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 application_number US 201514674177 A description referring to fig1 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 referring to fig3 with continued reference to fig2 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.