Tek taraflı delmede kuvvet ve malzeme akışına etkiyen faktörlerin incelenmesi

Abstract

Dövme yöntemi, metale kazandırdığı mekanik özellikleri, kısa üretim zamanı ve çok miktarda ürün elde edebilme açısından üretim yöntemleri içinde önemli bir yere sahiptir. Ancak diğer yön testler deki gelişmeler karşısında bu özelliklerini korumak zorundadır. Gerek bu özelliklerin korunması ve gerekse çapak ve tufal olarak malzeme kaybının fazla olması bu konuda bazı yeni çalışmaların ya pılmasına neden olmaktadır» Bu çalışmalar, malzeme kaybının azaltılması ve şekil değiştir me mekanizmasının daha iyi anlaşılması üzerine yoğunlaşmaktadır. Dövmede malzeme akışının incelenmesi ve sonuçların pratikte uygulan ması en önemli aşamayı oluşturmaktadır. Bu çalışmada genel olarak, dövmede önemli bir yere sahip olan delme konusu işlenmiştir. Hammadde boyutları ve delme zımbasının çapı üzerinde durularak dövme küvetleri araştırılmıştır. Yapılan deneylerde Ç1040 çeliğinden d» 38 mm çap ve değişik yüksekliklerde silindirik deney parçaları kullanılmış ve işlem K sıcaklıkta tek taraftan delme şeklinde gerçekleştirilmiştir. Sıcak dövmede parça boyutlarının büyümesi, soğuma hızını düşür mesi nedeniyle malzemenin şekil değişimini kolaylaştırmaktadır. Çünkü yavaş soğuma hızı malzemenin akma gerilmesinin yavaş artması na neden olmakta ve işlem düşük akma gerilmesinde gerçekleşmektedir. Delmede zımba çapının delme basıncım azalttığı sonucuna varıl mış ve delme gerilmesinin gerçek yığma gerilmesinden daha yüksek " olduğu gözlenmiştir. Daha önce yapılan iki taraflı delme deneyle rinde bulunan kuvvetlerle bu çalışmada bulunan kuvvetler aynı şart lar altında karşılaştırıldığında, iki taraflı delmenin daha kolay ' olduğu sonucuna varılmıştır. Deneylerde kullanılan en büyük parça (h = 49,4 mm) ve en küçük çaplı delme zımbası (d = 20 mm) için de İme de °p la s tik şekil değiştir menin başlangıcında bulunan tf*Ak " 2'86 tfAk değeri teorik sonuç olan dv2»92 tfAk değerine oldukça yakındır.

In forging, which has atı important role among the manu facturing methods because of high mechanic properties and manu facturing rate, a controlled plastic deformation occours by means of application of an impact or pressure on metals using the tools called "dies". Forging is the oldest metal shaping method used by humanity. In 20 th century. A rapid progress took place in forging industry like in all technological progresses. Besides them, the greatest disadvantage of the method is the wasting about 25 to 30 percent of metal as flash and scale lost. In general, a typical die in forging consist of two parts. When upper and lower parts of the die close together, the gap between them is in the same shape as that of the forged part. Investigations about forging concentrate basically on decreasing the material lost and increasing the quality. Investigating the factors affecting the material flow in forging brings some advantages like filling the die completely, rising the quality and reducing abrassion of the dies. According to the previous work, the factors affecting the material flow in forging can be listed as follows: 1- Shape and dimensions of the billet. 2- Shape and dimensions of the forged pari 3- Forging temperature. 4- Characteristics of the forging machines. 5- Friction conditions. 6- Flash geometry. Arrangement of production lines and automation in forging technology are important objects to examine because of the fact that they improve effectiveness by decreasing the production time, VI la.this work the effects of billet dimensions and punch diameter on the indentation force and pressure, and material flow with open indentation were investigated. Upsetting tests "were also conducted at the same temperature and with the same material in order to compare the results with those of the indentation test. AISİ 1040 medium corbon steel was used as the test material. In these experiments cylindi rical specimens with initial diameter of d = 38 mm and heights of 19 mm, 38 mm and 49, 4 mm were used. In the indentation test; the specimens were placed centerally on a flat-end die of 45 mm diameter, and then indented to various using punches with sharp: corners. The punch diameters used in the tests were 20 mm, 24 mm and 28 mms, Dies and punches which were used in the experiments are made of AISI Hİ3 Hot work tool steel, hardened and tempered. Tool hardnesses after tempering were measured to be about 55 RC. Upsetting and indentation tests were conducted using a double acting 150 metric tons capacity hydraulic press. In the tests, the press was used as single acting and specimen tempera ture, friction conditions and r$m speed were kept constant. Deformation forces were measured and recorded electronically, This was achieved by using a wheats tone bridge which was built by adhering $wo strfiin-geges on the press and two more on another steel piece. Then output signals of the bridge were calibrated. A Clip -gage was made for recording the press strok. A 20x20x70 mm aluminium piece and two leaf springs of dimensions 1x14x170 mm were used for the gage. Another wheats-lone bridge was built by adhering four strain- gages on the centilever end of the springs* The output signals of the bridges were magnified by means of a dynamic indicator and were put in a x-y recorder, thus "Deformation Force-press strok" curves were plotted. This mea suring and recording circuit is shown in Figure 1. Specimens placed in graphite powder were heated in on electric furnice to 105 0 C temperature. In the first stage of the experimental work, "Force-Strok" curves were obtained, by using the circuit Shown in Pig,l and placing the specimens heated up to 105 0°C centerally between flat-end punches and upsetting to 60 percent ratio. Instanteneous heights and specimens cross-sections vii viii were calculated using the curves volume constancy and neglecting the barrelling in upsetting. Then "True s tress -strain" curves were plotted using the calculated strain, stress walues. In the second stage of the experiments, "Indentation force-strok" curves were obtained by placing the specimens, which were heated to the same temperature in those of upsetting tests, on a flat-end punch of 45 mm diameter cent e rally and indenting down to various ratios by means of sharp-cornered pun* ches. Indentation pressures were calculated dividing the indentati on forces to punch cross-sections. Instanteneous heights were also calculated using the stroke values and specimen initial heights. Natural logaritma of the ratio of initial height to instanteneous height is defined as the indentation ratio. Then "Indentation pressure-indentation ratio" curves -were plotted by means of calculated values. Material flow was also examined using some of the chosen upset and indented specimens. For this aim, these specimens were cut and grounded in order to produce the cross sections parallel to their deformation axes. Then these parts were boiled 60 minutes in a "50 % H2 0 + 50 % HC1* Solution therefore the flow Lines were detected, and macro photographs were taken. Conclusions. Results of the experimental investigation may be listed as follows. 1- In single punch indentation, force and pressure values are higher compared to double punch indentation tests [9]for the same material, temperature and indentation ratio. 2- When the deflections from linearity in the diagrams which show- "true stres-strain" and "Indentation pressure - inden tation ratio" are taken as the yield points the ratios of inden tation yield point to yield stress increase with decreasing punch diameter as expected. It is interesting that, this ratio for the minimum punch diameter and maximum specimen height reaches up to a value given below. This value compares well with the values given by other investigators [13» is]. ax 3- For the small deformations, barreling is small and dead zone is not definite. 4- The dead zone becomes definite as the increasing of the deformation, and the area which does not contact with the flat-end of the punch becomes conical to invsrd. The angle increases as increasing of the specimen height for the same indentation ratio. 5- She material beneath the punch drags the material around itself, when it mo-res outwards and makes a curved surface drround the punch,. 6- During the indentation tests, barrelling begins at nearest zone to the punch and propagetes as the punch moves down