Study on grain refinement of automobile aluminum alloy casting parts

ZHY casting takes a new type of alnbb refiner for casting aluminum alloy as the research object, and uses optical microscope, electron probe X-ray microanalyzer and other analysis equipment to study the effect of alnbb Refiner on the microstructure of cast aluminum alloy auto parts and the principle of grain refinement. The results show that the grain refinement effect of alnbb is more obvious than that of al5tib, and the grain size is 50% of that of al5tib, which makes the casting defects in the cast aluminum alloy significantly reduced and the microstructure more uniform, which lays a foundation for the lightweight of the cast aluminum alloy parts.

Aluminum alloy casting auto parts is one of the main means to lighten the chassis, power assembly and body of the car. At present, it is widely used in car wheels, steering knuckle, swing arm, engine bracket and so on. Cast aluminum alloy is based on binary Al Si phase diagram, which is widely used because of its low melting point, good fluidity, good surface treatment and limited solubility of gas (except hydrogen). Generally, the microstructure of aluminum alloy casting auto parts is relatively coarse, so the grain refinement of aluminum alloy is usually carried out by using refiner and modifier.

In general, grain refinement is achieved by adding commercial grain refiners, such as adding Al-5Ti-1B master alloy to the melt before casting. Ti is the element with the highest growth limiting factor. It plays an important role in refining the grain size of Al through heterogeneous nucleation. However, the mechanism of Al grain refinement has always been the main topic of debate. Researchers have put forward various theories: phase diagram / peritectic theory, peritectic theory, hypernucleation theory and solute theory. To sum up, the use of commercial Al-Ti-B Master Alloy is based on the scientific theoretical basis of TiB2 particles formed by the reaction of Ti with B and Al3Ti intermetallic compound particles formed by the reaction of Al with Ti.

When commercial grain refiners are added to molten Al alloys, TiB2 particles act as heterogeneous nucleation sites, while Al3Ti intermetallic compounds dissolve in the solution during the peritectic reaction of α – al: Liquid Al + Al3Ti → α – Al (solid solution). By observing the interface layer between TiB2 particles and Al3Ti, it is found that the combination of TiB2 particles and Al3Ti results in the non-uniform nucleation of α – Al grains. Commercial Al-Ti-B Master Alloy is a very effective grain refiner for forging Al alloy, and its Si content is usually lower than 2wt%. However, the efficiency of commercial Al-Ti-B Master Alloy is relatively poor in the case of casting Al alloy (Si content is more than 4wt%). This is due to the interaction between Ti and Si to form titanium compounds (i.e. TiSi, TiSi2 and Ti5Si3), which consumes the melt of Ti, thus preventing the refinement of alloy grains. This phenomenon has been a hot topic of different studies, known as toxic effect.

The potential heterogeneous nucleation matrix for grain refinement must have three main characteristics:

① High melting point to prevent melting in contact with the molten metal to be refined;

② The lattice mismatch with nucleation phase is low;

③ Stable chemical properties (no interaction with alloy elements).

This paper mainly discusses the effect and mechanism of grain refinement of Al Si casting alloy by means of al-nb-b, a new refining agent.

In this paper, low-pressure casting aluminum alloy auto parts are used as the research object, and A356 is used as the casting alloy. In order to compare the refining effect of AlTiB and alnbb, al5tib and al4.5nbb are used as the refining agents respectively. After preheating, they are added to the melting furnace, and the niobium content of the final parts is 0.0375%. See Table 1 for the composition after adding the refining agent.

The technological process of low pressure casting is as follows: melting of casting alloy (740 ℃ + 5H) → degassing by refining (AR, 10mint) → addition of refiner (al5tib and al4.5nbb, 740 ℃) → addition of aluminum strontium bar modifier (alsr10 rod, 740 ℃) → addition of slag remover → slag raking → transfer to ladle → casting (680-720 ℃) → heat treatment (solution 550 ℃ + 240Min), quenching (40 ℃ water temperature), aging (165 ℃) ℃+120min)。 The samples were analyzed by metallography and electron microscope.

1) The new aluminum alloy refiner al4.5nbb can refine the grains of the parts of AlSi casting car more effectively. Compared with al5tib, the grain size is 50% of that of al5tib.

2) The Nb containing precipitates are mainly in the form of AlNb intermetallics. At the same time, alti intermetallics and AlNb intermetallics are combined to precipitate.

3) The intermetallic compound al3nb has a tetragonal structure with eight atoms in each crystal cell, which is exactly the same as Al3Ti phase, and they have the same lattice mismatch with al.

4) Niobium silicide is a high temperature intermetallic compound, and its melting temperature is higher than that of titanium silicide. At 700-800 ℃, the possibility of forming niobium silicide is less than titanium silicide, and the reaction kinetics of niobium silicide formation is slower. Therefore, niobium based phase can be used as heterogeneous nucleation site of Al without poisoning.

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