPDC and HPDC reached nearly the same TMF lifetimes as the GDC samples. The maximum stress of the LPDC specimens is approximately equal to that observed at the GDC material. However, during the very first TMF cycles, HPDC-specimens show higher maximum stress than the reference material. This is attributed to the very fine, nearly defect free microstructure at the outer shape of the HPDC material. Afterwards, the maximum stress of the HPDC samples is decreasing faster than that of the GDC material due to early formation of crack networks starting from fine pores in the HPDC microstructure. In TMF/HCF-testing the HPDC material shows the same effect. Furthermore, the HPDC samples show pronounced swelling during temperature cycling and TMF testing. The root cause was identified as the high internal pressure of air encased in the pores formed during HPDC.
机译：对于本研究工作，采用三种不同的工艺（重力压铸-GDC，低压压铸-LPDC和高压压铸-HPDC）和T5热处理制造了著名的Al-活塞合金AlSi12Cu4Ni3。分析了每个过程的材料的微观结构，在有和没有叠加高频疲劳的情况下，在OP-TMF载荷下测试了样品，并比较了使用寿命。 GDC试样的显微组织显示出原始的Si相和金属间相的均匀分布。 LPDC材料在整个样品上也显示出均匀的微观结构。但是，与GDC材料相比，Al混合晶体形成了更大的枝晶臂。 HPDC材料在微观结构中显示出从中心到外部形状逐渐变细的梯度。在试样外部区域的非常精细的微观结构中，没有形成原始的Si，Al混合晶体形成了球状的树枝状结构，被共晶凝固的熔体包围。与GDC材料相比，LPDC和HPDC中的孔隙率更高。为了模拟起停过程中活塞“热侧”的热载荷，进行了应变控制的异相热机械疲劳（TMF）试验和高周疲劳（HCF）载荷叠加。 TMF循环的最低温度T_（min）= 200℃，最高温度T_（max）= 440℃。 TMF周期的机械应变幅度ε_（a，t）〜（me）保持等于热应变幅度ε_a〜（th）和叠加的HCF周期ε_（a，t）〜（50％）的50％ HCF）在0.03％和0.05％之间变化。在GDC工艺中生产的样品的寿命代表了当前的技术水平。相比之下，在LPDC和HPDC中制造的样品达到了与GDC样品几乎相同的TMF寿命。 LPDC样品的最大应力大约等于在GDC材料上观察到的最大应力。但是，在最初的TMF循环中，HPDC样品显示出比参考材料更高的最大应力。这归因于HPDC材料外形上非常精细的，几乎没有缺陷的微观结构。之后，由于从HPDC微观结构中的细孔开始的裂纹网络的早期形成，HPDC样品的最大应力下降速度快于GDC材料。在TMF / HCF测试中，HPDC材料显示出相同的效果。此外，HPDC样品在温度循环和TMF测试期间显示出明显的溶胀。根本原因被确定为HPDC期间形成的孔中包裹的空气内部压力高。

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