UDK 669.245:621.74.042:621.7.016.2	ISSN 1580-2949
Original scientific article/Izvirni znanstveni članek	MTAEC9, 49(1)15(2015)
INFLUENCE OF THE HIP PROCESS ON THE PROPERTIES OF AS-CAST Ni-BASED ALLOYS
VPLIV VROČEGA IZOSTATSKEGA STISKANJA NA LASTNOSTI Ni-ZLITIN Z LITO STRUKTURO
Jitka Malcharczikova1, Martin Pohludka1, Vit Michenka2, Tomaš Cegan1, Jan Jurica1, Miroslav Kursa1
1VŠB-Technical University of Ostrava, Faculty of Metallurgy and Materials Engineering, Department of Non-Ferrous Metals, Refining Processes and Materials Recycling, Regional Materials Science and Technology Centre, 17. listopadu 15/2172, 708 33 Ostrava - Poruba, Czech
Republic
2VÜH@ a.s., Dobra 240, 739 51 Dobra, Czech Republic jitka.malcharczikova@vsb.cz
Prejem rokopisa - received: 2013-09-30; sprejem za objavo - accepted for publication: 2014-01-30
The main goal of this work was to evaluate the application of the samples prepared by centrifugal casting as the test samples for a tensile test. Selected types of modified superalloys were prepared as experimental samples. The samples were molten by vacuum-induction melting and then cast centrifugally into a shaped graphite mould. The final castings had the shape corresponding approximately to the test specimens. Some of the samples were subjected to hot isostatic pressing (HIP). After HIP the castings contained substantially fewer casting defects. Selected mechanical properties were compared for the samples in the as-cast state and after HIP. For the majority of the investigated alloys the HIP process led to an increase in the strength and ductility.
Keywords: Ni-based alloys, centrifugal casting, hot isostatic pressing, mechanical properties
Glavni cilj tega dela je bila ocena uporabnosti vzorcev za natezni preizkus, izdelanih s centrifugalnim litjem. Preizkusni vzorci so bili izdelani iz izbrane vrste modificiranih superzlitin. Vzorci so bili staljeni v vakuumski indukcijski peči in centrifugalno uliti v izoblikovane grafitne forme. Ulitki so imeli obliko, ki ustreza vzorcem za natezne preizkuse. Del vzorcev je bil vroče izostatsko stisnjen (HIP). Ulitki so imeli po obdelavi HIP občutno manj livarskih napak. Izbrane mehanske lastnosti so bile primerjane pri vzorcih v litem stanju in po obdelavi HIP. Pri večini preiskovanih zlitin je obdelava HIP povzročila povečanje trdnosti in duktilnosti.
Ključne besede: Ni-zlitine, centrifugalno litje, vroče izostatsko stiskanje, mehanske lastnosti
damaged8 and the testing of the mechanical properties of 1 INTRODUCTION	this type of material is, therefore, very difficult.
, , „ .... , „ , , .	The most frequent use of hot isostatic pressing is a
Ni-based alloys, including the alloys based on inter^	^^ „ , , • ,4019 t,. ,
' , „ .„ .	compaction of powder materials4,912. There are several metallic compounds (IMC) and superalloys are still in
papers dealing with the compaction of nickel-alloy
the forefront of interest. These alloys that can be use^	^ ^^^ ^ Z tttf. .u ,ii315 m- 1 1 n
...	castings with the HIP method1315. Nickel-alloy castings even at high temperatures are continuously investigated
^ , . „ , . , , .	consist of large and irregular grains of the y phase. This
thanks to1 toeir excellent mechanical and corrosion phase continues to coarse during the HIP process at a
properties1,2. The technology of hot isostatic pressing high temperature and pressure4,9,12,15. In some cases,
(HIP) belongs, within the powder metallurgy, to the carbides and the ^ phase precipitate at the grain methods of hot sintering, during which the powder boundaries after
a HIP process11. All of the above has a achieves the required properties as a reesult of the effect negative influence on the mechanical properties of nickel of predominantly physical processes. lhe HIP method is alloys. Therefore, after a HIP process, nickel alloys
successfully used also for additional compacting of	should be heat treated by solution annealing and
castings, ensuring a homogenisation of the structure and	aging11,12.
a reduction of the pores and shrinkage cavities. This
method is used also for the alloys based on nickel and
Ni3Al. Numerous data were published on sintering	2 EXPERIMENT
powder-pressed pieces made of the materials based on
Ni3-6. Kim7 describes a procedure for additional com- Selected types of modified superalloys IC50, IC396,
pacting of the castings made of alloys IC396. Many	IC221M and IC438 were prepared as the experimental
problems occur when processing the samples with the	samples. They were molten by vacuum-induction melt-
usual production methods. The methodology for process-	ing and then cast centrifugally into a shaped graphite
ing to the specified dimensions is highly demanding and	mould (Figure 1). The final castings had the shape
expensive. A large amount of experimental material is	corresponding approximately to the test specimens
Figure 1: Graphite-mould drawing Slika 1: Risba grafitne forme
(Figure 2). The compositions of the alloys were derived from the already verified and industrially used types of alloys for the given applications, and their compositions are presented in Table 1 (A: the state after HIP, B: the as-cast state).
Table 1: Chemical composition of the used samples in mass fractions, w/%
Tabela 1: Kemijska sestava uporabljenih vzorcev v masnih deležih,
w/%
Alloy	Sample	Amounts of elements					
		Al	Cr	Mo	Zr	B	Ni
IC50	3A, 3B	11.30	-	-	0.60	0.01	88.08
IC396	4A, 4B	7.98	7.72	3.02	0.85	0.01	80.42
IC221M	5A, 5B	8.00	7.70	1.43	1.70	0.01	81.10
IC438	6A, 6B	8.10	5.23	7.02	0.13	0.01	79.52
A flaw-detection method with X-ray radiation was used for determining the quality of the castings. The pattern of distribution and the type of casting defects were similar in all the centrifugally cast samples. Figure 3 schematically shows their location and size. Defects occurred on the tapered part of the test piece (the tensile bar body), or in the direction towards the head, on the part of the casting without a riser.
2.1 Hot isostatic pressing (HIP)
It was established after a flaw-detection analysis, that the castings contained numerous casting defects. That is why some of the samples were subjected to hot isostatic pressing (HIP). The conditions for HIP were derived from the literature sources3-7. Hot isostatic pressing was performed on a press of the EPSI company. In the first stage of the additional compacting we chose a pressure of 100 MPa, the time of 4 h and a temperature of 1100
Figure 3: Scheme of casting defects Slika 3: Shematski prikaz livarskih napak
°C. Using flaw detection, we established that the castings subjected to HIP under the pressure of 100 MPa still contained a considerable amount of casting defects. For this reason we increased the pressure to 150 MPa for the HIP process, while the time of 4 h and the temperature of 1100 °C remained unchanged. However, even after applying a higher pressure the castings still contained casting defects, but to a much smaller extent than in the state just after centrifugal casting.
Due to sufficient compaction, hot isostatic pressing can create a homogenous structure with a reduced number of pores and casting defects (shrinkage cavities and contractions) even in castings. This was confirmed in our case as well. After HIP, the castings contained substantially fewer casting defects.
2.2 Testing of the mechanical properties
Some of the samples in the as-cast state and the samples after HIP were ground to precise dimensions of the test piece and the tensile testing was performed. Short tensile bars with a length of 55 mm and a diameter of the central part of the bar of 5 mm were prepared by lathe turning. The strain rate was approximately 9.1 • 10-4 s-1 for all the samples. Table 2 gives the measured values of the tensile stress at yield, the ultimate engineering stress and the proportional elongation after braking. The table also contains the average values of the porosity and micro-hardness.
Table 2: Obtained mechanical characteristics Tabela 2: Dobljene mehanske lastnosti
Figure 2: Sample after the casting Slika 2: Vzorec po ulivanju
Sample No.	Rp0.2/ MPa	Rm/ MPa	A/ %	Porosity %	HV 0.05
3A	239	949	32	0.0309	302
3B	313	824	19	0.0393	249
4A	539	668	6	0.0123	353
4B	720	754	6	0.0743	336
5A	550	995	28	0.0486	354
5B	653	776	7	0.0496	324
6A	463	807	10	0.0589	359
6B	771	817	5	0.0614	351
3 DISCUSSION
We performed a comparison of the selected mechanical properties in the as-cast state and after HIP. In the majority of the investigated alloys the HIP process led to an increase in the determined strength and elongation. It is evident from the comparison of the values in Table 2 that a significant enhancement of the mechanical properties after the HIP process occurred in samples 3 and 5, while no significant improvement of these values took place in samples 4 and 6. The Rm value of sample 4 does not correspond to the expected values. The casting probably contained a larger volume of casting defects before the HIP process. Figure 4 shows the stress-strain curves for samples 3 and 5 in the as-cast state and after HIP. It may be concluded from the results that the process of additional compaction of the castings with the HIP method under the given conditions has a positive influence.
Figures 5 to 8 show the structures of samples 3 and 5 in the as-cast state and after HIP. The HIP process caused a coarsening of the grains as observed on Figures 5 and 7. This change in the structure was reflected by the
changes in the yield strength. The value of the yield strength dropped for all the types of alloys (Table 2, Figure 4). The gained results were already confirmed in the conclusions of the papers of other authors4-15. According to these results every nickel alloy reacts differently to the HIP process. To avoid a decrease in the mechanical properties of the castings, their grains should be as small as possible and the microstructural changes
Figure 5: Microstructure of sample 3A Slika 5: Mikrostruktura vzorca 3A
Figure 6: Microstructure of sample 3B Slika 6: Mikrostruktura vzorca 3B
Figure 4: Stress-strain curves for: a) samples No. 3 and b) samples No. 5
Slika 4: Krivulje napetost - raztezek za: a) vzorca st. 3 in b) vzorca st. 5
Figure 7: Microstructure of sample 5A Slika 7: Mikrostruktura vzorca 5A
Acknowledgement
Figure 8: Microstructure of sample 5B Slika 8: Mikrostruktura vzorca 5B
of these alloys should be studied during the HIP process. The porosity and micro-hardness are approximately the same for both the as-cast state and the HIP state. However, for all the experimental alloys the value of porosity decreased after the HIP process.
In practise it is possible to prepare the samples for evaluating the selected mechanical properties using the method of centrifugal casting of precision castings. The use of precision castings as the test specimens brings considerable savings of the materials and machining costs. The alloys of this type are used for high-temperature applications and a modification of the alloy composition is still performed16.
4 CONCLUSIONS
Modified Ni superalloys were prepared as the experimental samples. The samples were molten by vacuum-induction melting and then cast centrifugally into a shaped graphite mould. The final castings had the shape corresponding to the test specimens. After a flaw-detection analysis, it was established that the castings contained numerous casting defects. As a result, some of the samples were subjected to hot isostatic pressing (HIP). Hot isostatic pressing reduced the number of the pores and casting defects in the castings. We performed a comparison of selected mechanical properties in the as-cast state and after HIP. For the majority of the investigated alloys, the HIP process led to an increase in the strength and ductility.
The presented results were obtained within research project TA 01011128 "Research and development of centrifugal casting technology of the Ni-based inter-metallic compounds" and project CZ.1.05/2.1.00/01.0040 "Regional materials science and technology centre".
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