FRICTION STIR WELDING OF MA 957 OXIDE DISPERSION STRENGTHENED FERRITIC STEEL

Jata, et al.TMS The Minerals, Metals & Materials Society, 2005 FRICTION STIR WELDING OF MA 957 OXIDE DISPERSION STRENGTHENED FERRITIC STEEL Stanley M.. of Materials and Metallurgical En

Friction Stir Welding and Processing III, Edited by Kumar V Jata, et al.

TMS (The Minerals, Metals & Materials Society), 2005

FRICTION STIR WELDING OF MA 957 OXIDE

DISPERSION STRENGTHENED FERRITIC STEEL

Stanley M Howard1, Bharat K Jasthi1, William J Arbegast2, Glenn J Grant3, Santosh Koduri3, Darrell R Herling3

1South Dakota School of Mines and Tech, Dept of Materials and

Metallurgical Eng; Rapid City, SD 57701 USA

2Advanced Materials Processing and Joining Laboratory South Dakota School of Mines and Tech; Rapid City, SD 57701 USA

3Pacific Northwest National Laboratory; Materials Science Div, K2-03

902 Battelle Blvd, Richland, WA 99352 USA Keywords: Friction Stir Welding, ODS Alloys, Friction Plasticization

Abstract

A 1-in (25.4 mm) diameter yttria-dispersion-strengthened MA 957 ferritic steel alloy tube with a 0.125" (3.18 mm) wall thickness was successfully plasticized by friction stir welding The pin tool was a W-Re tool with 0.125" (3.17 mm) diameter tip It showed

no discernable wear for the total 12" (305 mm) of weld Weld conditions were 1000 and

1400 RPM, 4 in/min (101 mm/min), with and without preheating to 135 ºC Metallographic analysis of the post friction-stir welded material showed a decrease in material hardness ranging from 225±22 HV compared to the parent material, which was 373±21 HV All weld conditions produced plasticization; however, improved plasticization was observed for preheated samples

Introduction

The objective of this investigation was is to determine if MA 957 oxide dispersion strengthened ferritic steel could be plasticized by friction stir welding MA 957 is a yttria(Y2O3)-dispersion-strengthened ferritic steel alloy Conventional welding of the material disturbs the extremely fine oxide particles within the alloy, because the less dense particles rise to the surface of the molten weld pool1 A means of joining such alloys that preserves the dispersion and its concomitant properties would extend the alloy’s usefulness and offer important additional applications One method by which the alloy might be joined while preserving the dispersion structure is friction stir welding Before proceeding with a description of the work, a brief synopsis of the production and morphology of ODS MA 957 and the advantages of friction stir welding are useful

Oxide dispersion strengthened (ODS) alloys are generally produced by mechanical alloying (MA) The elemental or alloy powders are attrited by ball milling2 The powder

is consolidated and homogenized by hot isostatic pressing or by hot extrusion The ODS materials contains an extremely fine oxide particle dispersion of yttria ranging from 5 to

50 nm The yttria content ranges from 0.25 to 0.5 percent by weight as in the case of ODS

MA 957

The yttria dispersoids are very stable at high temperatures and inhibit dislocation motion

in the metal matrix thereby increasing the resistance the material’s creep deformation resistance The dispersoid particles also inhibit the recovery and recrystallization process This produces very stable and large grains During the high temperature deformation process, the large grains resist grain rotation The dispersion of stable oxides provides good creep rupture strength4 at temperatures above 980 oC (1253 K)

Since fiction stir welding is a solid phase technique of joining metals and alloys, the problem of dispersoid rising to the weld pool surface in fusion welding might be avoided The primary advantage of friction stir welding of steels compared to other fusion welding technique is the lower heat input and avoidance of the liquid-to-solid transformation This translates to less severe metallurgical changes in the weld’s heat affected zone, less residual stress3, and avoidance of problems arising from hydrogen embrittlement and gas entrapment

Experimental

The material used in this work consisted of a MA 957 cylinder fitted tightly with an Inconel 625 rod core material as shown in Figure 1 This ODS MA 957 steel was friction stir welded according to the selected processing parameters shown in Table I The material was friction stir processed with and without preheating to avoid the ductile-to-brittle transition temperature (DBT), which is near room temperature for this alloy Below the DBT this material exhibits notch sensitivity

The rod was clamped into a V-groove fixture as shown in Figure 2 No attempt was made

to restrain the material alongside each curved side of the cylinder, as occurs naturally in the welding of a flat plate Therefore, considerable flash was expected on either side of the weld as well as poor backfilling of the pin tool track Even so, a determination of plasticization extent was believed possible by this arrangement The pin tool used was a W-Re tool with 1/8” tip The specimen was preheating using a hot air gun Hardness measurements were performed on the completed specimens in selected zones using a Vickers hardness tester Each weld was approximately 3 inches (76.2 mm) long

After the four welds completed, the resulting friction stir processed cylindrical specimens were sectioned perpendicular to the axis with water cooling The samples were mounted and polished for metallographic examination An etching solution of 2 g CuCl2, 40 ml HCl, and 40-80 ml ethanol was used

Plasticization was achieved for all weld conditions The W-Re pin tool showed no noticeable damage or wear; however, some ODS MA 957 metal adhered to the pin tool Figure 3 shows micrographs of the parent and the nugget zones created by friction stir welding All of the figures are at the same magnification and have a size bar that is 200 microns in length Hardness measurements of the plasticized, parent, and HAZ materials are provided in Table 2 Some of the measurements are averages The table also shows the number of measurements included in each average and the standard deviations

Table 1 Weld Process Parameters

(IPM)

Spindle speed (rpm)

Condition

Discussion

All welds conditions resulted in considerable plasticity of the ODS MA 957 alloy This work largely dispels concerns that a dispersion-hardened material such as MA 957 cannot

be plasticized by friction stir welding It remains undetermined if friction stir welded ODS material retains its valued mechanical properties However, the hardness measurements do offer some information worth summarizing here

It was observed that the degree of plasticization was greatest when the material was preheated (to 135oC) and welded at 1400 RPM Also, higher rotational speed resulted in more plasticization regardless of preheating Hardness measurements indicated a significant decrease in the plasticized zone compared to the parent material Furthermore, the nugget hardness is directly related to the weld pitch as shown in Figure 4 Weld pitch

0.75" (19.0 mm)Dia Inconel 625 Rod

1.00" (25.4 mm) Dia

MA 957 Cylinder

9.0"

(229 mm) Figure 1: Dimensions of MA 957 ODS steel specimen

Figure 3: Micrographs of ODS MA 957: (a) nugget (right)-TMZ interface, (b) parent, (c) nugget zone

Table 2 Hardness Measurements

ODS specimen W-Re Pin tool

Figure 2: Friction stir welding setup for the MA 957

200 µm

200 µm

is likely related to the weld temperature, in which case the nugget hardness is directly related to weld temperature

There has been an apparent increase in grain size in the plasticized zone for all the welds; however, more work is needed to confirm this The decrease in hardness resulting from friction stir processing may be the result of this apparent increase in grain size in the nugget If additional investigation confirms this presumed increase in grain size, one might control such growth by liquid nitrogen quenching behind the weld Also, hardness may be regained by doing post weld heat-treatment It is the intent of the authors to perform TEM work on the nugget material to determine the grain size and yttria distribution

Figure 4: Comparison between Weld Pitch and Hardness

There is a decrease in the average weld nugget hardness compared to the parent material from approximately 373±21 HV to 225±22 HV Greater weld pitch, and likely related weld temperature, resulted in more plasticization, slightly greater nugget hardness No definitive grain size determination was made but morphology suggests an increase in grain size in the nugget TEM work is needed to determine the yttria morphology and distribution in the nugget

Conclusions

ODS MA 957 ferritic steel is plasticized without difficulty when stirred with W-Re pin tool Preheating to 135 OC improves the plasticization compared to not preheating Preheating has little effect on the resulting weld The value of preheating the material

above the ductile-brittle transition temperature to prevent pin tool damage was not established

Acknowledgements

Pacific Northwest National Laboratories and Advanced Materials Processing Center support this work under the contract number (4-12750)

Support from the AMP Center and Department of Materials and Metallurgical Engineering of SDSM&T is also acknowledged

References

2 Y.L Chen and A.R.Jones ‘Reduction of Porosity in Oxide Dispersion – Strengthened Alloys Produced by Powder Metallurgy’ Metallurgical and Materials Transactions A Volume 32A, August 2001-2077

3 Bimal K Kad, et al, ‘Optimization of ODS-Fe3Al Alloy properties for Heat Exchanger Applications’ University of California-San Diego, La Jolla, CA 92093

4 C Zakine, C Prioul, D Francois, ‘ Creep Behavior of ODS steels’ Materials Science and Engineering A 219 (1996) 102-108