Hot Tearing Variables Alloy Factors Effects of Alloy Chemistry


Verö carried out a systematic investigation of the effects of alloy composition on hot tearing.12


He studied the Al-Si al-


loys by measuring the average length of hot tearing of a “U” shaped permanent mold casting. The work showed that the severity of hot tearing increased with increasing Si content (up to 1.9%), and then tearing rapidly decreased with further increase of Si. This is the earliest record of the effect of alloy chemistry on hot tearing.


In the early 1940’s, Pumphrey et al. carried out a systematic study of six Al binary alloys: Al-Si, Al-Cu, Al-Mg, Al-Fe, Al-Mn, and Al-Zn, all made from high purity Al and high purity master alloys or virgin metals.21


The ring-casting test


was used, and the total length appearing on the ring surfaces were measured as the severity index of cracking. In the ring test each alloy was cast at three pouring temperatures with superheats of 20C (68F), 60C (140F), and 100C (212F), respectively. In all systems an initial increase in cracking was observed from the initial addition of the alloying ele- ment to super-purity Al, followed by a subsequent decrease to zero cracking at some higher element level. Alloy com- position and superheat determined alloy grain structure and thus hot tearing susceptibility. In Al-Si, Al-Cu, Al-Mg, and Al-Zn systems, at a given or fixed superheat, the grain structure changed from columnar to transitional and then to equiaxed with increasing amount of alloy element, which corresponds to decreasing cracking length.


In a study of hot tearing of nonferrous binary alloys, Rosenberg et al. developed a test and studied Al-Mg, Al-Sn, Al-Cu, Mg-Al and Mg-Zn alloy systems.19


Hot


tear resistance was rated as the maximum length of test casting which could be made free of tears; the greater the length, the greater the resistance to tearing. The lengths for pure Al and pure Mg were about 12” (30.48 cm). With a small addition of the solute to pure metals the hot tearing resistance decreased. Various alloying elements affected hot tearing in different ways, some more dramatically than others. For example, the resis- tance to hot tearing (the maximum length) was reduced by a factor of 3 when 0.5% Sn was added in Al, while similar additions of Cu had relatively small effect. In all alloys studied, minimum tear resistance was ob- tained at one or more compositions in the range of 0.25 to 10% alloy additions. After this point the resistance to hot tears starts to increase with further increase of alloy addition. For example, it was 0.25-5% Sn for Al-Sn alloy, 5% Cu for Al-Cu alloy and 4-6% Mg for Al-Mg alloy. The experimental results of hot tearing susceptibility of Al-Sn and Al-Cu alloys are shown in Figure 2.


Rosenberg’s work helped establish the effect of alloy composition on hot tearing. For most binary alloys, the re- lationship between hot tearing tendency and alloy compo- sition can be represented by the so-called lambda curve2,22 (Figure 3). Generally, the larger the freezing range, the more the alloy is prone to hot tearing as the residence time in the vulnerable stage is longer than if the alloy had a narrower freezing range.


Figure 2. Hot tearing of restrained bar test in binary Al-Sn and Al-Cu systems.19 26 International Journal of Metalcasting/Winter 11


Page 1  |  Page 2  |  Page 3  |  Page 4  |  Page 5  |  Page 6  |  Page 7  |  Page 8  |  Page 9  |  Page 10  |  Page 11  |  Page 12  |  Page 13  |  Page 14  |  Page 15  |  Page 16  |  Page 17  |  Page 18  |  Page 19  |  Page 20  |  Page 21  |  Page 22  |  Page 23  |  Page 24  |  Page 25  |  Page 26  |  Page 27  |  Page 28  |  Page 29  |  Page 30  |  Page 31  |  Page 32  |  Page 33  |  Page 34  |  Page 35  |  Page 36  |  Page 37  |  Page 38  |  Page 39  |  Page 40  |  Page 41  |  Page 42  |  Page 43  |  Page 44  |  Page 45  |  Page 46  |  Page 47  |  Page 48  |  Page 49  |  Page 50  |  Page 51  |  Page 52  |  Page 53  |  Page 54  |  Page 55  |  Page 56  |  Page 57  |  Page 58  |  Page 59  |  Page 60  |  Page 61  |  Page 62  |  Page 63  |  Page 64  |  Page 65  |  Page 66  |  Page 67  |  Page 68  |  Page 69  |  Page 70  |  Page 71  |  Page 72  |  Page 73  |  Page 74  |  Page 75  |  Page 76  |  Page 77  |  Page 78  |  Page 79  |  Page 80  |  Page 81  |  Page 82  |  Page 83  |  Page 84  |  Page 85  |  Page 86  |  Page 87