ONM Welding Propeller Shaft

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MIG  vis-à-vis Friction Welding in Propeller Shaft : a case study

AUTHOR DETAILS

Omkar Nath Mohanty
Director
Technology & Academic Initiative
RSB Group, India

Susant Kumar Sahoo
Deputy Manager
RSB Group, India
Technology & Academic Initiative

RSB Group, Pune

KEYWORDS
Propeller Shaft, Friction Welding of Fork and Tube, MIG welding using CO2, Hardness Profile,
Requirement of Torque, Fatigue Properties

Abstract

Apart from basic strength, ductility, and torsional characteristics, a propeller shaft requires adequate fatigue strength, which is often evaluated using full-length assembly. A hardness profile on the welded joint of the shaft can however be taken as a preliminary, but good indicator of the fatigue behaviour. A drop, i.e. softening, in the hardness-traverse, particularly in the HAZ area, would correlate well with a lowering in the fatigue resistance. 

Solid state friction welding of automotive propeller shaft, based on medium carbon – manganese steel, for achieving superior fatigue properties is commonly adopted by several industries. However, it is worthwhile looking into the more conventional and less expensive MIG welding for a comparison. A case is described, where a controlled MIG welding with CO2 was carried out, and a hardness profile on the joint was investigated
along with characterization of the microstructure in each zone. Full scale fatigue life tests were also undertaken. Overall, it was observed that acceptable mechanical properties, even comparable to the friction-welded situation, can be achieved through MIG welding.

1.0 Introduction

Propeller shaft is an integral part of an automotive that transmits power from the engine to the wheels. Presently, most OEMs use propeller shafts based on medium carbon-manganese steels, particularly for heavy vehicles, HCV/MCV. The propeller shaft, primarily comprises two components, viz. a fork and a tube that are welded to eachother. Commonly, the welding makes use of a MIG process based on carbon dioxide. Some discerning customers, however, prefer to use a solid state welding process, such as friction welding, for greater reliability (primarily by obviating the use of a filler wire, and avoiding a re-solidified zone), and increased fatigue life. 

2.0 Objective
The main purpose of this study was to investigate and examine if a cost-effective MIG  CO2 process of welding could serve as an  alternative to the expensive friction welding, fulfilling all requirements of mechanical properties. 

3.0 Material
An Isuzu propeller shaft fitted with a friction-welded fork and a tube, both steel,  was taken  as  reference. A portion of the Isuzu tube was cut out from the shaft, CO2 welded to a ‘dummy’ flange available in the shop floor, and was subjected to microscopic and mechanical characterization. The shaft, put through CO2 welding, was  compared with the original friction welded case, in terms of the desired mechanical properties.

Flange / Fork material

The flange material for CO2 welding was 37 C 15 steel ( broadly, C ~ 0.37; Mn ~ 1.5; Si~ 0.24 % ). The ISUZU fork was made from 45C8 steel ( i.e. CK 45 DIN ; with  C~ 0.45 ; Mn ~ 0.95, Si ~ 0.3 %). Both were in the normalized condition.

Tube material 

The ISUZU- tube is designated as STAM- 540H  ( with C 0.30; Mn 0.30 – 1.00, Si 0.35; S 0.035max; P 0.035max. ,
all in % ). This tube has an outer dia. of 63.5 mm & wall thickness of 1.8 mm
 

The filler (MIG ) wire used: 1.2 mm Ф (composition i : C – 0,06;  Mn – 1,47; Si-0.8 %  )

4.0 Experimental

Welding

The Isuzu shaft ( with 45C8 as fork and STAM-540 H as tube ) was available in the friction-welded condition. As stated earlier, portions of the tube from the Isuzu shaft were cut out from the shaft and re-welded to a dummy coupling flange ( based on 37 C 15 steel ) . The CO2 welding was carried out using an automated welding machine iof M/s. Li=. The parameters of welding are shown 

Table I: Welding parameters for Isuzu tube welding

Description                             Parameters
Voltage (volt) Current

( amp.)

Time

(sec.)

FlowRate               (lpm)
Isuzu Tube Welded

to Dummy Flange

28 – 35 230 – 270 35 – 38 18 – 22

Hardness measurement

Hardness measurement was done using a Zwick Roel Micro Vickers Hardness machine on samples cut from the welded shaft. The indenter was made to traverse relative to the sample, from the base of the tube or from the base of the fork, in steps of 0.2 mm. The load used was 0.5 kg., hence all hardness values are reported as HV 0.5 The mapping of hardness was done along the length, covering the weld pool , heat-affected zone (HAZ); and the base material . Hardness was measured on Isuzu tubes on two samples , one CO2 welded (Sample # 1), and the other friction-welded (Sample # 2 ).

Optical Microstructure
The optical microstructure was examined on Samples # 1 on the base tube, tube HAZ , weld-pool, fork HAZ and the fork base. Similar study was done on Sample # 2 as well, except that there was no weld-pool. The microstructures were observed in a range of magnifications, from 50 X up to 1000 X.

5.0 Result and Observation 

    5.1 Hardness and microstructural data on Sample # 1

Table II : Hardness data on  Sample # 1 in steps of 0.2 mm

Sample # 1
Sl No. Hardness in HV 0.5 Sl No. Hardness in HV 0.5
1 182 Tube base 12 211
2 181 13 216
3 178 14 221
4 181 15 228 Start of Weld
5 183 16 224
6 189 17 245
7 193 Start of HAZ 18 250
8 197 19 247
9 202 20 236
10 206 21 241
11 201

(Isuzu tube CO2 welded to dummy flange; hardness data starting from tube-base to weld-pool)

The hardness data in the base tube seem to fall in the range of HV 175 – HV 190. The HAZ shows a band of hardness, HV 190 – HV 225, and the weld-pool hardness is in a range of HV 225 – HV 250. All these hardness bands are similar to those observed in traditional propeller shafts, using conventional medium carbon-manganese steels, the width of the HAZ appears to be ~ 1.6 mm. also in the same range as in the regular shafts.

Macro & microstructure on Sample # 1

                                                                        

                         

   Detailed microstructures in the sequence of fork to tube, shown in Fig. 3 below:

              Fig. 3.1 (a) Flange Base, 50X                                                           Fig. 3.1 (b) Flange Base, 1000 X

           ferrite and unresolved pearlite ferrite at prior austenite grain boundary & resolved pearlite

         

Correlation between Hardness and Micro Structure in Sample# 1 (ISUZU tube, CO2 Welded)

The hardness values shown in Table II, agree broadly with the microstructural details; one has to bear in mind that the phases
in the sequence of ferrite, coarse pearlite, fine pearlite, bainite and martensite are in     ascending order of hardness

5.2  Hardness and microstructural data on Sample # 2

Table – III : Micro hardness data on Sample # 2  in steps of 0.2 mm. 

Sample # 2
Sl No. Hardness in HV(0.5) Sl No. Hardness in HV(0.5)
1 170 tube base 12 331
2 170 13 317 Fork HAZ continues
3 173 14 312
4 175 15 304
5 180 16 296
6 194 Start of Tube HAZ 17 278 End of Fork HAZ
7 206 18 270
8 321 19 255
9 420 Tube HAZ & Fork HAZ 20 248 Fork base
10 368 21
11 348

(hardness traverse on ISUZU tube, friction welded to ISUZU Fork, 45C8)

From the Table III, the following major observations could be made :

  1. The tube base hardness is HV 170 – HV 190 
  2. The fork base hardness is in the range of HV 245 – HV 270; this is slightly on the higher side compared to the
        conventional flange in the same air-cooled condition( as would be shown in the micrograph)
  3. There is a very sharp rise in hardness as one moves into the HAZ region starting from the tube base; in     steps of
              0.2 mm, the rise in hardness is by HV 115 (from HV 206 to HV 321 in step 7 to 8 ) and again, another sudden rise of
              around HV100 ( from HV 321 to HV 420 ) in a step of 0.2 mm. This is not observed in the CO2 weld conditions. 
  4. The peak  hardness itself is lower compared the same in the HAZ of 37C15 flange / fork ( ~ HV 540 – 560 )
  5. The combined HAZ region shows a sharp peak in the hardness profile ; the peak of hardness in the HAZ is  of the same
              order as the peak on the fork-HAZ of CO2 welding, with the same tube
  1. The width of the combined HAZ appears to be ~ 2.4 mm ( steps 6 – 17 ), this appears to be similar to the combined
              HAZ in the tube and in the fork in the CO2 welding case as well.

The hardness traverse above was taken along lines, starting from tube to the fork, friction-welded to each other.
A macro picture of the same is given in Fig. 3 below (a thicker flash on the tube side, as is expected, is seen)

        Fig.4 :  Macro photograph of Friction Welded Isuzu Shaft ( Sample  #2)

The microstructural details in the sequence from Fork base to tube base are given below, in Fig. 5 :

Microstructure of the HAZ in the Fork is represented both in Fig.5.3 as well as in Fig. 5.4. They contain martensitic needles One  can also observe more of bainitic structure in the HAZ of the tube. ( Fig. 5.4). Further, one could observe the change in the size of the hardness indentation marks (small to large, as one move from fork HAZ to tube HAZ ); the intermediate size is also seen. Some more microstructures are shown in 

 Tube HAZ primarily displays a bainitic structure ( in both Figs. 5.5 & in Fig. 5.6 ), while the tube base ( Fig. 5.6) shows  a ferrite-pearlite mixture, as is to be expected

Ferrite and fine Pearlite is seen in Fig. 5.7. Probably this area  is close to Tube HAZ, hence also bearing upper bainitic features. The features of the tube flash and the fork flash are shown in Fig. 5.8. It may be observed that the tube flash ( in Fig. 5.8) is thinner, compared to the flash of the thicker fork.   A bainitic / martensitic  feature along with ferrite in both portions are there due to faster cooling experienced here. In the tube part , more   of ferrite (white feature ) is observed.

            

Correlation between hardness and micro structure in ISUZU Shaft, friction Welded  / CO   welded

  • The hardness values shown in Table III agree broadly with the microstructural details shown in Fig. 5; one has to bear
              in mind that the phases in the sequence of ferrite, coarse pearlite, fine pearlite, bainite and martensite are in
              ascending order of hardness.
  • There is a very sharp rise in hardness as one moves into the HAZ region starting from the tube – base. For example,
              in a step of 0.2 mm, the rise in hardness increses by HV 115 ( from HV 206 to HV 321 in step 7 to 8 ) , and again
              another sharp rise of around HV 100 ( from HV 321 to HV 420 ) in a step of 0.2 mm. This correlates well with the
              microstructure, particularly that in Fig.5.4. Such an occurrence appears to be due to the friction welding process
              that has caused the two materials, viz. tube and the fork, mingle with each other at the interface. 
  • The hardness and micro structure investigations have shown that in the case of friction welding the peak hardness in
              the HAZ is ~ HV 420 ( which is the interface between the fork & the tube ).  In the CO2 welding case,  for both the
              traditional (K3) tube / Isuzu tube and 37 C15 fork, one gets a peak hardness in the fork HAZ of ~ HV 540. 

The hardness profile for a CO2 -welded traditional tube, and friction welded Isuzu tube are given below in Fig. 6 (a) and  6(b) respectively.

Fig.6 (a) Hardness Profile for CO2- welded K3 tube ( traditional)                      

Fig.6 (b) Hardness Profile for friction welded Isuzu Tube

The point to note here is that, in both cases, there is no softening, going from the tube base into the joint. Thus, it would be expected that the fatigue life for controlled  CO   welded shafts might fulfill the prescribed life by the customer. Hence, full scale fatigue test was conducted.

5.3 Dyanamic ( Fatigue ) test 

The soundness of the weld from the perspective of fatigue life is significant. A typical  dynamic (fatigue) test was
carried out ( shown in Fig.7) under the test conditions as shown in Table – IV.

Assy. Sr No Product Description  Torque applied Frequency Pressure for forward stroke  Pressure for backward stroke  Remarks for a minimum
1 lakh cycle
1 Propeller Shaft 325 rear +7000Nm to -7000Nm  ~0.3Hz 74 Bar 99 Bar passed  
2 Propeller Shaft 325 rear +7000Nm to -7000Nm ~0.3Hz 74 Bar 99 Bar passed 
3 Propeller Shaft 325 rear +7000Nm to -7000Nm ~0.3Hz 74 Bar 99 Bar passes

Table IV : Fatigue
test result for Isuzu shaft

Concluding Remarks

  1. The tube base ( starting material ) micro hardness variation is in the range of HV 175 – HV 190 is essentially due to a
        varying microstructure from place to place.
  1. The Tube HAZ shows a band of hardness HV 190 – HV 225 in the case of CO2 welding; the corresponding microstructure
        shows a change from ferrite, pearlite to additional occurrence of bainite.
  1. In the case of CO2 welding , the weld-pool hardness is in a range of HV 225 – HV 250 ( for a low carbon filler wire );
        this shows a combination of bainite along with ferrite & pearlite.
  1. Flange / Fork HAZ shows a micro hardness as high as HV 540 – HV 560 ( in both 320 & 490 Jts. ), This is consistent with
        the formation of martensite , apart of other phases.
  1. The micro-hardness profile from sample to sample shows a band ( in agreement with the local microstructure where the
        micro- indenter fall ). However broadly, there is a continuous rise in micro hardness from tube base to weld-pool; there is
        NO HAZ SOFTENING.
  1. In friction welding one observes a sudden rise in micro-hardness from about HV 200 to HV 420 in a space of 0.4 mm, as
        one moves from start of tube HAZ to the fusion interface between the tube and the fork. The corresponding
        microstructure changes from ferrite, pearlite / bainite to martensite at the interface.
  1. Broadly, the two cases (CO2 welded, friction welded ) would present a similar hardness profile if one notionally removed
        the weld-pool from the hardness profile. 
  1.  In the dynamic (fatigue) test, the shaft with CO2 welding passed the minimum prescribed 1 lakh cycle without any failure 

          One can therefore conclude that in the case investigated, it is possible to use the conventional CO   welding as a
      substitute for friction welding.

         

Bibiliography

[1] Tanabe, H.,  Anabi I, Miyasaka A., Taniyoka S. , “Haz-softening -Resistance High Strength Steel tube for automotive 

      propeller shafts” Nippon steel Technical Report,64,(1995)

[2] Tanabe, H.,  Yamajaki K.Camp ISIJ 3, 1465(1990)

[3] Tanabe, H.,  Miyasaka A., Camp ISIJ 6,1842(1993)

[4] Kobayashi, K.,et al, Toyota Engineering,39,126(1989)

[5]  Hasui.J,  et al , “Friction Welding” First Edition, Tokyo, Corona Publishing,11(1979)