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A note on the workability of porous-steel preforms
Journal of Materials Processing Technology, 25 (1991) 229-233
229
Elsevier
A note on the workability of porous-steel preforms Aly E1-Domiaty and Mostafa Shaker Faculty of Engineering and Technology, Suez Canal University, Port-Said, Egypt (Received November 11, 1989; accepted in revised form March 28, 1990)
Industrial Summary Powder forging may be defined as the process of converting a porous preform into a fully dense part by forging in an enclosed die cavity. The material utilization is high because flash is avoided, and complex components can be forged to near-net or net shape. Cracking during forging is a major problem and a method of characterizing workability where free-surface fracture is dominant, as is the case during upsetting, has been developed for wrought metals. In the present work, a trial has been undertaken to apply the well-established workability technique for fully-dense materials (wrought materials) to porous materials. The workability limits have been determined for different compacts having different densities, from which it is shown that the density is the parameter controlling the workability of porous preforms.
Introduction
Cracking and defects in metalworkingprocesses occur frequently because of the trend toward forming more complex shapes in difficult-to-form alloys. Historically, successful preform designs have been developedthrough iterative trialand-error procedures, using prior experience as a guide, but computer-aided manufacturing techniques are now emerging that minimize trial-and-error. Most bulk-forming processes such as forging involve stress states that are primarily compressive; however, secondary tensile-stresses interact with the local microstructural features to initiate fractures. The material plays a major role in determining workability, but the process is an equally important factor through control of the local tensile-stress state. Thus, both material and process parameters must be considered in an overall evaluation of workability. A workability evaluation method must therefore involve a test technique that subjects the material to secondary tensile stress/microstructure combinations, similar to those encountered in the actual processes. For the production of low-alloy steel parts, powder forging has received the most attention commercially. The basic steps of the powder-forging process are: (i) production of a preform; (ii) preheating of the preform and tooling 0924-0136/91/$03.50 © 1991--Elsevier Science Publishers B.V.
230 (for warm- and hot-forging); (iii) forging; and (iv) part finishing, heat treatment and coating. The first step is identical to conventional press/sinter practice. The shape and density of the preform are important with regard to the final properties of the product. Forging involves the deformation of a simple preform-shape into a final forged-shape. Overall, the deformation of the preform is predominantly strain in the axial direction. The selection of a preform to be forged to a final part-shape is the critical step in powder forging. The preform-shape and -density directly affect the magnitude of the mechanical properties of the final part. Porosity in the preform reduces the workability of the preform material drastically, which means that cracking during forging can be a serious problem. A method of characterizing workability where free surface fracture is dominant, as is the case during upsetting, has been developed for wrought materials [ 1 ]. Figure 1 shows: a typical workability line (labeled the 'fracture locus' ); a typical strain-path, which is the local surface strain-state generated during forging; and the effects that process variables have on these lines. Parts that can be forged with a strain level that remains below the workability line can be forged without cracking. If the strain levels are above the line (i.e. in the unsafe region), surface cracking will take place during forging. Surface cracking may be avoided by adjusting the preform density, the forging temperature or the preform shape. The position of the workability line is dependent on preform density and on the forging temperature. The strain to which a preform
UPSETTING
!
UPSET TESTS Fracture~ tocus
%~
I
I0 8
h/d =1.5 Lubricated
~"~ '~''. frictiofliess ~..
h/d= 1.0 smoothdies~
~. ~,'~4b. ~. '~--~
1 _~0.6 uJ"
hid=0.75 | roughd~s |
( h o m o g e ~ ' ~ , ' ~ ldl~%.
~
c" "~
"-{o.~ o2
-L0
-0.8
-06
-0./,
-0.2
Compressive Strain Ezz Fig. 1. The workability limit from the upsetting test (after Ref. [ 1 ] ).
0
231 is subjected is dependent on the aspect ratio (geometry), the friction condition between the preform and forging dies and the preform density. The object of the present work, therefore, is to determine the forgability (workability) line at room temperature, considering the effect of change in the density of the preform. Experimental work Steel powder with a chemical composition (wt. To) of C: 0.2; Mn: 0.5; S: 0.04; P: 0.04; Fe: balance; and a range of particle size of from 75-105 ~m, was used to prepare twenty-seven preforms. Three preforms were compacted under the same pressure and sintered using the same temperature and time, this procedure giving nine different groups of preforms, each group consisting of three preforms having the same density. The conditions used in compaction and sintering are shown in Table 1. Upsetting of a cylindrical preform provides a deformation test in which the average stress-state is inherently similar to that of bulk-deformation processes. Barreling of the cylindrical surface, which is a disadvantage for flowstress measurements, actually furnishes considerable flexibility for workability testing. Variation of the frictional conditions of the contact surface and of the aspect ratio of the cylinder, leads to change of barrel curvature and, consequently, to change of the secondary tensile hoop-stress developed at the bulge surface. Thus, the upset can be used to provide a wide range of stresses and strains and can be used to evaluate workability. In the present work, upset tests were carried out under different frictional conditions using cylindrical preforms, of 20 mm diameter. Different strainTABLE 1 The compactingand sinteringparameters CompactingPressure Sintering (MN/m2) Temperature ( °C) 283
424.5
566
900 1000 1100 900 1000 1100 900 1000 1100
Time(min) 15 30 60 15 30 60 15 30 60
232 paths were generated by using different height-to-diameter ratios and frictional conditions. For the determination of the limit strains, the tests were stopped when cracks were observed by eye on the surface of the specimen during the forming process. The preforms were taken out from the machine and the final height and the final diameter were measured. The diameter was measured at three locations: one measurement at the mid-height and two measurements at positions very close to the top and the bottom of the preform• The average diameter was then calculated. Axial and hoop strains were calculated from: Axial strain ez = In (Hf/Ho) Hoop strain eo = in (Df/Do) where Do and Ho are the initial diameter and the initial height of the preform; and Df and Hf are the final diameter and the final height after the visual detection of cracking.
Experimental results The workability limits for the steel powder preforms are shown in Fig. 2, which represents the forming-limit diagram for free-surface fracture in the upsetting deformation process. The higher forming-limit (line 1 ) was obtained
4,
0.5 U.Ja .E 0.,~
~
0.3
a. 0 0 Z
g/crn ~
0.2
0.1 -01g
-0'.S
-~,
-0'.6
-0'.S
-0'.~
Axial Strain
Fig. 2. The experimental workability-limits.
-01.3 Ezz
-0~
-01.,
o!,
i 0.2
233
for higher-density preforms (p=6.6 gr/cm3). As the density decreases, the forming limit reduces. Line 3 is the lowest forming-limit, which was obtained for preform densities of 4.8 gr/cm 3, and again a decrease in density reduces the forming limit. The density effect is more obvious close to the plane-strain condition (e~z= 0), whilst the effect is not clear near to the homogeneous-compression zone, (see Fig. 2). The surface fracture-strain loci (Lines 1, 2 and 3) given in Fig. 2 can be considered as forming-limit diagrams, such as those for sheet-metal forming. In this form the data can be treated as an empirical fracture-criterion and utilized for trouble-shooting workability problems. The forming limits (Lines 1, 2 and 3 ) offer a graphical method for the presentation of the material and the process influences on workability. Material effects are embodied in each fracture line, the height of which depends on microstructural fractures, composition and density. Process parameters of temperature and strain rate also affect the position of the fracture line, but in the present work the process parameters were the same for the three different fracture lines (Lines 1, 2 and 3). Workability analysis is usually carried out by superimposing the strain path for any forming process of interest onto the material forming-limit. Intersection of the strain path with the forming limit indicates that fracture is likely and gives the strains at fracture. Alteration of the strain path (through modification of the geometry of the component or of the friction conditions between the component and the forming dies), or movement of the material forminglimit (through re-selection or modification of the material by choosing a greater preform-density), can then be attempted, to reach combinations where the strain path does not intersect the forming limit before the component is completely formed. Conclusions
The main conclusion that can be drawn out from the present work is the material density has a great effect on the forming-limit diagram. It has been shown that as the material density increases, the height of the forming limit increases and the ability of the material to deform increases.
References Kuhn, H.A., Workability in hot and cold deformation processes - Test methods, Criteria and Applications, Proc. Syrup. Formability Analysis, Modeling and Experimentation, S.S. Hecker, A.K. Ghosh and H.L. Gegel, (Eds.), A&M Metals Flow and fracture Committee, Chicago, Illinois, Oct. 24/25. 1977, pp. 259-280.
229
Elsevier
A note on the workability of porous-steel preforms Aly E1-Domiaty and Mostafa Shaker Faculty of Engineering and Technology, Suez Canal University, Port-Said, Egypt (Received November 11, 1989; accepted in revised form March 28, 1990)
Industrial Summary Powder forging may be defined as the process of converting a porous preform into a fully dense part by forging in an enclosed die cavity. The material utilization is high because flash is avoided, and complex components can be forged to near-net or net shape. Cracking during forging is a major problem and a method of characterizing workability where free-surface fracture is dominant, as is the case during upsetting, has been developed for wrought metals. In the present work, a trial has been undertaken to apply the well-established workability technique for fully-dense materials (wrought materials) to porous materials. The workability limits have been determined for different compacts having different densities, from which it is shown that the density is the parameter controlling the workability of porous preforms.
Introduction
Cracking and defects in metalworkingprocesses occur frequently because of the trend toward forming more complex shapes in difficult-to-form alloys. Historically, successful preform designs have been developedthrough iterative trialand-error procedures, using prior experience as a guide, but computer-aided manufacturing techniques are now emerging that minimize trial-and-error. Most bulk-forming processes such as forging involve stress states that are primarily compressive; however, secondary tensile-stresses interact with the local microstructural features to initiate fractures. The material plays a major role in determining workability, but the process is an equally important factor through control of the local tensile-stress state. Thus, both material and process parameters must be considered in an overall evaluation of workability. A workability evaluation method must therefore involve a test technique that subjects the material to secondary tensile stress/microstructure combinations, similar to those encountered in the actual processes. For the production of low-alloy steel parts, powder forging has received the most attention commercially. The basic steps of the powder-forging process are: (i) production of a preform; (ii) preheating of the preform and tooling 0924-0136/91/$03.50 © 1991--Elsevier Science Publishers B.V.
230 (for warm- and hot-forging); (iii) forging; and (iv) part finishing, heat treatment and coating. The first step is identical to conventional press/sinter practice. The shape and density of the preform are important with regard to the final properties of the product. Forging involves the deformation of a simple preform-shape into a final forged-shape. Overall, the deformation of the preform is predominantly strain in the axial direction. The selection of a preform to be forged to a final part-shape is the critical step in powder forging. The preform-shape and -density directly affect the magnitude of the mechanical properties of the final part. Porosity in the preform reduces the workability of the preform material drastically, which means that cracking during forging can be a serious problem. A method of characterizing workability where free surface fracture is dominant, as is the case during upsetting, has been developed for wrought materials [ 1 ]. Figure 1 shows: a typical workability line (labeled the 'fracture locus' ); a typical strain-path, which is the local surface strain-state generated during forging; and the effects that process variables have on these lines. Parts that can be forged with a strain level that remains below the workability line can be forged without cracking. If the strain levels are above the line (i.e. in the unsafe region), surface cracking will take place during forging. Surface cracking may be avoided by adjusting the preform density, the forging temperature or the preform shape. The position of the workability line is dependent on preform density and on the forging temperature. The strain to which a preform
UPSETTING
!
UPSET TESTS Fracture~ tocus
%~
I
I0 8
h/d =1.5 Lubricated
~"~ '~''. frictiofliess ~..
h/d= 1.0 smoothdies~
~. ~,'~4b. ~. '~--~
1 _~0.6 uJ"
hid=0.75 | roughd~s |
( h o m o g e ~ ' ~ , ' ~ ldl~%.
~
c" "~
"-{o.~ o2
-L0
-0.8
-06
-0./,
-0.2
Compressive Strain Ezz Fig. 1. The workability limit from the upsetting test (after Ref. [ 1 ] ).
0
231 is subjected is dependent on the aspect ratio (geometry), the friction condition between the preform and forging dies and the preform density. The object of the present work, therefore, is to determine the forgability (workability) line at room temperature, considering the effect of change in the density of the preform. Experimental work Steel powder with a chemical composition (wt. To) of C: 0.2; Mn: 0.5; S: 0.04; P: 0.04; Fe: balance; and a range of particle size of from 75-105 ~m, was used to prepare twenty-seven preforms. Three preforms were compacted under the same pressure and sintered using the same temperature and time, this procedure giving nine different groups of preforms, each group consisting of three preforms having the same density. The conditions used in compaction and sintering are shown in Table 1. Upsetting of a cylindrical preform provides a deformation test in which the average stress-state is inherently similar to that of bulk-deformation processes. Barreling of the cylindrical surface, which is a disadvantage for flowstress measurements, actually furnishes considerable flexibility for workability testing. Variation of the frictional conditions of the contact surface and of the aspect ratio of the cylinder, leads to change of barrel curvature and, consequently, to change of the secondary tensile hoop-stress developed at the bulge surface. Thus, the upset can be used to provide a wide range of stresses and strains and can be used to evaluate workability. In the present work, upset tests were carried out under different frictional conditions using cylindrical preforms, of 20 mm diameter. Different strainTABLE 1 The compactingand sinteringparameters CompactingPressure Sintering (MN/m2) Temperature ( °C) 283
424.5
566
900 1000 1100 900 1000 1100 900 1000 1100
Time(min) 15 30 60 15 30 60 15 30 60
232 paths were generated by using different height-to-diameter ratios and frictional conditions. For the determination of the limit strains, the tests were stopped when cracks were observed by eye on the surface of the specimen during the forming process. The preforms were taken out from the machine and the final height and the final diameter were measured. The diameter was measured at three locations: one measurement at the mid-height and two measurements at positions very close to the top and the bottom of the preform• The average diameter was then calculated. Axial and hoop strains were calculated from: Axial strain ez = In (Hf/Ho) Hoop strain eo = in (Df/Do) where Do and Ho are the initial diameter and the initial height of the preform; and Df and Hf are the final diameter and the final height after the visual detection of cracking.
Experimental results The workability limits for the steel powder preforms are shown in Fig. 2, which represents the forming-limit diagram for free-surface fracture in the upsetting deformation process. The higher forming-limit (line 1 ) was obtained
4,
0.5 U.Ja .E 0.,~
~
0.3
a. 0 0 Z
g/crn ~
0.2
0.1 -01g
-0'.S
-~,
-0'.6
-0'.S
-0'.~
Axial Strain
Fig. 2. The experimental workability-limits.
-01.3 Ezz
-0~
-01.,
o!,
i 0.2
233
for higher-density preforms (p=6.6 gr/cm3). As the density decreases, the forming limit reduces. Line 3 is the lowest forming-limit, which was obtained for preform densities of 4.8 gr/cm 3, and again a decrease in density reduces the forming limit. The density effect is more obvious close to the plane-strain condition (e~z= 0), whilst the effect is not clear near to the homogeneous-compression zone, (see Fig. 2). The surface fracture-strain loci (Lines 1, 2 and 3) given in Fig. 2 can be considered as forming-limit diagrams, such as those for sheet-metal forming. In this form the data can be treated as an empirical fracture-criterion and utilized for trouble-shooting workability problems. The forming limits (Lines 1, 2 and 3 ) offer a graphical method for the presentation of the material and the process influences on workability. Material effects are embodied in each fracture line, the height of which depends on microstructural fractures, composition and density. Process parameters of temperature and strain rate also affect the position of the fracture line, but in the present work the process parameters were the same for the three different fracture lines (Lines 1, 2 and 3). Workability analysis is usually carried out by superimposing the strain path for any forming process of interest onto the material forming-limit. Intersection of the strain path with the forming limit indicates that fracture is likely and gives the strains at fracture. Alteration of the strain path (through modification of the geometry of the component or of the friction conditions between the component and the forming dies), or movement of the material forminglimit (through re-selection or modification of the material by choosing a greater preform-density), can then be attempted, to reach combinations where the strain path does not intersect the forming limit before the component is completely formed. Conclusions
The main conclusion that can be drawn out from the present work is the material density has a great effect on the forming-limit diagram. It has been shown that as the material density increases, the height of the forming limit increases and the ability of the material to deform increases.
References Kuhn, H.A., Workability in hot and cold deformation processes - Test methods, Criteria and Applications, Proc. Syrup. Formability Analysis, Modeling and Experimentation, S.S. Hecker, A.K. Ghosh and H.L. Gegel, (Eds.), A&M Metals Flow and fracture Committee, Chicago, Illinois, Oct. 24/25. 1977, pp. 259-280.