Materials Science & Technology

FATIGUE CRACK PROPAGATION BEHAVIOR OF BRAZED STEEL JOINTS Dr. Tanya Aycan Başer EMPA-Swiss Federal Laboratories for Materials Testing and Research Laboratory for Joining Interface and Technology, Dübendorf, Switzerland Materials Science & Technology

Outline Introduction Mechanical properties of materials What is fatigue? Why brazing? Problems occurred during brazing Application area of brazing Materials and Methods Base material and filler metals Brazing and heat treatment Experimental procedures Results and discussion Fatigue Crack Propagation (da/dn-δk) Curves Microstructural Investigations Fractographic Investigations Brazing Quality Effect of shield gas on mechanical properties Effect of sample geometry on mechanical properties

Introduction Stress / MPa Elastik bolge () MALZEMELERİN MEKANİK ÖZELLİKLERİ GERILIM-GERINIM DIYAGRAMI GEVREK-SUNEK DAVRANIS 2000 1500 Cekme dayanimi x Akma dayanimi x kopma dayanimi 1000 500 Plastik bolge 0 0 2 4 6 8 10 12 14 16 18 20 Strain / % Cekme testi ()

Introduction YORULMA NEDİR? Malzemeyi zorlayan gerilmeler zaman icinde degisecek olursa malzeme cekme deneyinde elde edilen kopma degerinin altinda bir gerilmede sunek de olsa plastik sekil degistirmeden kirilabilir. Bu olaya yorulma denir. Yukleme ve bosalmanin periyodik olarak cok sayida tekrari sonucunda malzemede yipranmalar meydana gelir. Bunun nedeni yukun siddetinden cok onun periyodik olarak uzun bir sure uygulanmasidir. İc mekanizmasi oldukca karisik olan bu olaya malzemenin yorulmasi denir. Yorulma 3 asamada gerceklesir: 1-Catlak baslangici 2-Catlak ilerlemesi 3-Kirilma

Introduction WHY BRAZING? Welding fusion takes place with melting of both the base metal and usually a filler metal. To join metals by applying heat, sometimes with pressure and sometimes with an intermediate or filler metal having a high melting point. Brazing is a process for joining similar or dissimilar metals using a filler metal and heating them the liquidus of the filler metals above 840 F (450 C), and below the solidus of the base metals Soldering has the same definition as brazing except for the fact that the filler metal used has a liquidus below 840 F (450 C) and below the solidus of the base metals. Brazing process is used because of the compressor impeller geometry Defects such as incomplete gap filling, pores or cracks may be formed during brazing and they can act as stress concentration sites in the brazing zone. Brazing Under cyclic mechanical loading, fatigue cracks can initiate and propagate from these defects, leading to spontaneous failure.

Introduction Brazing is a quick and low-cost brazing method to produce strong joints and it is used in the aerospace and other industries as well as for power generation, e.g. turbine parts or compressor impellers. /www.nasa.org/ /www.airbus.com/ /www.geae.com/ /www.manturbo.com/

Materials and Method Base material and filler metals The soft martensitic stainless steel X3CrNiMo13-4 was used as base material. Chemical composition of X3CrNiMo13-4 Element C Si Mn P S Cr Mo N Ni Min. 12.00 0.30 0.02 3.50 Max. 0.05 0.70 1.50 0.04 0.01 14.00 0.70 4.50 As filler metal, foils of the binary alloy Au-18Ni with a thickness of 100 μm were applied (Melting temperature is around 800 C). Brazing and heat treatment Brazing was performed in an industrial shielding gas (93 vol.-% Ar, 7 vol.-% H 2 ) furnace at a temperature of 1020 C for 20 minutes. After brazing, the specimens were tempered at 520 C for 5.5 h in nitrogen atmosphere.

Materials and Method Experimental procedures Fatigue crack propagation tests were performed on a resonant testing machine. Geometry of the DCB specimen (90 x 60 x 8 mm) Fractured specimens were investigated by SEM. Set-up of the fatigue crack propagation test ASTM E647

da/dn [m/cycle] Results Fatigue Crack Propagation (da/dn-δk) Curves 1E-4 1E-5 1E-6 1E-7 The Paris equation: da dn CK 1E-8 da: difference of crack length dn: difference of number of cycles 1E-9 C,n: experimentally measured material constants R = 0.1 1E-10 R = 0.3 R = 0.5 R = 0.7 1E-11 1 10 100 K [MPa m 1/2 ] Calculated C and n parameters at different R values n R C n K I F Bh a h 813.25 12 0.5 2 a h 0.5 0.1 1.309E-22 11.17 0.3 4.071E-23 12.17 0.5 7.234E-22 12.64 0.7 8.489E-21 12.81

da/dn [m/cycle] Results The Paris Exponent 1E-5 1E-6 1E-7 Material n 1E-8 1E-9 1E-10 1E-11 1 10 K [MPa m 1/2 ] R = 0.1 R = 0.3 R = 0.5 R = 0.7 R. H. Dauskardt, Acta Metall Mater. Vol 41 9 (1993) 2765. Metals 3-4 Ceramics -50 Brazed components 11-13?

SEM Taramali elektron mikroskobu Çalışma Prensibi Taramalı Elektron Mikroskobu üç temel kısımdan oluşmaktadır: Optik Kolonda elektron demetinin kaynağı olan elektron tabancası, elektronları numuneye doğru hızlandırmak için yüksek gerilimin uygulandığı anot plakası, ince elektron demeti elde etmek için yoğunlaştırıcı mercekler, demeti numune üzerinde odaklamak için objektif merceği, bu merceğe bağlı çeşitli çapta apatürler ve elektron demetinin numune yüzeyini taraması için tarama bobinleri yer almaktadır. Numune hucresine numune yerlestirilir. Görüntü sisteminde elektron demeti ile numune girişimi sonucunda oluşan çeşitli elektron ve ışımaları toplayan dedektörler, bunların sinyal çoğaltıcıları ve numune yüzeyinde elektron demetini görüntü ekranıyla senkronize tarayan manyetik bobinler bulunmaktadır. Nasil goruntu elde edilir? Taramalı Elektron Mikroskobunda (SEM) görüntü, yüksek gerilim ile hızlandırılmış elektronların numune üzerine odaklanması, bu elektron demetinin numune yüzeyinde taratılması sırasında elektron ve numune atomları arasında oluşan çeşitli girişimler sonucunda meydana gelen etkilerin uygun algılayıclarda toplanması ve sinyal güçlendiricilerinden geçirildikten sonra bir katot ışınları tüpünün ekranına aktarılmasıyla elde edilir.

Results Microstructural Investigations SEM-cross section X3CrNiMo13-4 steel Ni-rich Solid solution Au-18Ni braze Diffusion zone 100 µm Au-rich Solid solution 20 µm

Results Microstructural Investigations SEM-cross section 20 µm 20 µm 20 µm Crack tip 5 µm

Results Microstructural Investigations SEM-cross section 200 µm 200 µm 100 µm 20 µm

Results Microstructural Investigations SEM-cross section Pre-damaged zones well ahead of the crack tip 20 µm 95 µm Crack tip 32 µm 20 µm

Results Fractographic Investigations SEM-cross section 200 µm 20 µm 20 µm 50 µm

Results Fractographic Investigations stereo SEM 200 µm pores 3 mm 200 µm The stepped nature of the fracture pattern is clearly evident

Fractographic Investigations Brittle fracture ductile fracture transcrystalline steel Steel wire 1018 steel Al alloy BMG Cu alloy

Results Fractographic Investigations Brittle or ductile?

Results Fractographic Investigations 50 µm 20 µm Plastic deformation features containing ductile dimples

Discussion Damage and Fracture Behaviour of Brazed Joints Under Cyclic Loading notch X3CrNiMo13-4 Au-18Ni X3CrNiMo13-4 After crack initiation, high stresses can lead to the formation of cavities well ahead of the crack tip. New cavities develop and grow every loading cycle. The fatigue crack then propagates along these predamaged zones and coalescence and final failure occurs. High Paris exponent, n, was explained by the triaxial stress state in the filler metal, which is a result of the different elastic-plastic material properties of the filler metal and the base material.

Brazing quality Defects such as incomplete gap filling, pores or cracks may be formed during brazing and they can act as stress concentration sites in the brazing zone. Under cyclic mechanical loading, fatigue cracks can initiate and propagate from these defects, leading to spontaneous failure. Therefore, defect assesment of brazed components should be considered. 200 µm

Brazing quality Brazed steel plates of different batches The addition of hydrogen to the argon allows removing the oxide film on the stainless steel surface, which is essential for filler metal wetting. BUT... brazed steel plates shielding gas Batch A 93 vol.-% Ar, 7 vol.-% H 2 Batch B, C 93 vol.-% Ar, 7 vol.-% H 2 Batch D, E 100 vol.-% H 2 Batch F, G 100 vol.-% H 2

Brazing quality (93 vol.-% Ar, 7 vol.-% H 2 ) 100 µm (100 vol.-% H 2 ) 100 µm

Materials and Method Base material and filler metals The soft martensitic stainless steel X3CrNiMo13-4 was used as base material. As filler metal, foils of the binary alloy Au-18Ni with a thickness of 100 μm were applied. Brazing and heat treatment Brazing was performed in an industrial shielding gas (93 vol.-% Ar, 7 vol.-% H 2 and 100 vol.-% H 2 ) furnace at a temperature of 1020 C for 20 minutes. After brazing, the specimens were tempered at 520 C for 5.5 h in nitrogen atmosphere. Experimental procedures Fatigue tests were performed on a standard electro-mechanical and servohydrolic testing machine. Fractured specimens were investigated by SEM. Geometry of the t-joint specimen (t=16mm, W=8 mm) standart electro-mechanical testing machine Servohydrolic testing machine

Brazing quality brazing σ nom = 700 MPa N = 8304 σ nom = 650 MPa N = 11787 σ nom = 700 MPa N = 8642 crack standart round specimen (Ø 1 =5 mm, Ø 2 =4 mm) Brazing zone! In general, fracture occured on the base material instead of the brazing zone (100 vol.-% H 2 ) Specimen geometry shielding gas R m [MPa] Base material - 975±25 T-joint 93 vol.-% Ar, 7 vol.-% H 2 882±15.7 100 vol.-% H 2 1120±4.8 Standart round 100 vol.-% H 2 1084±3.6 The addition of hydrogen to the argon allows removing the oxide film on the stainless steel surface Joint strength was improved under 100% H 2 atmosphere

Brazing quality S-N curves nom, max (MPa) 1200 1100 1000 900 800 700 600 500 400 300 T-joint-defect free-93 vol.-% Ar, 7 vol.-% H 2 T-joint-defect free-100 vol.-% H 2 round shape 100 vol.-% H 2 200 10 2 10 3 10 4 10 5 N f N u =20 000 cycles The only difference in between these specimens is different shielding gases. Brazing quality was improved under 100 vol.-% H 2 Fracture occurred on the base material in specimens which were brazed under 100 vol.-% H 2

Results Fractographic Investigations- Comparison stereo SEM σ nom = 400 MPa N = 5630 (93 vol.-% Ar, 7 vol.-% H 2 ) The step fractured pattern σ nom = 650 MPa N = 11787 3 mm 200 µm Stronger bonding was obtained and better interface reaction occured under 100% H 2 atmosphere The step fractured pattern could not be observed (100 vol.-% H 2 ) 3 mm 200 µm

TEŞEKKÜRLER

Additional-I Table 1. Chemical composition of X3CrNiMo13-4. Element C Si Mn P S Cr Mo N Ni Min. 12.00 0.30 0.02 3.50 Max. 0.05 0.70 1.50 0.04 0.01 14.00 0.70 4.50 Table 2. Mechanical properties of X3CrNiMo 13-4 and X3CrNiMo13-4 Au-18Ni braze joints. R p0.2 [MPa] R m [MPa] A 5 [%] τ e [MPa] τ max [MPa] K Ic [MPa m 0.5 ] X3CrNiMo-13-4 920 ± 5 975 ± 25 17.5 ± 2.5 620 ± 5 660 ± 10 ~270 X3CrNiMo13-4 -AuNi18 923 ± 7 976 ± 15 6 ± 0.5 245 ± 10 539 ± 7 49 ± 1.5

Additional-II The stress intensity for mode I loading, KI, as a function of the specimen geometry and the applied load can be calculated according to; K I F Bh a h 813.25 12 0.5 a h 2 0.5 where F is the applied force, B and h the specimen geometry and a the total crack length measured from the load initiation point.

a [mm] Results Fatigue Crack Growth Curves 40 30 ΔF 1 = 6.8 kn ΔK 1 = 23 MPa m 1/2 ΔF 1, ΔF 2, ΔF 3 =constant 20 10 0 40 30 ΔF 2 = 5.7 kn ΔK 2 = 19 MPa m 1/2 batch 1 batch 2 batch 3 20 10 0 40 30 ΔF 3 = 4.6 kn ΔK 3 = 16 MPa m 1/2 batch 1 batch 2 batch 3 20 10 0 batch 1 batch 2 batch 3 0 75 150 225 300 N (10 3 )

Results a (mm) Average of Fatigue Crack Growth Curves (batch 1, 2 and 3) 35 30 25 20 15 10 5 0 F 1 =6.8 kn F 2 =5.7 kn F 3 =4.6 kn 0 50 100 150 200 250 300 N [10 3 ] The fatigue crack growth rate of brazed components is extremely sensitive to the load range.

Additional-III Nucleation based theory for ductile fracture under high triaxial stress Nucleation based theory for ductile fracture under high triaxial stress Mechanism of ductile fracture in pure silver under high-triaxial stress states under static loading (a) A few nanometer-sized cavities nucleate with small plastic strain. (b) Additional nucleation occurs with a small additional macroscopic strain. (c) At a critical stage, cavities get sufficiently close, and there is a coalescence between the small nanometer-sized cavities, and larger cavities form. (d) Additional nucleation occurs in regions near the cavities and further interlinkage occurs. (e) The interlinkage of larger cavities through continued nearby nucleation leads to final failure. M. C. Tolle, M. E. Kassner, Acta Metall. Mater. Vol 43 (1995) 287.

Additional-IV Damage and Fracture Behaviour of Brazed Joints Under Cyclic Loading ΔK 2 BULK MATERIALS a 1 ΔK 1 Δa ΔK 1 < ΔK 2 X3CrNiMo13-4 Δa` >> Δa BRAZED JOINTS Au-18Ni ΔK 1 a 1 Δa` ΔK 2 X3CrNiMo13-4 ΔK 1 < ΔK 2