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An impeller for a turbo-machine is provided. The impeller includes a metallic base having a back surface and a front surface opposite to the back surface; at least one blade extending on the front surface of the metallic base; a composite shroud attached to the at least one blade such that plural cl

An impeller for a turbo-machine is provided. The impeller includes a metallic base having a back surface and a front surface opposite to the back surface; at least one blade extending on the front surface of the metallic base; a composite shroud attached to the at least one blade such that plural closed paths are formed by the front surface of the metallic base, the at least one blade and the composite shroud; and a connector configured to attach the composite shroud to the metallic base or the at least one blade.

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1. An impeller for a turbo-machine, the impeller comprising: a metallic base having a back surface and a front surface opposite to the back surface;at least one blade extending on the front surface of the metallic base;a composite shroud attached to the at least one blade such that plural closed pat

1. An impeller for a turbo-machine, the impeller comprising: a metallic base having a back surface and a front surface opposite to the back surface;at least one blade extending on the front surface of the metallic base;a composite shroud attached to the at least one blade such that plural closed paths are formed by the front surface of the metallic base, the at least one blade and the composite shroud; anda connector configured to attach the composite shroud to the metallic base or the at least one blade. 2. The impeller of claim 1, wherein a leading edge of the at least one blade is made of the composite material and the remaining part is made of the metallic material, or a trailing edge of the at least one blade is made of the composite material and the remaining part is made of the metallic material, or both the leading and trailing edges of the at least one blade is made of the composite material and the remaining part is made of the metallic material. 3. The impeller of claim 2, wherein the connector comprises: an adhesive or silicon or resin distributed between those parts made of the composite material. 4. The impeller of claim 1, wherein the composite shroud includes brackets made of the composite material that are configured to fit inner parts of the at least one blade, wherein the connector includes an adhesive or silicon or resin that connects the brackets to the inner parts of the at least one blade, and the brackets and the inner parts of the at least one blade form a smooth profile of the at least one blade. 5. The impeller of claim 1, further comprising: composite stripes connected to and extending from the composite shroud to the back surface of the metallic base over a trailing edge of the at least one blade. 6. The impeller of claim 1, wherein the composite shroud includes plural depressions and the connector includes plural pins attached to the at least one blade and configured to fit inside the plural depressions such that the plural pins have an open position and a closed position, the open position allowing the plural pins to enter or exit the depressions while the closed position locks the plural pins inside the plural depressions. 7. The impeller of claim 1, wherein the connector includes plural hooks formed on the at least one blade and corresponding plural holes in the composite shroud that are configured to receive the plural hooks. 8. The impeller of claim 1, wherein the connector includes plural pins formed on the at least one blade and corresponding plural holes in the composite shroud that are configured to receive the plural pins. 9. The impeller of claim 1, wherein the connector includes plural pins provided in plural holes formed in the at least one blade and corresponding holes in the composite shroud. 10. The impeller of claim 9, wherein the plural pins are riveted or threaded to receive nuts or both to fix the composite shroud to the at least one blade. 11. The impeller of claim 1, further comprising: plural composite blades integrally formed with the composite shroud and configured to connect to the at least one blade on the front surface of the metallic base so that an entire blade is made of a composite material at a trailing edge and made of a metallic material at a leading edge, wherein the leading edge first encounters a fluid to be processed by the impeller and the trailing edge encounters last the fluid. 12. The impeller of claim 11, wherein the connector includes a blind bolt or a metallic wire or rivets or bolts that are configured to connect the plural composite blades to the metallic base. 13. The impeller of claim 1, further comprising: blade end portions integrally formed with the composite shroud and configured to cover a trailing edge of the at least one blade and a portion of a side surface of the metallic base, wherein the side surface of the metallic base is provided between the front surface and the back surface. 14. The impeller of claim 13, wherein the blade end portions form a continuous surface around the side surface of the metallic base. 15. The impeller of claim 13, wherein the connector includes a bolt or a pin that attaches the blade end portions to the side surface of the metallic base. 16. The impeller of claim 13, wherein the connector includes a bolt or a pin that attaches with a bracket the blade end portions to the side surface of the metallic base. 17. The impeller of claim 1, further comprising: plural connecting elements formed on the at least one blade such that the each of the plural connecting elements has a hole, wherein the composite shroud has plural holes configured to receive the plural connecting elements, and wherein the connector includes a band that enters the holes of the plural connecting elements to fix the composite shroud to the at least one blade. 18. The impeller of claim 17, wherein the holes of the plural connecting elements are open on one side. 19. The impeller of claim 17, wherein the band is configured to encircle the entire composite shroud. 20. The impeller of claim 1, wherein the connector comprises: plural brackets configured to be attached to the at least one blade; andplural pins configured to attach the plural brackets to the at least one blade, wherein the plural pins are configured to fixedly receive the composite shroud. 21. A centrifugal compressor, comprising: a casing;a shaft provided in the casing and configured to rotate relative to the casing; andat least the impeller of claim 1, wherein the impeller is attached to the shaft. 22. A method for attaching a composite shroud to an open or closed metallic impeller for a turbo-machine, wherein the impeller comprises a metallic base having a back surface, a front surface opposite to the back surface, and at least one blade extending on the front surface of the metallic base, the method comprising: contacting the composite shroud with the at least one blade such that plural closed paths are formed by the front surface of the metallic base, the at least one blade and the composite shroud; andfixing the composite shroud to the metallic base and/or the at least one blade using a connector. 23. The method of claim 22, wherein a leading edge of the at least one blade is made of the composite material and the remaining part is made of the metallic material, or a trailing edge of the at least one blade is made of the composite material and the remaining part is made of the metallic material, or both the leading and trailing edges of the at least one blade is made of the composite material and the remaining part is made of the metallic material. 24. The method of claim 22, further comprising: applying an adhesive or silicon or resin distributed between those parts made of the composite material. 25. The method of claim 22, further comprising: bonding brackets of the composite shroud and made of the composite material to inner parts of the at least one blade by using an adhesive or silicon or resin, wherein the brackets and the inner parts of the at least one blade form a smooth profile of the blades. 26. The method of claim 22, further comprising: connecting composite stripes extending from the composite shroud to the back surface of the metallic base over a trailing edge of the at least one blade; andcuring the composite strips to harden. 27. The method of claim 22, further comprising: forming plural depressions into the composite shroud; andattaching plural pins to the at least one blade, wherein the plural pins are configured to fit inside the plural depressions such that the plural pins have an open position and a closed position, the open position allowing the plural pins to enter or exit the depressions while the closed position locks the plural pins inside the plural depressions. 28. The method of claim 22, further comprising: forming plural hooks on the at least one blade and forming corresponding plural holes in the composite shroud that are configured to receive the plural hooks. 29. The method of claim 22, further comprising: forming plural pins on the at least one blade and forming corresponding plural holes in the composite shroud that are configured to receive the plural pins. 30. The method of claim 22, further comprising: forming plural pins in plural holes one the at least one blade and corresponding holes in the composite shroud; andriveting or threading the plural pins to receive nuts or both to fix the composite shroud to the at least one blades. 31. The method of claim 22, further comprising: forming plural composite blades integrally with the composite shroud; andconnecting the plural composite blades to the at least one blade on the front surface of the metallic base so that an entire blade is made of a composite material at a trailing edge and made of a metallic material at a leading edge, wherein the leading edge first encounters a fluid to be processed by the impeller and the trailing edge encounters last the fluid. 32. The method of claim 31, further comprising: connecting with a blind bolt or a metallic wire or rivets or bolts the plural composite blades to the metallic base. 33. The method of claim 22, further comprising: forming blade end portions integrally with the composite shroud such that the blade end portions are configured to cover a trailing edge of the at least one blade and a portion of a side surface of the metallic base, wherein the side surface of the metallic base is provided between the front surface and the back surface. 34. The method of claim 33, wherein the blade end portions form a continuous surface around the side surface of the metallic base. 35. The method of claim 33, further comprising: attaching the blade end portions with a bolt or a pin to the side surface of the metallic base. 36. The method of claim 33, further comprising: attaching a bracket with a bolt or a pin to the metallic base so that blade end portions are fixed by the bracket to the side surface of the metallic base. 37. The method of claim 33, further comprising: forming plural connecting elements on the at least one blade such that each of the plural connecting elements has a hole;covering the at least one blade with the composite shroud; andproviding a band through holes of the plural connecting elements to fix the composite shroud to the at least one blade. 38. The method of claim 37, wherein the holes of the plural connecting elements are open on one side. 39. The method of claim 22, wherein the band is configured to encircle an entire periphery of the composite shroud. 40. The method of claim 22, further comprising: attaching plural brackets to the at least one blade; andinserting plural pins through the plural brackets and the at least one blade to fix the composite shroud.