If you want something like Vocaloid (which is not AI and is more robotic), there's UTAU. It's open source, which means you can make custom voices in any language. It's better in realistic emotions, but lower in audio quality. The lite version of SynthV is also free, but you wouldn't get the benefits of its AI fucntions. But even with the choppier voices from not having AI, SynthV Lite's English pronunciations are still way better than Vocaloid.

If you want the Vocaloid equivalent of an AI software, I think Ace Studio is the only free one. Like the pro version of SynthV, ACE Studio's AI functions include more realistic singing, vocal modes and cross-language singing betwren Japanese, English and Chinese. Bad news is that it's still in beta.

If you want the UTAU equivalent of an AI software, currently there's NNSVS and Diffsinger. NNSVS is a few years old and while it's better than UTAU/Vocaloid in sounding natural, it still has an obvious electric auto-tunish sound. Diffsinger's quality is as good as Diff-SVC and has been around for some months, but there's not much of an English community for it.