EFFECT OF HIGH Li ADDITION ON THE STRUCTURE AND PROPERTIES OF Al5Mg2SiMn-TYPE CASTING ALLOY
https://doi.org/
Viktoriya Boyko1, https://orcid.org/0000-0003-0455-3924, Scopus ID 56026879900
Andrii Burmak2, https://orcid.org/0000-0003-2415-8032, Scopus ID 36456732200
Mykhailo Voron3, https://orcid.org/0000-0002-0804-9496, Scopus ID 57205261832
Olaf Kessler1, https://orcid.org/0000-0002-1087-4137, Scopus ID 55950231900
Kostiantyn Mykhalenkov3, https://orcid.org/0000-0002-2150-8837, Scopus ID 6025516800,
Armin Springer1, https://orcid.org/0000-0001-9878-7240, Scopus ID 8339395000
1 University of Rostock, Rostock
2 Igor Sikorsky Kyiv Polytechnic Institute, Kyiv
3 Physico-Technological Institute of Metals and Alloys of the NAS of Ukraine, Kyiv
Innovative class of Li-containing aluminium alloy materials is garnering considerable interest. The main objective is to develop alloys that not only exhibit exceptional strength but also achieve a significant reduction in density for a wide range of applications, especially in the aerospace and automotive sectors.
In this paper, the authors present recent results on the structural characterisation of a Li-containing Al casting alloy, along with changes in mechanical properties in the as-cast condition.
For the melting process, aluminium (Al 99.7%), magnesium (Mg 99.85%), and lithium (Li 99.9%) were utilised. Silicon and manganese were subsequently introduced by adding AlSi40 and AlMn20 master alloys.
It was found that increasing the Li content to 2.1 wt.% results in the complete elimination of the eutectic structure. The base alloy Li0 exhibited a hardness of 82.5 HV, and the addition of 0.8 wt.% Li did not result in a significant change. However, further increases in Li content to 3.0 and 4.0 wt.% produce a remarkable rise in hardness, reaching 157.6 HV5 for the Li4.0 alloy.
To account for the porosity effect, yield strength (YS) was determined from compression tests, which may collapse closed pores during plastic flow, thereby reducing the results' sensitivity to porosity. The key findings could be summarised as follows:
1. The addition of Li induces significant structural changes in the Al5Mg2SiMn-type alloy, transforming it from a eutectic structure to one in which only primary intermetallics are formed, and the formation of lamellas or fibres is entirely suppressed.
2. Alongside a-Al and primary Mg2Si phases, two lithium-containing intermetallics, LiAlSi and AlLi, are detected. These intermetallics exhibit coarse, euhedral, faceted polyhedral crystal morphologies, indicating primary crystallisation from the melt.
3. When only primary intermetallics are present, the Al-Mg-Si-Mn-type alloy exhibits high hardness, reaching up to 157 HV5, and a yield strength that is double that of the lithium-free alloy. Elevated-temperature compression tests show that the yield strength increases progressively with increasing lithium content up to 350 MPa at 4.0 wt.% Li at room temperature and 170 MPa at 300°C.
Keywords: aluminium alloy, microstructure, mechanical properties, casting, intermetallic.
References
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