Deep Understanding of the Influence of the Process Parameters During Linear Friction Welding on the Joint Quality and the Microstructural Changes of Two Mono-Material Titanium Alloy Joints: The β-Metastable Ti-5Al-2Sn-2Zr-4Mo-4Cr (Ti17) and the Near-α Ti-6Al-2Sn-4Zr-2Mo (Ti6242)