Abstract:
A method of fabricating a load compressor scroll housing includes the steps of forging near net shape parts that are machined and subsequently welded together. The forged portions are machined to define specific features of the scroll housing along with the mating surfaces. An interface between top and bottom portions is defined within the volute chamber. The weld joint at the interface provides a finished surface within the volute chamber such that airflow is not adversely affected and additional machine process are not required within the volute chamber after formation of the weld joint.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    This application is a divisional of U.S. application Ser. No. 13/189,691 filed on Jul. 25, 2011. 
     
    
     BACKGROUND 
       [0002]    This disclosure generally relates to assembly of a load compressor for an auxiliary power unit. More particularly, this disclosure relates to fabrication of a load compressor scroll housing for an auxiliary power unit. 
         [0003]    Aircraft utilize auxiliary power units for inflight restarting of main engines and to supply compressed air and electric power during ground operations. An auxiliary power unit typically includes a small gas turbine engine that drives a load compressor. The load compressor generates compressed air that can be used for environmental systems and to aid in starting a main engine. A load compressor includes an impeller driven by the small gas turbine engine that generates compressed air guided through a discharge scroll. The scroll is a volute passage defined by the load compressor housing that directs compressed air through a single outlet. 
         [0004]    The volute passage is typically formed through known lost wax casting processes. Lost wax casting can be inefficient for some materials and therefore limits the materials utilized for forming the scroll housing. 
       SUMMARY 
       [0005]    A disclosed method of fabricating a load compressor scroll housing includes the steps of forging near net shape parts that are machined and then welded together. The forged portions are machined to define specific features of the scroll housing along with the mating surfaces. 
         [0006]    The load compressor scroll housing includes a volute chamber and annular passages that receive airflow from an impeller rotating within a central cavity of the scroll housing. The interface between top and bottom portions is defined within the volute chamber. The weld joint at the interface provides a finished surface within the volute chamber such that airflow is not adversely affected and additional machining is not required within the volute chamber after formation of the weld joint. The top and bottom portions are formed in a near net shape forging process starting with an aluminum billet. The forging operation from a billet provides a homogenous material composition of the scroll housing not reliably obtainable using other processes. An energy beam is directed through an annular passage into the volute chamber and against an outer wall to form the desired weld joint. The finished weld joint provides a desired finish within the volute chamber that does not adversely alter flow properties. 
         [0007]    These and other features disclosed herein can be best understood from the following specification and drawings, the following of which is a brief description. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0008]      FIG. 1  is a schematic view of an example auxiliary power unit. 
           [0009]      FIG. 2  is a cross-sectional view of a portion of an example auxiliary power unit. 
           [0010]      FIG. 3  is a cross section of a load compressor for an auxiliary power unit. 
           [0011]      FIG. 4  is a sectional view of an example scroll housing for a load compressor. 
           [0012]      FIG. 5  is a perspective view of a top portion of the scroll housing. 
           [0013]      FIG. 6  is a schematic view of a bottom portion of the scroll housing. 
           [0014]      FIG. 7  is a cross-sectional view illustrating welding of the top and bottom halves of the example scroll housing. 
           [0015]      FIG. 8  is a perspective view of a completed example scroll compressor housing. 
       
    
    
     DETAILED DESCRIPTION 
       [0016]    Referring to  FIGS. 1 and 2 , an example auxiliary power unit  10  includes a gas turbine engine  12  that drives a load compressor  14  and an accessory drive unit  16 . The load compressor  14  generates compressed air that is driven through ducts  18 . The gas turbine  12  includes an inlet that draws air into a compressor  42 . The compressor  42  is an impeller that is mounted onto a shaft  40  that is driven by turbine  44 . Air drawn in by the compressor  42  is driven to combustor  46  where gasses are mixed with the compressed air and ignited. The ignited gasses produce a gas stream that is driven against the turbine  44  to drive the compressor  42 . Also mounted on the shaft  40  is an impeller  38  for the load compressor  14 . Accordingly, the load compressor  14  is driven by the turbine  44  on the common shaft  40 . 
         [0017]    Referring to  FIG. 3 , with continued reference to  FIG. 2 , the load compressor  14  includes a scroll housing  20  that defines a volute chamber  22 . Passage  24  provides for airflow from the impeller  38  into the volute chamber  22 . From the volute chamber  22  the compressed air is driven through the ducts  18  to various units and accessories that utilize the compressed air. As appreciated, compressed air is utilized for aircraft environmental control systems and also to aid in the starting of a main aircraft engine. Moreover, the terms scroll or volute are also used by those skilled in the art to describe the example scroll housing  20  and other structures that gather flow from a diffuser discharge annulus and deliver that flow to a downstream pipe. 
         [0018]    The example scroll housing  20  is fabricated from a top portion  50  and a bottom portion  52 . The top and bottom portions  50 ,  52  of the scroll housing  20  define both the volute chamber  22  and the annular passages  24 . 
         [0019]    Scroll housing  20  is fabricated by welding the top portion  50  to the bottom portion  52 . It is desired to provide a smooth surface finish within the volute chamber  22  such that airflow is not adversely affected by any inconsistencies within the surface finish. Accordingly, an interface  58  between mating surfaces defined on the top and bottom portions of the scroll housing  20  provides little interruption of the desired interior surface finish of the volute chamber  22 . 
         [0020]    An outer wall  28  of the volute chamber  22  includes the inner surface  30  and an outer surface  32 . The inner surface and the outer surface  32  are defined by the outer wall  28 . A boss  34  is disposed at the location of mating surfaces between the top and bottom portions  50 ,  52  of the scroll housing  20 . The boss  34  provides additional material for the welded joint interface  58 . 
         [0021]    Referring to  FIG. 4 , with continued reference to  FIG. 3 , the scroll housing  20  includes inner annular passage  24  open to an inner periphery of the inner cavity  36  that defines the space within which the impeller  38  rotates. 
         [0022]    Referring to  FIGS. 5 and 6 , the top portion  50  and the bottom portion  52  of the scroll housing  20  are formed from a forging operation. An aluminum alloy material is utilized to fabricate the example top and bottom portions  50 ,  52  of the example scroll housing  20 . Some aluminum alloys are not compatible with lost mold casting procedures. In such non-compatible alloys, differences in the melting temperature of the alloy components cannot provide for homogenous distribution of elements in a lost wax molding process and therefore result in undesirable manufacturing inefficiencies. 
         [0023]    The example scroll housing  20  is fabricated utilizing an extruded aluminum material. The aluminum material is extruded as a billet and shaped to a near complete configuration by a forging operation. After the forging operation is complete, various features are machined into each of the top and bottom portions  50 ,  52 . Mating surfaces  54  and  56  are formed after forging to define the weld joint and mating interface  58  between the top and bottom portions  50 ,  52  of the scroll housing  20 . Other surfaces are also machined to provide the desired configuration of the completed scroll housing  20  and include openings for a shaft bearing and also the inner cavity  36  within which the impeller  38  rotates. Once the desired machining operations are complete and provide the machine surfaces within desired tolerances the top and bottom portions are held together with the mating surfaces  54  and  56  in a desired alignment. 
         [0024]    Referring to  FIG. 7 , the now aligned top and bottom portions  50 ,  52  of the scroll housing  20  are welded utilizing an energy beam welding method. In this example, an electron beam  62  is utilized for forming the desired weld joint between the top and bottom portions  50 ,  52 . In this example, an electron beam welding machine  64  generates an electron beam  62  that is directed through the passage  24  against the outer wall  28 . The electron beam  62  is guided or directed through the passage  24  against an inner surface  30  of the outer wall  28 . The specific location that the electron beam  62  is directed is the interface  58  between the mating surfaces  54  and  56 . 
         [0025]    The directed energy beam provides a weld joint  60  that creates a substantially finished surface that does not affect the desired airflow properties through the volute chamber  22 . The weld beam  62  is applied in a continuous manner about the circumference of the volute chamber  22 . This is accomplished by either rotating the electron beam  62  such that it is directed through the passage  24  or by rotating the assembled top and bottom portions of the scroll housing  20 . 
         [0026]    The electron beam  62  provides a weld joint  60  through the entire thickness of the outer wall  28  to form the desired joint. On an outer surface of the outer wall  28 , a boss  34  is provided to further accommodate the weld joint  60 . Once the desired weld joint is completed, the scroll compressor is finish machined to complete fabrication of the scroll housing  20 . As appreciated, although an electron beam welding process is disclosed other beam welding processes could also be utilized such as for example laser welding. 
         [0027]    Referring to  FIG. 8 , completed scroll housing  20  as illustrated and includes all the features of a one piece housing without the inefficiencies caused by the lost wax molding manufacturing processes. 
         [0028]    Accordingly, the example scroll housing for a load compressor provides a desired surface finish within the volute chambers without sacrificing manufacturing inefficiencies. Moreover, the electron beam welding method allows for the precision application of a weld joint within the volute chamber without modification of the part configuration. 
         [0029]    Although an example embodiment has been disclosed, a worker of ordinary skill in this art would recognize that certain modifications would come within the scope of this disclosure. For that reason, the following claims should be studied to determine the scope and content of this invention.