Resistance Mash Welding for Joining of Copper Conductors2
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The automotive industry is developing designs and manufacturing processes for a new generation ofelectric motors intended for use in hybrid and electric vehicles. This paper is focused on using solid-statewelding to join rectangular wires in the fabrication of motor stators. Resistance welding has not typicallybeen applied to copper due to its very high electrical conductivity; however through optimization ofthe current and pressure profiles, excellent quality copper-to-copper joints have been demonstratedwith a technique known as resistance mash welding. A better understanding of resistance mash weldingcharacteristics will help advancements in its application for stators.
Transcript of Resistance Mash Welding for Joining of Copper Conductors2
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CONTENT
1.Introduction2.Objective3.Experimental plan4.Experimental approach5.Evaluation of welded joints6.Process analysis7.Results8.Conclusion
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1.INTRODUCTION
Vehicle electrification requires efficient motors with increased power and minimized losses.
While there are other forms of losses in the motor , minimizing resistive losses is crucial to boosting motor efficiency.
Bar wound stators have best packing density and has maximum conductive metal area.
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Cont..
Individual pieces of rectangular solid piece copper are bent into hairpin shape and joined together to create required electrical circuit.
Joining of copper can be complicated by the material’s very high thermal conductivity.
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2.OBJECTIVE
To identify and develop copper joining process to achieve high quality copper to copper joints with a minimum of time, energy, and applied force.
To assess the influence of variations in wire to wire alignment on weld strength
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3. EXPERIMENTAL PLAN
Resistance Mash Welding (RMW) joints are made using a conventional spot welding machine with a fast follow up head.
C101 copper wires with rectangular cross section 3.44mm x 3.66 mm were used.
A pneumatic spot welding machine was used.
7Cont..
Two styles of electrodes were used:1. A 16mm diameter flat faced electrode.2. A non-standard chisel faced electrode.
The electrode were produced from RWMA Class 20 AL-60 material with good anti-sticking and electrical conductivity properties.
A fast-follow-up head was used in the machine to maintain force on the wires due to rapid softening and collapse of the wires during welding.
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4.EXPERIMENTAL APPROACH
Iterative tests were performed to obtain the optimum process parameters and system design.
Variables manipulated: Weld and forge forces Timing between weld and forge initiation Weld current and time Pre-welding parameters Anneal(pulse welding) parameters Cool times Placement of wires relative to the chisel tip location
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For parallel wire joints: Variations in wire overlap Angular alignment Weld end position with respect to electrode face
position were studied.
For cross wire joints variation was studied at three percentages of full wire extension.
105.EVALUATION OF WELDED JOINTS
A weld schedule that achieved High average peak strengths A relatively small minimum to maximum range Final wire thickness greater than 2mmwas considered acceptable.
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6. PROCESS ANALYSIS
In RMW a multistep process was developed using control of applied force and current to manage the temperature and deformation of wires during the joining process.
12Cont..Resistance heating in a spot welder occurs when
current flows through the spot welding gun into the base metal, across the faying interface, into the second piece of base metal and back to ground through remainder of gun structure.
The low weld force levels increased the heat generation at the faying surface without collapsing the joint.
After heating the weld zone to forging temperature, forge force applied provided the deformation needed to strain the heated metal and produce a forge bond.
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7.RESULTS
Cross-wire weld Flat faced electrodes were used.
Achieved tensile strength values between 1.44 and 1.60 KN.
15Cont..Parallel-wire weld
Using the same parameters tensile strength was between 540 and 800N
Using the special trapezoidal designed tip the weld tensile strengths were between 780N and 1.15KN
16Cont..The strongest joints were generally
produced by the standard condition(Just under 1200N).
17Cont..The average weld thickness was about
2.5mm.The average joint width for the standard
condition was about 7mm.The wire extension of the final weld joint
with the standard condition of about 3.5mm
18Cont..For parallel-wire geometry, a critical value
of 2.5mm final joint thickness was observed to produce optimum weld tensile strength.
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8.CONCLUSION
For the parallel wire geometry, joints with acceptable strength can be produced by the following : The standard condition i.e., properly aligned ±10% width lateral misalignment Short extension -15% angular misalignments
20Cont..The cross-wire joint geometries appeared to
provide sufficient process capability, providing no insulation was entrapped in the weld joint.
In a properly controlled process, means of producing overheating of the weld joint can be avoided.
Smooth fast-follow-up and weld force head motions were required to prevent collapse of the wires during welding.
